ckeditor/ckeditor5-engine

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src/view/downcastwriter.js

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/**
 * @license Copyright (c) 2003-2020, CKSource - Frederico Knabben. All rights reserved.
 * For licensing, see LICENSE.md or https://ckeditor.com/legal/ckeditor-oss-license
 */

/**
 * @module module:engine/view/downcastwriter
 */

import Position from './position';
import Range from './range';
import Selection from './selection';
import ContainerElement from './containerelement';
import AttributeElement from './attributeelement';
import EmptyElement from './emptyelement';
import UIElement from './uielement';
import CKEditorError from '@ckeditor/ckeditor5-utils/src/ckeditorerror';
import DocumentFragment from './documentfragment';
import isIterable from '@ckeditor/ckeditor5-utils/src/isiterable';
import Text from './text';
import EditableElement from './editableelement';
import { isPlainObject } from 'lodash-es';

/**
 * View downcast writer.
 *
 * It provides a set of methods used to manipulate view nodes.
 *
 * Do not create an instance of this writer manually. To modify a view structure, use
 * the {@link module:engine/view/view~View#change `View#change()`} block.
 *
 * The `DowncastWriter` is designed to work with semantic views which are the views that were/are being downcasted from the model.
 * To work with ordinary views (e.g. parsed from a pasted content) use the
 * {@link module:engine/view/upcastwriter~UpcastWriter upcast writer}.
 *
 * Read more about changing the view in the {@glink framework/guides/architecture/editing-engine#changing-the-view Changing the view}
 * section of the {@glink framework/guides/architecture/editing-engine Editing engine architecture} guide.
 */
export default class DowncastWriter {
    /**
     * @param {module:engine/view/document~Document} document The view document instance.
     */
    constructor( document ) {
        /**
         * The view document instance in which this writer operates.
         *
         * @readonly
         * @type {module:engine/view/document~Document}
         */
        this.document = document;

        /**
         * Holds references to the attribute groups that share the same {@link module:engine/view/attributeelement~AttributeElement#id id}.
         * The keys are `id`s, the values are `Set`s holding {@link module:engine/view/attributeelement~AttributeElement}s.
         *
         * @private
         * @type {Map.<String,Set>}
         */
        this._cloneGroups = new Map();
    }

    /**
     * Sets {@link module:engine/view/documentselection~DocumentSelection selection's} ranges and direction to the
     * specified location based on the given {@link module:engine/view/selection~Selectable selectable}.
     *
     * Usage:
     *
     *        // Sets selection to the given range.
     *        const range = writer.createRange( start, end );
     *        writer.setSelection( range );
     *
     *        // Sets backward selection to the given range.
     *        const range = writer.createRange( start, end );
     *        writer.setSelection( range );
     *
     *        // Sets selection to given ranges.
     *         const ranges = [ writer.createRange( start1, end2 ), writer.createRange( start2, end2 ) ];
     *        writer.setSelection( range );
     *
     *        // Sets selection to the other selection.
     *        const otherSelection = writer.createSelection();
     *        writer.setSelection( otherSelection );
     *
     *         // Sets collapsed selection at the given position.
     *        const position = writer.createPositionFromPath( root, path );
     *        writer.setSelection( position );
     *
     *         // Sets collapsed selection at the position of given item and offset.
     *        const paragraph = writer.createContainerElement( 'p' );
     *        writer.setSelection( paragraph, offset );
     *
     * Creates a range inside an {@link module:engine/view/element~Element element} which starts before the first child of
      * that element and ends after the last child of that element.
     *
     *         writer.setSelection( paragraph, 'in' );
     *
     * Creates a range on the {@link module:engine/view/item~Item item} which starts before the item and ends just after the item.
     *
     *        writer.setSelection( paragraph, 'on' );
     *
     *         // Removes all ranges.
     *        writer.setSelection( null );
     *
     * `DowncastWriter#setSelection()` allow passing additional options (`backward`, `fake` and `label`) as the last argument.
     *
     *        // Sets selection as backward.
     *        writer.setSelection( range, { backward: true } );
     *
     *        // Sets selection as fake.
     *        // Fake selection does not render as browser native selection over selected elements and is hidden to the user.
     *         // This way, no native selection UI artifacts are displayed to the user and selection over elements can be
     *         // represented in other way, for example by applying proper CSS class.
     *        writer.setSelection( range, { fake: true } );
     *
     *         // Additionally fake's selection label can be provided. It will be used to describe fake selection in DOM
     *         // (and be  properly handled by screen readers).
     *        writer.setSelection( range, { fake: true, label: 'foo' } );
     *
     * @param {module:engine/view/selection~Selectable} selectable
     * @param {Number|'before'|'end'|'after'|'on'|'in'} [placeOrOffset] Sets place or offset of the selection.
     * @param {Object} [options]
     * @param {Boolean} [options.backward] Sets this selection instance to be backward.
     * @param {Boolean} [options.fake] Sets this selection instance to be marked as `fake`.
     * @param {String} [options.label] Label for the fake selection.
     */
    setSelection( selectable, placeOrOffset, options ) {
        this.document.selection._setTo( selectable, placeOrOffset, options );
    }

    /**
     * Moves {@link module:engine/view/documentselection~DocumentSelection#focus selection's focus} to the specified location.
     *
     * The location can be specified in the same form as {@link module:engine/view/view~View#createPositionAt view.createPositionAt()}
     * parameters.
     *
     * @param {module:engine/view/item~Item|module:engine/view/position~Position} itemOrPosition
     * @param {Number|'end'|'before'|'after'} [offset] Offset or one of the flags. Used only when
     * first parameter is a {@link module:engine/view/item~Item view item}.
     */
    setSelectionFocus( itemOrPosition, offset ) {
        this.document.selection._setFocus( itemOrPosition, offset );
    }

    /**
     * Creates a new {@link module:engine/view/text~Text text node}.
     *
     *        writer.createText( 'foo' );
     *
     * @param {String} data The text's data.
     * @returns {module:engine/view/text~Text} The created text node.
     */
    createText( data ) {
        return new Text( this.document, data );
    }

    /**
     * Creates new {@link module:engine/view/attributeelement~AttributeElement}.
     *
     *        writer.createAttributeElement( 'strong' );
     *        writer.createAttributeElement( 'a', { href: 'foo.bar' } );
     *
     *        // Make `<a>` element contain other attributes element so the `<a>` element is not broken.
     *        writer.createAttributeElement( 'a', { href: 'foo.bar' }, { priority: 5 } );
     *
     *        // Set `id` of a marker element so it is not joined or merged with "normal" elements.
     *        writer.createAttributeElement( 'span', { class: 'my-marker' }, { id: 'marker:my' } );
     *
     * @param {String} name Name of the element.
     * @param {Object} [attributes] Element's attributes.
     * @param {Object} [options] Element's options.
     * @param {Number} [options.priority] Element's {@link module:engine/view/attributeelement~AttributeElement#priority priority}.
     * @param {Number|String} [options.id] Element's {@link module:engine/view/attributeelement~AttributeElement#id id}.
     * @returns {module:engine/view/attributeelement~AttributeElement} Created element.
     */
    createAttributeElement( name, attributes, options = {} ) {
        const attributeElement = new AttributeElement( this.document, name, attributes );

        if ( options.priority ) {
            attributeElement._priority = options.priority;
        }

        if ( options.id ) {
            attributeElement._id = options.id;
        }

        return attributeElement;
    }

    /**
     * Creates new {@link module:engine/view/containerelement~ContainerElement}.
     *
     *        writer.createContainerElement( 'p' );
     *
     *        // Create element with custom attributes.
     *        writer.createContainerElement( 'div', { id: 'foo-bar', 'data-baz': '123' } );
     *
     *        // Create element with custom styles.
     *        writer.createContainerElement( 'p', { style: 'font-weight: bold; padding-bottom: 10px' } );
     *
     *        // Create element with custom classes.
     *        writer.createContainerElement( 'p', { class: 'foo bar baz' } );
     *
     * @param {String} name Name of the element.
     * @param {Object} [attributes] Elements attributes.
     * @returns {module:engine/view/containerelement~ContainerElement} Created element.
     */
    createContainerElement( name, attributes ) {
        return new ContainerElement( this.document, name, attributes );
    }

    /**
     * Creates new {@link module:engine/view/editableelement~EditableElement}.
     *
     *        writer.createEditableElement( 'div' );
     *        writer.createEditableElement( 'div', { id: 'foo-1234' } );
     *
     * @param {String} name Name of the element.
     * @param {Object} [attributes] Elements attributes.
     * @returns {module:engine/view/editableelement~EditableElement} Created element.
     */
    createEditableElement( name, attributes ) {
        const editableElement = new EditableElement( this.document, name, attributes );
        editableElement._document = this.document;

        return editableElement;
    }

    /**
     * Creates new {@link module:engine/view/emptyelement~EmptyElement}.
     *
     *        writer.createEmptyElement( 'img' );
     *        writer.createEmptyElement( 'img', { id: 'foo-1234' } );
     *
     * @param {String} name Name of the element.
     * @param {Object} [attributes] Elements attributes.
     * @returns {module:engine/view/emptyelement~EmptyElement} Created element.
     */
    createEmptyElement( name, attributes ) {
        return new EmptyElement( this.document, name, attributes );
    }

    /**
     * Creates new {@link module:engine/view/uielement~UIElement}.
     *
     *        writer.createUIElement( 'span' );
     *        writer.createUIElement( 'span', { id: 'foo-1234' } );
     *
     * Custom render function can be provided as third parameter:
     *
     *        writer.createUIElement( 'span', null, function( domDocument ) {
     *            const domElement = this.toDomElement( domDocument );
     *            domElement.innerHTML = '<b>this is ui element</b>';
     *
     *            return domElement;
     *        } );
     *
     * @param {String} name Name of the element.
     * @param {Object} [attributes] Elements attributes.
     * @param {Function} [renderFunction] Custom render function.
     * @returns {module:engine/view/uielement~UIElement} Created element.
     */
    createUIElement( name, attributes, renderFunction ) {
        const uiElement = new UIElement( this.document, name, attributes );

        if ( renderFunction ) {
            uiElement.render = renderFunction;
        }

        return uiElement;
    }

    /**
     * Adds or overwrite element's attribute with a specified key and value.
     *
     *        writer.setAttribute( 'href', 'http://ckeditor.com', linkElement );
     *
     * @param {String} key Attribute key.
     * @param {String} value Attribute value.
     * @param {module:engine/view/element~Element} element
     */
    setAttribute( key, value, element ) {
        element._setAttribute( key, value );
    }

    /**
     * Removes attribute from the element.
     *
     *        writer.removeAttribute( 'href', linkElement );
     *
     * @param {String} key Attribute key.
     * @param {module:engine/view/element~Element} element
     */
    removeAttribute( key, element ) {
        element._removeAttribute( key );
    }

    /**
     * Adds specified class to the element.
     *
     *        writer.addClass( 'foo', linkElement );
     *        writer.addClass( [ 'foo', 'bar' ], linkElement );
     *
     * @param {Array.<String>|String} className
     * @param {module:engine/view/element~Element} element
     */
    addClass( className, element ) {
        element._addClass( className );
    }

    /**
     * Removes specified class from the element.
     *
     *        writer.removeClass( 'foo', linkElement );
     *        writer.removeClass( [ 'foo', 'bar' ], linkElement );
     *
     * @param {Array.<String>|String} className
     * @param {module:engine/view/element~Element} element
     */
    removeClass( className, element ) {
        element._removeClass( className );
    }

    /**
     * Adds style to the element.
     *
     *        writer.setStyle( 'color', 'red', element );
     *        writer.setStyle( {
     *            color: 'red',
     *            position: 'fixed'
     *        }, element );
     *
     * **Note**: The passed style can be normalized if
     * {@link module:engine/controller/datacontroller~DataController#addStyleProcessorRules a particular style processor rule is enabled}.
     * See {@link module:engine/view/stylesmap~StylesMap#set `StylesMap#set()`} for details.
     *
     * @param {String|Object} property Property name or object with key - value pairs.
     * @param {String} [value] Value to set. This parameter is ignored if object is provided as the first parameter.
     * @param {module:engine/view/element~Element} element Element to set styles on.
     */
    setStyle( property, value, element ) {
        if ( isPlainObject( property ) && element === undefined ) {
            element = value;
        }

        element._setStyle( property, value );
    }

    /**
     * Removes specified style from the element.
     *
     *        writer.removeStyle( 'color', element ); // Removes 'color' style.
     *        writer.removeStyle( [ 'color', 'border-top' ], element ); // Removes both 'color' and 'border-top' styles.
     *
     * **Note**: This method can work with normalized style names if
     * {@link module:engine/controller/datacontroller~DataController#addStyleProcessorRules a particular style processor rule is enabled}.
     * See {@link module:engine/view/stylesmap~StylesMap#remove `StylesMap#remove()`} for details.
     *
     * @param {Array.<String>|String} property
     * @param {module:engine/view/element~Element} element
     */
    removeStyle( property, element ) {
        element._removeStyle( property );
    }

    /**
     * Sets a custom property on element. Unlike attributes, custom properties are not rendered to the DOM,
     * so they can be used to add special data to elements.
     *
     * @param {String|Symbol} key
     * @param {*} value
     * @param {module:engine/view/element~Element} element
     */
    setCustomProperty( key, value, element ) {
        element._setCustomProperty( key, value );
    }

    /**
     * Removes a custom property stored under the given key.
     *
     * @param {String|Symbol} key
     * @param {module:engine/view/element~Element} element
     * @returns {Boolean} Returns true if property was removed.
     */
    removeCustomProperty( key, element ) {
        return element._removeCustomProperty( key );
    }

    /**
     * Breaks attribute nodes at provided position or at boundaries of provided range. It breaks attribute elements inside
     * up to a container element.
     *
     * In following examples `<p>` is a container, `<b>` and `<u>` are attribute nodes:
     *
     *        <p>foo<b><u>bar{}</u></b></p> -> <p>foo<b><u>bar</u></b>[]</p>
     *        <p>foo<b><u>{}bar</u></b></p> -> <p>foo{}<b><u>bar</u></b></p>
     *        <p>foo<b><u>b{}ar</u></b></p> -> <p>foo<b><u>b</u></b>[]<b><u>ar</u></b></p>
     *        <p><b>fo{o</b><u>ba}r</u></p> -> <p><b>fo</b><b>o</b><u>ba</u><u>r</u></b></p>
     *
     * **Note:** {@link module:engine/view/documentfragment~DocumentFragment DocumentFragment} is treated like a container.
     *
     * **Note:** Difference between {@link module:engine/view/downcastwriter~DowncastWriter#breakAttributes breakAttributes} and
     * {@link module:engine/view/downcastwriter~DowncastWriter#breakContainer breakContainer} is that `breakAttributes` breaks all
     * {@link module:engine/view/attributeelement~AttributeElement attribute elements} that are ancestors of given `position`,
     * up to the first encountered {@link module:engine/view/containerelement~ContainerElement container element}.
     * `breakContainer` assumes that given `position` is directly in container element and breaks that container element.
     *
     * Throws {@link module:utils/ckeditorerror~CKEditorError CKEditorError} `view-writer-invalid-range-container`
     * when {@link module:engine/view/range~Range#start start}
     * and {@link module:engine/view/range~Range#end end} positions of a passed range are not placed inside same parent container.
     *
     * Throws {@link module:utils/ckeditorerror~CKEditorError CKEditorError} `view-writer-cannot-break-empty-element`
     * when trying to break attributes
     * inside {@link module:engine/view/emptyelement~EmptyElement EmptyElement}.
     *
     * Throws {@link module:utils/ckeditorerror~CKEditorError CKEditorError} `view-writer-cannot-break-ui-element`
     * when trying to break attributes
     * inside {@link module:engine/view/uielement~UIElement UIElement}.
     *
     * @see module:engine/view/attributeelement~AttributeElement
     * @see module:engine/view/containerelement~ContainerElement
     * @see module:engine/view/downcastwriter~DowncastWriter#breakContainer
     * @param {module:engine/view/position~Position|module:engine/view/range~Range} positionOrRange Position where
     * to break attribute elements.
     * @returns {module:engine/view/position~Position|module:engine/view/range~Range} New position or range, after breaking the attribute
     * elements.
     */
    breakAttributes( positionOrRange ) {
        if ( positionOrRange instanceof Position ) {
            return this._breakAttributes( positionOrRange );
        } else {
            return this._breakAttributesRange( positionOrRange );
        }
    }

    /**
     * Breaks {@link module:engine/view/containerelement~ContainerElement container view element} into two, at the given position. Position
     * has to be directly inside container element and cannot be in root. Does not break if position is at the beginning
     * or at the end of it's parent element.
     *
     *        <p>foo^bar</p> -> <p>foo</p><p>bar</p>
     *        <div><p>foo</p>^<p>bar</p></div> -> <div><p>foo</p></div><div><p>bar</p></div>
     *        <p>^foobar</p> -> ^<p>foobar</p>
     *        <p>foobar^</p> -> <p>foobar</p>^
     *
     * **Note:** Difference between {@link module:engine/view/downcastwriter~DowncastWriter#breakAttributes breakAttributes} and
     * {@link module:engine/view/downcastwriter~DowncastWriter#breakContainer breakContainer} is that `breakAttributes` breaks all
     * {@link module:engine/view/attributeelement~AttributeElement attribute elements} that are ancestors of given `position`,
     * up to the first encountered {@link module:engine/view/containerelement~ContainerElement container element}.
     * `breakContainer` assumes that given `position` is directly in container element and breaks that container element.
     *
     * @see module:engine/view/attributeelement~AttributeElement
     * @see module:engine/view/containerelement~ContainerElement
     * @see module:engine/view/downcastwriter~DowncastWriter#breakAttributes
     * @param {module:engine/view/position~Position} position Position where to break element.
     * @returns {module:engine/view/position~Position} Position between broken elements. If element has not been broken,
     * the returned position is placed either before it or after it.
     */
    breakContainer( position ) {
        const element = position.parent;

        if ( !( element.is( 'containerElement' ) ) ) {
            /**
             * Trying to break an element which is not a container element.
             *
             * @error view-writer-break-non-container-element
             */
            throw new CKEditorError(
                'view-writer-break-non-container-element: Trying to break an element which is not a container element.',
                this.document
            );
        }

        if ( !element.parent ) {
            /**
             * Trying to break root element.
             *
             * @error view-writer-break-root
             */
            throw new CKEditorError( 'view-writer-break-root: Trying to break root element.', this.document );
        }

        if ( position.isAtStart ) {
            return Position._createBefore( element );
        } else if ( !position.isAtEnd ) {
            const newElement = element._clone( false );

            this.insert( Position._createAfter( element ), newElement );

            const sourceRange = new Range( position, Position._createAt( element, 'end' ) );
            const targetPosition = new Position( newElement, 0 );

            this.move( sourceRange, targetPosition );
        }

        return Position._createAfter( element );
    }

    /**
     * Merges {@link module:engine/view/attributeelement~AttributeElement attribute elements}. It also merges text nodes if needed.
     * Only {@link module:engine/view/attributeelement~AttributeElement#isSimilar similar} attribute elements can be merged.
     *
     * In following examples `<p>` is a container and `<b>` is an attribute element:
     *
     *        <p>foo[]bar</p> -> <p>foo{}bar</p>
     *        <p><b>foo</b>[]<b>bar</b></p> -> <p><b>foo{}bar</b></p>
     *        <p><b foo="bar">a</b>[]<b foo="baz">b</b></p> -> <p><b foo="bar">a</b>[]<b foo="baz">b</b></p>
     *
     * It will also take care about empty attributes when merging:
     *
     *        <p><b>[]</b></p> -> <p>[]</p>
     *        <p><b>foo</b><i>[]</i><b>bar</b></p> -> <p><b>foo{}bar</b></p>
     *
     * **Note:** Difference between {@link module:engine/view/downcastwriter~DowncastWriter#mergeAttributes mergeAttributes} and
     * {@link module:engine/view/downcastwriter~DowncastWriter#mergeContainers mergeContainers} is that `mergeAttributes` merges two
     * {@link module:engine/view/attributeelement~AttributeElement attribute elements} or {@link module:engine/view/text~Text text nodes}
     * while `mergeContainer` merges two {@link module:engine/view/containerelement~ContainerElement container elements}.
     *
     * @see module:engine/view/attributeelement~AttributeElement
     * @see module:engine/view/containerelement~ContainerElement
     * @see module:engine/view/downcastwriter~DowncastWriter#mergeContainers
     * @param {module:engine/view/position~Position} position Merge position.
     * @returns {module:engine/view/position~Position} Position after merge.
     */
    mergeAttributes( position ) {
        const positionOffset = position.offset;
        const positionParent = position.parent;

        // When inside text node - nothing to merge.
        if ( positionParent.is( 'text' ) ) {
            return position;
        }

        // When inside empty attribute - remove it.
        if ( positionParent.is( 'attributeElement' ) && positionParent.childCount === 0 ) {
            const parent = positionParent.parent;
            const offset = positionParent.index;

            positionParent._remove();
            this._removeFromClonedElementsGroup( positionParent );

            return this.mergeAttributes( new Position( parent, offset ) );
        }

        const nodeBefore = positionParent.getChild( positionOffset - 1 );
        const nodeAfter = positionParent.getChild( positionOffset );

        // Position should be placed between two nodes.
        if ( !nodeBefore || !nodeAfter ) {
            return position;
        }

        // When position is between two text nodes.
        if ( nodeBefore.is( 'text' ) && nodeAfter.is( 'text' ) ) {
            return mergeTextNodes( nodeBefore, nodeAfter );
        }
        // When position is between two same attribute elements.
        else if ( nodeBefore.is( 'attributeElement' ) && nodeAfter.is( 'attributeElement' ) && nodeBefore.isSimilar( nodeAfter ) ) {
            // Move all children nodes from node placed after selection and remove that node.
            const count = nodeBefore.childCount;
            nodeBefore._appendChild( nodeAfter.getChildren() );

            nodeAfter._remove();
            this._removeFromClonedElementsGroup( nodeAfter );

            // New position is located inside the first node, before new nodes.
            // Call this method recursively to merge again if needed.
            return this.mergeAttributes( new Position( nodeBefore, count ) );
        }

        return position;
    }

    /**
     * Merges two {@link module:engine/view/containerelement~ContainerElement container elements} that are before and after given position.
     * Precisely, the element after the position is removed and it's contents are moved to element before the position.
     *
     *        <p>foo</p>^<p>bar</p> -> <p>foo^bar</p>
     *        <div>foo</div>^<p>bar</p> -> <div>foo^bar</div>
     *
     * **Note:** Difference between {@link module:engine/view/downcastwriter~DowncastWriter#mergeAttributes mergeAttributes} and
     * {@link module:engine/view/downcastwriter~DowncastWriter#mergeContainers mergeContainers} is that `mergeAttributes` merges two
     * {@link module:engine/view/attributeelement~AttributeElement attribute elements} or {@link module:engine/view/text~Text text nodes}
     * while `mergeContainer` merges two {@link module:engine/view/containerelement~ContainerElement container elements}.
     *
     * @see module:engine/view/attributeelement~AttributeElement
     * @see module:engine/view/containerelement~ContainerElement
     * @see module:engine/view/downcastwriter~DowncastWriter#mergeAttributes
     * @param {module:engine/view/position~Position} position Merge position.
     * @returns {module:engine/view/position~Position} Position after merge.
     */
    mergeContainers( position ) {
        const prev = position.nodeBefore;
        const next = position.nodeAfter;

        if ( !prev || !next || !prev.is( 'containerElement' ) || !next.is( 'containerElement' ) ) {
            /**
             * Element before and after given position cannot be merged.
             *
             * @error view-writer-merge-containers-invalid-position
             */
            throw new CKEditorError( 'view-writer-merge-containers-invalid-position: ' +
                'Element before and after given position cannot be merged.', this.document );
        }

        const lastChild = prev.getChild( prev.childCount - 1 );
        const newPosition = lastChild instanceof Text ? Position._createAt( lastChild, 'end' ) : Position._createAt( prev, 'end' );

        this.move( Range._createIn( next ), Position._createAt( prev, 'end' ) );
        this.remove( Range._createOn( next ) );

        return newPosition;
    }

    /**
     * Insert node or nodes at specified position. Takes care about breaking attributes before insertion
     * and merging them afterwards.
     *
     * Throws {@link module:utils/ckeditorerror~CKEditorError CKEditorError} `view-writer-insert-invalid-node` when nodes to insert
     * contains instances that are not {@link module:engine/view/text~Text Texts},
     * {@link module:engine/view/attributeelement~AttributeElement AttributeElements},
     * {@link module:engine/view/containerelement~ContainerElement ContainerElements},
     * {@link module:engine/view/emptyelement~EmptyElement EmptyElements} or
     * {@link module:engine/view/uielement~UIElement UIElements}.
     *
     * @param {module:engine/view/position~Position} position Insertion position.
     * @param {module:engine/view/text~Text|module:engine/view/attributeelement~AttributeElement|
     * module:engine/view/containerelement~ContainerElement|module:engine/view/emptyelement~EmptyElement|
     * module:engine/view/uielement~UIElement|Iterable.<module:engine/view/text~Text|
     * module:engine/view/attributeelement~AttributeElement|module:engine/view/containerelement~ContainerElement|
     * module:engine/view/emptyelement~EmptyElement|module:engine/view/uielement~UIElement>} nodes Node or nodes to insert.
     * @returns {module:engine/view/range~Range} Range around inserted nodes.
     */
    insert( position, nodes ) {
        nodes = isIterable( nodes ) ? [ ...nodes ] : [ nodes ];

        // Check if nodes to insert are instances of AttributeElements, ContainerElements, EmptyElements, UIElements or Text.
        validateNodesToInsert( nodes, this.document );

        const container = getParentContainer( position );

        if ( !container ) {
            /**
             * Position's parent container cannot be found.
             *
             * @error view-writer-invalid-position-container
             */
            throw new CKEditorError( 'view-writer-invalid-position-container', this.document );
        }

        const insertionPosition = this._breakAttributes( position, true );
        const length = container._insertChild( insertionPosition.offset, nodes );

        for ( const node of nodes ) {
            this._addToClonedElementsGroup( node );
        }

        const endPosition = insertionPosition.getShiftedBy( length );
        const start = this.mergeAttributes( insertionPosition );

        // When no nodes were inserted - return collapsed range.
        if ( length === 0 ) {
            return new Range( start, start );
        } else {
            // If start position was merged - move end position.
            if ( !start.isEqual( insertionPosition ) ) {
                endPosition.offset--;
            }

            const end = this.mergeAttributes( endPosition );

            return new Range( start, end );
        }
    }

    /**
     * Removes provided range from the container.
     *
     * Throws {@link module:utils/ckeditorerror~CKEditorError CKEditorError} `view-writer-invalid-range-container` when
     * {@link module:engine/view/range~Range#start start} and {@link module:engine/view/range~Range#end end} positions are not placed inside
     * same parent container.
     *
     * @param {module:engine/view/range~Range|module:engine/view/item~Item} rangeOrItem Range to remove from container
     * or an {@link module:engine/view/item~Item item} to remove. If range is provided, after removing, it will be updated
     * to a collapsed range showing the new position.
     * @returns {module:engine/view/documentfragment~DocumentFragment} Document fragment containing removed nodes.
     */
    remove( rangeOrItem ) {
        const range = rangeOrItem instanceof Range ? rangeOrItem : Range._createOn( rangeOrItem );

        validateRangeContainer( range, this.document );

        // If range is collapsed - nothing to remove.
        if ( range.isCollapsed ) {
            return new DocumentFragment( this.document );
        }

        // Break attributes at range start and end.
        const { start: breakStart, end: breakEnd } = this._breakAttributesRange( range, true );
        const parentContainer = breakStart.parent;

        const count = breakEnd.offset - breakStart.offset;

        // Remove nodes in range.
        const removed = parentContainer._removeChildren( breakStart.offset, count );

        for ( const node of removed ) {
            this._removeFromClonedElementsGroup( node );
        }

        // Merge after removing.
        const mergePosition = this.mergeAttributes( breakStart );
        range.start = mergePosition;
        range.end = mergePosition.clone();

        // Return removed nodes.
        return new DocumentFragment( this.document, removed );
    }

    /**
     * Removes matching elements from given range.
     *
     * Throws {@link module:utils/ckeditorerror~CKEditorError CKEditorError} `view-writer-invalid-range-container` when
     * {@link module:engine/view/range~Range#start start} and {@link module:engine/view/range~Range#end end} positions are not placed inside
     * same parent container.
     *
     * @param {module:engine/view/range~Range} range Range to clear.
     * @param {module:engine/view/element~Element} element Element to remove.
     */
    clear( range, element ) {
        validateRangeContainer( range, this.document );

        // Create walker on given range.
        // We walk backward because when we remove element during walk it modifies range end position.
        const walker = range.getWalker( {
            direction: 'backward',
            ignoreElementEnd: true
        } );

        // Let's walk.
        for ( const current of walker ) {
            const item = current.item;
            let rangeToRemove;

            // When current item matches to the given element.
            if ( item.is( 'element' ) && element.isSimilar( item ) ) {
                // Create range on this element.
                rangeToRemove = Range._createOn( item );
                // When range starts inside Text or TextProxy element.
            } else if ( !current.nextPosition.isAfter( range.start ) && item.is( 'textProxy' ) ) {
                // We need to check if parent of this text matches to given element.
                const parentElement = item.getAncestors().find( ancestor => {
                    return ancestor.is( 'element' ) && element.isSimilar( ancestor );
                } );

                // If it is then create range inside this element.
                if ( parentElement ) {
                    rangeToRemove = Range._createIn( parentElement );
                }
            }

            // If we have found element to remove.
            if ( rangeToRemove ) {
                // We need to check if element range stick out of the given range and truncate if it is.
                if ( rangeToRemove.end.isAfter( range.end ) ) {
                    rangeToRemove.end = range.end;
                }

                if ( rangeToRemove.start.isBefore( range.start ) ) {
                    rangeToRemove.start = range.start;
                }

                // At the end we remove range with found element.
                this.remove( rangeToRemove );
            }
        }
    }

    /**
     * Moves nodes from provided range to target position.
     *
     * Throws {@link module:utils/ckeditorerror~CKEditorError CKEditorError} `view-writer-invalid-range-container` when
     * {@link module:engine/view/range~Range#start start} and {@link module:engine/view/range~Range#end end} positions are not placed inside
     * same parent container.
     *
     * @param {module:engine/view/range~Range} sourceRange Range containing nodes to move.
     * @param {module:engine/view/position~Position} targetPosition Position to insert.
     * @returns {module:engine/view/range~Range} Range in target container. Inserted nodes are placed between
     * {@link module:engine/view/range~Range#start start} and {@link module:engine/view/range~Range#end end} positions.
     */
    move( sourceRange, targetPosition ) {
        let nodes;

        if ( targetPosition.isAfter( sourceRange.end ) ) {
            targetPosition = this._breakAttributes( targetPosition, true );

            const parent = targetPosition.parent;
            const countBefore = parent.childCount;

            sourceRange = this._breakAttributesRange( sourceRange, true );

            nodes = this.remove( sourceRange );

            targetPosition.offset += ( parent.childCount - countBefore );
        } else {
            nodes = this.remove( sourceRange );
        }

        return this.insert( targetPosition, nodes );
    }

    /**
     * Wraps elements within range with provided {@link module:engine/view/attributeelement~AttributeElement AttributeElement}.
     * If a collapsed range is provided, it will be wrapped only if it is equal to view selection.
     *
     * If a collapsed range was passed and is same as selection, the selection
     * will be moved to the inside of the wrapped attribute element.
     *
     * Throws {@link module:utils/ckeditorerror~CKEditorError} `view-writer-invalid-range-container`
     * when {@link module:engine/view/range~Range#start}
     * and {@link module:engine/view/range~Range#end} positions are not placed inside same parent container.
     *
     * Throws {@link module:utils/ckeditorerror~CKEditorError} `view-writer-wrap-invalid-attribute` when passed attribute element is not
     * an instance of {@link module:engine/view/attributeelement~AttributeElement AttributeElement}.
     *
     * Throws {@link module:utils/ckeditorerror~CKEditorError} `view-writer-wrap-nonselection-collapsed-range` when passed range
     * is collapsed and different than view selection.
     *
     * @param {module:engine/view/range~Range} range Range to wrap.
     * @param {module:engine/view/attributeelement~AttributeElement} attribute Attribute element to use as wrapper.
     * @returns {module:engine/view/range~Range} range Range after wrapping, spanning over wrapping attribute element.
    */
    wrap( range, attribute ) {
        if ( !( attribute instanceof AttributeElement ) ) {
            throw new CKEditorError( 'view-writer-wrap-invalid-attribute', this.document );
        }

        validateRangeContainer( range, this.document );

        if ( !range.isCollapsed ) {
            // Non-collapsed range. Wrap it with the attribute element.
            return this._wrapRange( range, attribute );
        } else {
            // Collapsed range. Wrap position.
            let position = range.start;

            if ( position.parent.is( 'element' ) && !_hasNonUiChildren( position.parent ) ) {
                position = position.getLastMatchingPosition( value => value.item.is( 'uiElement' ) );
            }

            position = this._wrapPosition( position, attribute );
            const viewSelection = this.document.selection;

            // If wrapping position is equal to view selection, move view selection inside wrapping attribute element.
            if ( viewSelection.isCollapsed && viewSelection.getFirstPosition().isEqual( range.start ) ) {
                this.setSelection( position );
            }

            return new Range( position );
        }
    }

    /**
     * Unwraps nodes within provided range from attribute element.
     *
     * Throws {@link module:utils/ckeditorerror~CKEditorError CKEditorError} `view-writer-invalid-range-container` when
     * {@link module:engine/view/range~Range#start start} and {@link module:engine/view/range~Range#end end} positions are not placed inside
     * same parent container.
     *
     * @param {module:engine/view/range~Range} range
     * @param {module:engine/view/attributeelement~AttributeElement} attribute
     */
    unwrap( range, attribute ) {
        if ( !( attribute instanceof AttributeElement ) ) {
            /**
             * Attribute element need to be instance of attribute element.
             *
             * @error view-writer-unwrap-invalid-attribute
             */
            throw new CKEditorError( 'view-writer-unwrap-invalid-attribute', this.document );
        }

        validateRangeContainer( range, this.document );

        // If range is collapsed - nothing to unwrap.
        if ( range.isCollapsed ) {
            return range;
        }

        // Break attributes at range start and end.
        const { start: breakStart, end: breakEnd } = this._breakAttributesRange( range, true );
        const parentContainer = breakStart.parent;

        // Unwrap children located between break points.
        const newRange = this._unwrapChildren( parentContainer, breakStart.offset, breakEnd.offset, attribute );

        // Merge attributes at the both ends and return a new range.
        const start = this.mergeAttributes( newRange.start );

        // If start position was merged - move end position back.
        if ( !start.isEqual( newRange.start ) ) {
            newRange.end.offset--;
        }

        const end = this.mergeAttributes( newRange.end );

        return new Range( start, end );
    }

    /**
     * Renames element by creating a copy of renamed element but with changed name and then moving contents of the
     * old element to the new one. Keep in mind that this will invalidate all {@link module:engine/view/position~Position positions} which
     * has renamed element as {@link module:engine/view/position~Position#parent a parent}.
     *
     * New element has to be created because `Element#tagName` property in DOM is readonly.
     *
     * Since this function creates a new element and removes the given one, the new element is returned to keep reference.
     *
     * @param {String} newName New name for element.
     * @param {module:engine/view/containerelement~ContainerElement} viewElement Element to be renamed.
     */
    rename( newName, viewElement ) {
        const newElement = new ContainerElement( this.document, newName, viewElement.getAttributes() );

        this.insert( Position._createAfter( viewElement ), newElement );
        this.move( Range._createIn( viewElement ), Position._createAt( newElement, 0 ) );
        this.remove( Range._createOn( viewElement ) );

        return newElement;
    }

    /**
     * Cleans up memory by removing obsolete cloned elements group from the writer.
     *
     * Should be used whenever all {@link module:engine/view/attributeelement~AttributeElement attribute elements}
     * with the same {@link module:engine/view/attributeelement~AttributeElement#id id} are going to be removed from the view and
     * the group will no longer be needed.
     *
     * Cloned elements group are not removed automatically in case if the group is still needed after all its elements
     * were removed from the view.
     *
     * Keep in mind that group names are equal to the `id` property of the attribute element.
     *
     * @param {String} groupName Name of the group to clear.
     */
    clearClonedElementsGroup( groupName ) {
        this._cloneGroups.delete( groupName );
    }

    /**
     * Creates position at the given location. The location can be specified as:
     *
     * * a {@link module:engine/view/position~Position position},
     * * parent element and offset (offset defaults to `0`),
     * * parent element and `'end'` (sets position at the end of that element),
     * * {@link module:engine/view/item~Item view item} and `'before'` or `'after'` (sets position before or after given view item).
     *
     * This method is a shortcut to other constructors such as:
     *
     * * {@link #createPositionBefore},
     * * {@link #createPositionAfter},
     *
     * @param {module:engine/view/item~Item|module:engine/model/position~Position} itemOrPosition
     * @param {Number|'end'|'before'|'after'} [offset] Offset or one of the flags. Used only when
     * first parameter is a {@link module:engine/view/item~Item view item}.
     */
    createPositionAt( itemOrPosition, offset ) {
        return Position._createAt( itemOrPosition, offset );
    }

    /**
     * Creates a new position after given view item.
     *
     * @param {module:engine/view/item~Item} item View item after which the position should be located.
     * @returns {module:engine/view/position~Position}
     */
    createPositionAfter( item ) {
        return Position._createAfter( item );
    }

    /**
     * Creates a new position before given view item.
     *
     * @param {module:engine/view/item~Item} item View item before which the position should be located.
     * @returns {module:engine/view/position~Position}
     */
    createPositionBefore( item ) {
        return Position._createBefore( item );
    }

    /**
     * Creates a range spanning from `start` position to `end` position.
     *
     * **Note:** This factory method creates its own {@link module:engine/view/position~Position} instances basing on passed values.
     *
     * @param {module:engine/view/position~Position} start Start position.
     * @param {module:engine/view/position~Position} [end] End position. If not set, range will be collapsed at `start` position.
     * @returns {module:engine/view/range~Range}
     */
    createRange( start, end ) {
        return new Range( start, end );
    }

    /**
     * Creates a range that starts before given {@link module:engine/view/item~Item view item} and ends after it.
     *
     * @param {module:engine/view/item~Item} item
     * @returns {module:engine/view/range~Range}
     */
    createRangeOn( item ) {
        return Range._createOn( item );
    }

    /**
     * Creates a range inside an {@link module:engine/view/element~Element element} which starts before the first child of
     * that element and ends after the last child of that element.
     *
     * @param {module:engine/view/element~Element} element Element which is a parent for the range.
     * @returns {module:engine/view/range~Range}
     */
    createRangeIn( element ) {
        return Range._createIn( element );
    }

    /**
     Creates new {@link module:engine/view/selection~Selection} instance.
     *
     *         // Creates empty selection without ranges.
     *        const selection = writer.createSelection();
     *
     *        // Creates selection at the given range.
     *        const range = writer.createRange( start, end );
     *        const selection = writer.createSelection( range );
     *
     *        // Creates selection at the given ranges
     *         const ranges = [ writer.createRange( start1, end2 ), writer.createRange( star2, end2 ) ];
     *        const selection = writer.createSelection( ranges );
     *
     *        // Creates selection from the other selection.
     *        const otherSelection = writer.createSelection();
     *        const selection = writer.createSelection( otherSelection );
     *
     *        // Creates selection from the document selection.
     *        const selection = writer.createSelection( editor.editing.view.document.selection );
     *
     *         // Creates selection at the given position.
     *        const position = writer.createPositionFromPath( root, path );
     *        const selection = writer.createSelection( position );
     *
     *        // Creates collapsed selection at the position of given item and offset.
     *        const paragraph = writer.createContainerElement( 'p' );
     *        const selection = writer.createSelection( paragraph, offset );
     *
     *        // Creates a range inside an {@link module:engine/view/element~Element element} which starts before the
     *        // first child of that element and ends after the last child of that element.
     *        const selection = writer.createSelection( paragraph, 'in' );
     *
     *        // Creates a range on an {@link module:engine/view/item~Item item} which starts before the item and ends
     *        // just after the item.
     *        const selection = writer.createSelection( paragraph, 'on' );
     *
     * `Selection`'s constructor allow passing additional options (`backward`, `fake` and `label`) as the last argument.
     *
     *        // Creates backward selection.
     *        const selection = writer.createSelection( range, { backward: true } );
     *
     * Fake selection does not render as browser native selection over selected elements and is hidden to the user.
     * This way, no native selection UI artifacts are displayed to the user and selection over elements can be
     * represented in other way, for example by applying proper CSS class.
     *
     * Additionally fake's selection label can be provided. It will be used to describe fake selection in DOM
     * (and be  properly handled by screen readers).
     *
     *        // Creates fake selection with label.
     *        const selection = writer.createSelection( range, { fake: true, label: 'foo' } );
     *
     * @param {module:engine/view/selection~Selectable} [selectable=null]
     * @param {Number|'before'|'end'|'after'|'on'|'in'} [placeOrOffset] Offset or place when selectable is an `Item`.
     * @param {Object} [options]
     * @param {Boolean} [options.backward] Sets this selection instance to be backward.
     * @param {Boolean} [options.fake] Sets this selection instance to be marked as `fake`.
     * @param {String} [options.label] Label for the fake selection.
     * @returns {module:engine/view/selection~Selection}
     */
    createSelection( selectable, placeOrOffset, options ) {
        return new Selection( selectable, placeOrOffset, options );
    }

    /**
     * Wraps children with provided `wrapElement`. Only children contained in `parent` element between
     * `startOffset` and `endOffset` will be wrapped.
     *
     * @private
     * @param {module:engine/view/element~Element} parent
     * @param {Number} startOffset
     * @param {Number} endOffset
     * @param {module:engine/view/element~Element} wrapElement
     */
    _wrapChildren( parent, startOffset, endOffset, wrapElement ) {
        let i = startOffset;
        const wrapPositions = [];

        while ( i < endOffset ) {
            const child = parent.getChild( i );
            const isText = child.is( 'text' );
            const isAttribute = child.is( 'attributeElement' );
            const isEmpty = child.is( 'emptyElement' );
            const isUI = child.is( 'uiElement' );

            //
            // (In all examples, assume that `wrapElement` is `<span class="foo">` element.)
            //
            // Check if `wrapElement` can be joined with the wrapped element. One of requirements is having same name.
            // If possible, join elements.
            //
            // <p><span class="bar">abc</span></p>  -->  <p><span class="foo bar">abc</span></p>
            //
            if ( isAttribute && this._wrapAttributeElement( wrapElement, child ) ) {
                wrapPositions.push( new Position( parent, i ) );
            }
            //
            // Wrap the child if it is not an attribute element or if it is an attribute element that should be inside
            // `wrapElement` (due to priority).
            //
            // <p>abc</p>                   -->  <p><span class="foo">abc</span></p>
            // <p><strong>abc</strong></p>  -->  <p><span class="foo"><strong>abc</strong></span></p>
            //
            else if ( isText || isEmpty || isUI || ( isAttribute && shouldABeOutsideB( wrapElement, child ) ) ) {
                // Clone attribute.
                const newAttribute = wrapElement._clone();

                // Wrap current node with new attribute.
                child._remove();
                newAttribute._appendChild( child );

                parent._insertChild( i, newAttribute );
                this._addToClonedElementsGroup( newAttribute );

                wrapPositions.push( new Position( parent, i ) );
            }
            //
            // If other nested attribute is found and it wasn't wrapped (see above), continue wrapping inside it.
            //
            // <p><a href="foo.html">abc</a></p>  -->  <p><a href="foo.html"><span class="foo">abc</span></a></p>
            //
            else if ( isAttribute ) {
                this._wrapChildren( child, 0, child.childCount, wrapElement );
            }

            i++;
        }

        // Merge at each wrap.
        let offsetChange = 0;

        for ( const position of wrapPositions ) {
            position.offset -= offsetChange;

            // Do not merge with elements outside selected children.
            if ( position.offset == startOffset ) {
                continue;
            }

            const newPosition = this.mergeAttributes( position );

            // If nodes were merged - other merge offsets will change.
            if ( !newPosition.isEqual( position ) ) {
                offsetChange++;
                endOffset--;
            }
        }

        return Range._createFromParentsAndOffsets( parent, startOffset, parent, endOffset );
    }

    /**
     * Unwraps children from provided `unwrapElement`. Only children contained in `parent` element between
     * `startOffset` and `endOffset` will be unwrapped.
     *
     * @private
     * @param {module:engine/view/element~Element} parent
     * @param {Number} startOffset
     * @param {Number} endOffset
     * @param {module:engine/view/element~Element} unwrapElement
     */
    _unwrapChildren( parent, startOffset, endOffset, unwrapElement ) {
        let i = startOffset;
        const unwrapPositions = [];

        // Iterate over each element between provided offsets inside parent.
        // We don't use tree walker or range iterator because we will be removing and merging potentially multiple nodes,
        // so it could get messy. It is safer to it manually in this case.
        while ( i < endOffset ) {
            const child = parent.getChild( i );

            // Skip all text nodes. There should be no container element's here either.
            if ( !child.is( 'attributeElement' ) ) {
                i++;

                continue;
            }

            //
            // (In all examples, assume that `unwrapElement` is `<span class="foo">` element.)
            //
            // If the child is similar to the given attribute element, unwrap it - it will be completely removed.
            //
            // <p><span class="foo">abc</span>xyz</p>  -->  <p>abcxyz</p>
            //
            if ( child.isSimilar( unwrapElement ) ) {
                const unwrapped = child.getChildren();
                const count = child.childCount;

                // Replace wrapper element with its children
                child._remove();
                parent._insertChild( i, unwrapped );

                this._removeFromClonedElementsGroup( child );

                // Save start and end position of moved items.
                unwrapPositions.push(
                    new Position( parent, i ),
                    new Position( parent, i + count )
                );

                // Skip elements that were unwrapped. Assuming there won't be another element to unwrap in child elements.
                i += count;
                endOffset += count - 1;

                continue;
            }

            //
            // If the child is not similar but is an attribute element, try partial unwrapping - remove the same attributes/styles/classes.
            // Partial unwrapping will happen only if the elements have the same name.
            //
            // <p><span class="foo bar">abc</span>xyz</p>  -->  <p><span class="bar">abc</span>xyz</p>
            // <p><i class="foo">abc</i>xyz</p>            -->  <p><i class="foo">abc</i>xyz</p>
            //
            if ( this._unwrapAttributeElement( unwrapElement, child ) ) {
                unwrapPositions.push(
                    new Position( parent, i ),
                    new Position( parent, i + 1 )
                );

                i++;

                continue;
            }

            //
            // If other nested attribute is found, look through it's children for elements to unwrap.
            //
            // <p><i><span class="foo">abc</span></i><p>  -->  <p><i>abc</i><p>
            //
            this._unwrapChildren( child, 0, child.childCount, unwrapElement );

            i++;
        }

        // Merge at each unwrap.
        let offsetChange = 0;

        for ( const position of unwrapPositions ) {
            position.offset -= offsetChange;

            // Do not merge with elements outside selected children.
            if ( position.offset == startOffset || position.offset == endOffset ) {
                continue;
            }

            const newPosition = this.mergeAttributes( position );

            // If nodes were merged - other merge offsets will change.
            if ( !newPosition.isEqual( position ) ) {
                offsetChange++;
                endOffset--;
            }
        }

        return Range._createFromParentsAndOffsets( parent, startOffset, parent, endOffset );
    }

    /**
     * Helper function for `view.writer.wrap`. Wraps range with provided attribute element.
     * This method will also merge newly added attribute element with its siblings whenever possible.
     *
     * Throws {@link module:utils/ckeditorerror~CKEditorError} `view-writer-wrap-invalid-attribute` when passed attribute element is not
     * an instance of {@link module:engine/view/attributeelement~AttributeElement AttributeElement}.
     *
     * @private
     * @param {module:engine/view/range~Range} range
     * @param {module:engine/view/attributeelement~AttributeElement} attribute
     * @returns {module:engine/view/range~Range} New range after wrapping, spanning over wrapping attribute element.
     */
    _wrapRange( range, attribute ) {
        // Break attributes at range start and end.
        const { start: breakStart, end: breakEnd } = this._breakAttributesRange( range, true );
        const parentContainer = breakStart.parent;

        // Wrap all children with attribute.
        const newRange = this._wrapChildren( parentContainer, breakStart.offset, breakEnd.offset, attribute );

        // Merge attributes at the both ends and return a new range.
        const start = this.mergeAttributes( newRange.start );

        // If start position was merged - move end position back.
        if ( !start.isEqual( newRange.start ) ) {
            newRange.end.offset--;
        }
        const end = this.mergeAttributes( newRange.end );

        return new Range( start, end );
    }

    /**
     * Helper function for {@link #wrap}. Wraps position with provided attribute element.
     * This method will also merge newly added attribute element with its siblings whenever possible.
     *
     * Throws {@link module:utils/ckeditorerror~CKEditorError} `view-writer-wrap-invalid-attribute` when passed attribute element is not
     * an instance of {@link module:engine/view/attributeelement~AttributeElement AttributeElement}.
     *
     * @private
     * @param {module:engine/view/position~Position} position
     * @param {module:engine/view/attributeelement~AttributeElement} attribute
     * @returns {module:engine/view/position~Position} New position after wrapping.
     */
    _wrapPosition( position, attribute ) {
        // Return same position when trying to wrap with attribute similar to position parent.
        if ( attribute.isSimilar( position.parent ) ) {
            return movePositionToTextNode( position.clone() );
        }

        // When position is inside text node - break it and place new position between two text nodes.
        if ( position.parent.is( 'text' ) ) {
            position = breakTextNode( position );
        }

        // Create fake element that will represent position, and will not be merged with other attributes.
        const fakePosition = this.createAttributeElement();
        fakePosition._priority = Number.POSITIVE_INFINITY;
        fakePosition.isSimilar = () => false;

        // Insert fake element in position location.
        position.parent._insertChild( position.offset, fakePosition );

        // Range around inserted fake attribute element.
        const wrapRange = new Range( position, position.getShiftedBy( 1 ) );

        // Wrap fake element with attribute (it will also merge if possible).
        this.wrap( wrapRange, attribute );

        // Remove fake element and place new position there.
        const newPosition = new Position( fakePosition.parent, fakePosition.index );
        fakePosition._remove();

        // If position is placed between text nodes - merge them and return position inside.
        const nodeBefore = newPosition.nodeBefore;
        const nodeAfter = newPosition.nodeAfter;

        if ( nodeBefore instanceof Text && nodeAfter instanceof Text ) {
            return mergeTextNodes( nodeBefore, nodeAfter );
        }

        // If position is next to text node - move position inside.
        return movePositionToTextNode( newPosition );
    }

    /**
     *     Wraps one {@link module:engine/view/attributeelement~AttributeElement AttributeElement} into another by
     *     merging them if possible. When merging is possible - all attributes, styles and classes are moved from wrapper
     *     element to element being wrapped.
     *
     *     @private
     *     @param {module:engine/view/attributeelement~AttributeElement} wrapper Wrapper AttributeElement.
     *     @param {module:engine/view/attributeelement~AttributeElement} toWrap AttributeElement to wrap using wrapper element.
     *     @returns {Boolean} Returns `true` if elements are merged.
     */
    _wrapAttributeElement( wrapper, toWrap ) {
        if ( !canBeJoined( wrapper, toWrap ) ) {
            return false;
        }

        // Can't merge if name or priority differs.
        if ( wrapper.name !== toWrap.name || wrapper.priority !== toWrap.priority ) {
            return false;
        }

        // Check if attributes can be merged.
        for ( const key of wrapper.getAttributeKeys() ) {
            // Classes and styles should be checked separately.
            if ( key === 'class' || key === 'style' ) {
                continue;
            }

            // If some attributes are different we cannot wrap.
            if ( toWrap.hasAttribute( key ) && toWrap.getAttribute( key ) !== wrapper.getAttribute( key ) ) {
                return false;
            }
        }

        // Check if styles can be merged.
        for ( const key of wrapper.getStyleNames() ) {
            if ( toWrap.hasStyle( key ) && toWrap.getStyle( key ) !== wrapper.getStyle( key ) ) {
                return false;
            }
        }

        // Move all attributes/classes/styles from wrapper to wrapped AttributeElement.
        for ( const key of wrapper.getAttributeKeys() ) {
            // Classes and styles should be checked separately.
            if ( key === 'class' || key === 'style' ) {
                continue;
            }

            // Move only these attributes that are not present - other are similar.
            if ( !toWrap.hasAttribute( key ) ) {
                this.setAttribute( key, wrapper.getAttribute( key ), toWrap );
            }
        }

        for ( const key of wrapper.getStyleNames() ) {
            if ( !toWrap.hasStyle( key ) ) {
                this.setStyle( key, wrapper.getStyle( key ), toWrap );
            }
        }

        for ( const key of wrapper.getClassNames() ) {
            if ( !toWrap.hasClass( key ) ) {
                this.addClass( key, toWrap );
            }
        }

        return true;
    }

    /**
     * Unwraps {@link module:engine/view/attributeelement~AttributeElement AttributeElement} from another by removing
     * corresponding attributes, classes and styles. All attributes, classes and styles from wrapper should be present
     * inside element being unwrapped.
     *
     * @private
     * @param {module:engine/view/attributeelement~AttributeElement} wrapper Wrapper AttributeElement.
     * @param {module:engine/view/attributeelement~AttributeElement} toUnwrap AttributeElement to unwrap using wrapper element.
     * @returns {Boolean} Returns `true` if elements are unwrapped.
     **/
    _unwrapAttributeElement( wrapper, toUnwrap ) {
        if ( !canBeJoined( wrapper, toUnwrap ) ) {
            return false;
        }

        // Can't unwrap if name or priority differs.
        if ( wrapper.name !== toUnwrap.name || wrapper.priority !== toUnwrap.priority ) {
            return false;
        }

        // Check if AttributeElement has all wrapper attributes.
        for ( const key of wrapper.getAttributeKeys() ) {
            // Classes and styles should be checked separately.
            if ( key === 'class' || key === 'style' ) {
                continue;
            }

            // If some attributes are missing or different we cannot unwrap.
            if ( !toUnwrap.hasAttribute( key ) || toUnwrap.getAttribute( key ) !== wrapper.getAttribute( key ) ) {
                return false;
            }
        }

        // Check if AttributeElement has all wrapper classes.
        if ( !toUnwrap.hasClass( ...wrapper.getClassNames() ) ) {
            return false;
        }

        // Check if AttributeElement has all wrapper styles.
        for ( const key of wrapper.getStyleNames() ) {
            // If some styles are missing or different we cannot unwrap.
            if ( !toUnwrap.hasStyle( key ) || toUnwrap.getStyle( key ) !== wrapper.getStyle( key ) ) {
                return false;
            }
        }

        // Remove all wrapper's attributes from unwrapped element.
        for ( const key of wrapper.getAttributeKeys() ) {
            // Classes and styles should be checked separately.
            if ( key === 'class' || key === 'style' ) {
                continue;
            }

            this.removeAttribute( key, toUnwrap );
        }

        // Remove all wrapper's classes from unwrapped element.
        this.removeClass( Array.from( wrapper.getClassNames() ), toUnwrap );

        // Remove all wrapper's styles from unwrapped element.
        this.removeStyle( Array.from( wrapper.getStyleNames() ), toUnwrap );

        return true;
    }

    /**
     * Helper function used by other `DowncastWriter` methods. Breaks attribute elements at the boundaries of given range.
     *
     * @private
     * @param {module:engine/view/range~Range} range Range which `start` and `end` positions will be used to break attributes.
     * @param {Boolean} [forceSplitText=false] If set to `true`, will break text nodes even if they are directly in container element.
     * This behavior will result in incorrect view state, but is needed by other view writing methods which then fixes view state.
     * @returns {module:engine/view/range~Range} New range with located at break positions.
     */
    _breakAttributesRange( range, forceSplitText = false ) {
        const rangeStart = range.start;
        const rangeEnd = range.end;

        validateRangeContainer( range, this.document );

        // Break at the collapsed position. Return new collapsed range.
        if ( range.isCollapsed ) {
            const position = this._breakAttributes( range.start, forceSplitText );

            return new Range( position, position );
        }

        const breakEnd = this._breakAttributes( rangeEnd, forceSplitText );
        const count = breakEnd.parent.childCount;
        const breakStart = this._breakAttributes( rangeStart, forceSplitText );

        // Calculate new break end offset.
        breakEnd.offset += breakEnd.parent.childCount - count;

        return new Range( breakStart, breakEnd );
    }

    /**
     * Helper function used by other `DowncastWriter` methods. Breaks attribute elements at given position.
     *
     * Throws {@link module:utils/ckeditorerror~CKEditorError CKEditorError} `view-writer-cannot-break-empty-element` when break position
     * is placed inside {@link module:engine/view/emptyelement~EmptyElement EmptyElement}.
     *
     * Throws {@link module:utils/ckeditorerror~CKEditorError CKEditorError} `view-writer-cannot-break-ui-element` when break position
     * is placed inside {@link module:engine/view/uielement~UIElement UIElement}.
     *
     * @private
     * @param {module:engine/view/position~Position} position Position where to break attributes.
     * @param {Boolean} [forceSplitText=false] If set to `true`, will break text nodes even if they are directly in container element.
     * This behavior will result in incorrect view state, but is needed by other view writing methods which then fixes view state.
     * @returns {module:engine/view/position~Position} New position after breaking the attributes.
     */
    _breakAttributes( position, forceSplitText = false ) {
        const positionOffset = position.offset;
        const positionParent = position.parent;

        // If position is placed inside EmptyElement - throw an exception as we cannot break inside.
        if ( position.parent.is( 'emptyElement' ) ) {
            /**
             * Cannot break inside EmptyElement instance.
             *
             * @error view-writer-cannot-break-empty-element
             */
            throw new CKEditorError( 'view-writer-cannot-break-empty-element', this.document );
        }

        // If position is placed inside UIElement - throw an exception as we cannot break inside.
        if ( position.parent.is( 'uiElement' ) ) {
            /**
             * Cannot break inside UIElement instance.
             *
             * @error view-writer-cannot-break-ui-element
             */
            throw new CKEditorError( 'view-writer-cannot-break-ui-element', this.document );
        }

        // There are no attributes to break and text nodes breaking is not forced.
        if ( !forceSplitText && positionParent.is( 'text' ) && isContainerOrFragment( positionParent.parent ) ) {
            return position.clone();
        }

        // Position's parent is container, so no attributes to break.
        if ( isContainerOrFragment( positionParent ) ) {
            return position.clone();
        }

        // Break text and start again in new position.
        if ( positionParent.is( 'text' ) ) {
            return this._breakAttributes( breakTextNode( position ), forceSplitText );
        }

        const length = positionParent.childCount;

        // <p>foo<b><u>bar{}</u></b></p>
        // <p>foo<b><u>bar</u>[]</b></p>
        // <p>foo<b><u>bar</u></b>[]</p>
        if ( positionOffset == length ) {
            const newPosition = new Position( positionParent.parent, positionParent.index + 1 );

            return this._breakAttributes( newPosition, forceSplitText );
        } else {
            // <p>foo<b><u>{}bar</u></b></p>
            // <p>foo<b>[]<u>bar</u></b></p>
            // <p>foo{}<b><u>bar</u></b></p>
            if ( positionOffset === 0 ) {
                const newPosition = new Position( positionParent.parent, positionParent.index );

                return this._breakAttributes( newPosition, forceSplitText );
            }
            // <p>foo<b><u>b{}ar</u></b></p>
            // <p>foo<b><u>b[]ar</u></b></p>
            // <p>foo<b><u>b</u>[]<u>ar</u></b></p>
            // <p>foo<b><u>b</u></b>[]<b><u>ar</u></b></p>
            else {
                const offsetAfter = positionParent.index + 1;

                // Break element.
                const clonedNode = positionParent._clone();

                // Insert cloned node to position's parent node.
                positionParent.parent._insertChild( offsetAfter, clonedNode );
                this._addToClonedElementsGroup( clonedNode );

                // Get nodes to move.
                const count = positionParent.childCount - positionOffset;
                const nodesToMove = positionParent._removeChildren( positionOffset, count );

                // Move nodes to cloned node.
                clonedNode._appendChild( nodesToMove );

                // Create new position to work on.
                const newPosition = new Position( positionParent.parent, offsetAfter );

                return this._breakAttributes( newPosition, forceSplitText );
            }
        }
    }

    /**
     * Stores the information that an {@link module:engine/view/attributeelement~AttributeElement attribute element} was
     * added to the tree. Saves the reference to the group in the given element and updates the group, so other elements
     * from the group now keep a reference to the given attribute element.
     *
     * The clones group can be obtained using {@link module:engine/view/attributeelement~AttributeElement#getElementsWithSameId}.
     *
     * Does nothing if added element has no {@link module:engine/view/attributeelement~AttributeElement#id id}.
     *
     * @private
     * @param {module:engine/view/attributeelement~AttributeElement} element Attribute element to save.
     */
    _addToClonedElementsGroup( element ) {
        // Add only if the element is in document tree.
        if ( !element.root.is( 'rootElement' ) ) {
            return;
        }

        // Traverse the element's children recursively to find other attribute elements that also might got inserted.
        // The loop is at the beginning so we can make fast returns later in the code.
        if ( element.is( 'element' ) ) {
            for ( const child of element.getChildren() ) {
                this._addToClonedElementsGroup( child );
            }
        }

        const id = element.id;

        if ( !id ) {
            return;
        }

        let group = this._cloneGroups.get( id );

        if ( !group ) {
            group = new Set();
            this._cloneGroups.set( id, group );
        }

        group.add( element );
        element._clonesGroup = group;
    }

    /**
     * Removes all the information about the given {@link module:engine/view/attributeelement~AttributeElement attribute element}
     * from its clones group.
     *
     * Keep in mind, that the element will still keep a reference to the group (but the group will not keep a reference to it).
     * This allows to reference the whole group even if the element was already removed from the tree.
     *
     * Does nothing if the element has no {@link module:engine/view/attributeelement~AttributeElement#id id}.
     *
     * @private
     * @param {module:engine/view/attributeelement~AttributeElement} element Attribute element to remove.
     */
    _removeFromClonedElementsGroup( element ) {
        // Traverse the element's children recursively to find other attribute elements that also got removed.
        // The loop is at the beginning so we can make fast returns later in the code.
        if ( element.is( 'element' ) ) {
            for ( const child of element.getChildren() ) {
                this._removeFromClonedElementsGroup( child );
            }
        }

        const id = element.id;

        if ( !id ) {
            return;
        }

        const group = this._cloneGroups.get( id );

        if ( !group ) {
            return;
        }

        group.delete( element );
        // Not removing group from element on purpose!
        // If other parts of code have reference to this element, they will be able to get references to other elements from the group.
    }
}

// Helper function for `view.writer.wrap`. Checks if given element has any children that are not ui elements.
function _hasNonUiChildren( parent ) {
    return Array.from( parent.getChildren() ).some( child => !child.is( 'uiElement' ) );
}

/**
 * Attribute element need to be instance of attribute element.
 *
 * @error view-writer-wrap-invalid-attribute
 */

// Returns first parent container of specified {@link module:engine/view/position~Position Position}.
// Position's parent node is checked as first, then next parents are checked.
// Note that {@link module:engine/view/documentfragment~DocumentFragment DocumentFragment} is treated like a container.
//
// @param {module:engine/view/position~Position} position Position used as a start point to locate parent container.
// @returns {module:engine/view/containerelement~ContainerElement|module:engine/view/documentfragment~DocumentFragment|undefined}
// Parent container element or `undefined` if container is not found.
function getParentContainer( position ) {
    let parent = position.parent;

    while ( !isContainerOrFragment( parent ) ) {
        if ( !parent ) {
            return undefined;
        }
        parent = parent.parent;
    }

    return parent;
}

// Checks if first {@link module:engine/view/attributeelement~AttributeElement AttributeElement} provided to the function
// can be wrapped otuside second element. It is done by comparing elements'
// {@link module:engine/view/attributeelement~AttributeElement#priority priorities}, if both have same priority
// {@link module:engine/view/element~Element#getIdentity identities} are compared.
//
// @param {module:engine/view/attributeelement~AttributeElement} a
// @param {module:engine/view/attributeelement~AttributeElement} b
// @returns {Boolean}
function shouldABeOutsideB( a, b ) {
    if ( a.priority < b.priority ) {
        return true;
    } else if ( a.priority > b.priority ) {
        return false;
    }

    // When priorities are equal and names are different - use identities.
    return a.getIdentity() < b.getIdentity();
}

// Returns new position that is moved to near text node. Returns same position if there is no text node before of after
// specified position.
//
//        <p>foo[]</p>  ->  <p>foo{}</p>
//        <p>[]foo</p>  ->  <p>{}foo</p>
//
// @param {module:engine/view/position~Position} position
// @returns {module:engine/view/position~Position} Position located inside text node or same position if there is no text nodes
// before or after position location.
function movePositionToTextNode( position ) {
    const nodeBefore = position.nodeBefore;

    if ( nodeBefore && nodeBefore.is( 'text' ) ) {
        return new Position( nodeBefore, nodeBefore.data.length );
    }

    const nodeAfter = position.nodeAfter;

    if ( nodeAfter && nodeAfter.is( 'text' ) ) {
        return new Position( nodeAfter, 0 );
    }

    return position;
}

// Breaks text node into two text nodes when possible.
//
//        <p>foo{}bar</p> -> <p>foo[]bar</p>
//        <p>{}foobar</p> -> <p>[]foobar</p>
//        <p>foobar{}</p> -> <p>foobar[]</p>
//
// @param {module:engine/view/position~Position} position Position that need to be placed inside text node.
// @returns {module:engine/view/position~Position} New position after breaking text node.
function breakTextNode( position ) {
    if ( position.offset == position.parent.data.length ) {
        return new Position( position.parent.parent, position.parent.index + 1 );
    }

    if ( position.offset === 0 ) {
        return new Position( position.parent.parent, position.parent.index );
    }

    // Get part of the text that need to be moved.
    const textToMove = position.parent.data.slice( position.offset );

    // Leave rest of the text in position's parent.
    position.parent._data = position.parent.data.slice( 0, position.offset );

    // Insert new text node after position's parent text node.
    position.parent.parent._insertChild( position.parent.index + 1, new Text( position.root.document, textToMove ) );

    // Return new position between two newly created text nodes.
    return new Position( position.parent.parent, position.parent.index + 1 );
}

// Merges two text nodes into first node. Removes second node and returns merge position.
//
// @param {module:engine/view/text~Text} t1 First text node to merge. Data from second text node will be moved at the end of
// this text node.
// @param {module:engine/view/text~Text} t2 Second text node to merge. This node will be removed after merging.
// @returns {module:engine/view/position~Position} Position after merging text nodes.
function mergeTextNodes( t1, t2 ) {
    // Merge text data into first text node and remove second one.
    const nodeBeforeLength = t1.data.length;
    t1._data += t2.data;
    t2._remove();

    return new Position( t1, nodeBeforeLength );
}

// Checks if provided nodes are valid to insert. Checks if each node is an instance of
// {@link module:engine/view/text~Text Text} or {@link module:engine/view/attributeelement~AttributeElement AttributeElement},
// {@link module:engine/view/containerelement~ContainerElement ContainerElement},
// {@link module:engine/view/emptyelement~EmptyElement EmptyElement} or
// {@link module:engine/view/uielement~UIElement UIElement}.
//
// Throws {@link module:utils/ckeditorerror~CKEditorError CKEditorError} `view-writer-insert-invalid-node` when nodes to insert
// contains instances that are not {@link module:engine/view/text~Text Texts},
// {@link module:engine/view/emptyelement~EmptyElement EmptyElements},
// {@link module:engine/view/uielement~UIElement UIElements},
// {@link module:engine/view/attributeelement~AttributeElement AttributeElements} or
// {@link module:engine/view/containerelement~ContainerElement ContainerElements}.
//
// @param Iterable.<module:engine/view/text~Text|module:engine/view/attributeelement~AttributeElement
// |module:engine/view/containerelement~ContainerElement> nodes
// @param {Object} errorContext
function validateNodesToInsert( nodes, errorContext ) {
    for ( const node of nodes ) {
        if ( !validNodesToInsert.some( ( validNode => node instanceof validNode ) ) ) { // eslint-disable-line no-use-before-define
            /**
             * Inserted nodes should be valid to insert. of {@link module:engine/view/attributeelement~AttributeElement AttributeElement},
             * {@link module:engine/view/containerelement~ContainerElement ContainerElement},
             * {@link module:engine/view/emptyelement~EmptyElement EmptyElement},
             * {@link module:engine/view/uielement~UIElement UIElement}, {@link module:engine/view/text~Text Text}.
             *
             * @error view-writer-insert-invalid-node
             */
            throw new CKEditorError( 'view-writer-insert-invalid-node', errorContext );
        }

        if ( !node.is( 'text' ) ) {
            validateNodesToInsert( node.getChildren(), errorContext );
        }
    }
}

const validNodesToInsert = [ Text, AttributeElement, ContainerElement, EmptyElement, UIElement ];

// Checks if node is ContainerElement or DocumentFragment, because in most cases they should be treated the same way.
//
// @param {module:engine/view/node~Node} node
// @returns {Boolean} Returns `true` if node is instance of ContainerElement or DocumentFragment.
function isContainerOrFragment( node ) {
    return node && ( node.is( 'containerElement' ) || node.is( 'documentFragment' ) );
}

// Checks if {@link module:engine/view/range~Range#start range start} and {@link module:engine/view/range~Range#end range end} are placed
// inside same {@link module:engine/view/containerelement~ContainerElement container element}.
// Throws {@link module:utils/ckeditorerror~CKEditorError CKEditorError} `view-writer-invalid-range-container` when validation fails.
//
// @param {module:engine/view/range~Range} range
// @param {Object} errorContext
function validateRangeContainer( range, errorContext ) {
    const startContainer = getParentContainer( range.start );
    const endContainer = getParentContainer( range.end );

    if ( !startContainer || !endContainer || startContainer !== endContainer ) {
        /**
         * Range container is invalid. This can happen if {@link module:engine/view/range~Range#start range start} and
         * {@link module:engine/view/range~Range#end range end} positions are not placed inside same container or
         * parent container for these positions cannot be found.
         *
         * @error view-writer-invalid-range-container
         */

        throw new CKEditorError( 'view-writer-invalid-range-container', errorContext );
    }
}

// Checks if two attribute elements can be joined together. Elements can be joined together if, and only if
// they do not have ids specified.
//
// @private
// @param {module:engine/view/element~Element} a
// @param {module:engine/view/element~Element} b
// @returns {Boolean}
function canBeJoined( a, b ) {
    return a.id === null && b.id === null;
}