ckeditor/ckeditor5-engine

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src/conversion/downcasthelpers.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
 */

/**
 * Contains downcast (model-to-view) converters for {@link module:engine/conversion/downcastdispatcher~DowncastDispatcher}.
 *
 * @module engine/conversion/downcasthelpers
 */

import ModelRange from '../model/range';
import ModelSelection from '../model/selection';
import ModelElement from '../model/element';

import ViewAttributeElement from '../view/attributeelement';
import DocumentSelection from '../model/documentselection';
import ConversionHelpers from './conversionhelpers';

import { cloneDeep } from 'lodash-es';
import CKEditorError from '@ckeditor/ckeditor5-utils/src/ckeditorerror';

/**
 * Downcast conversion helper functions.
 *
 * @extends module:engine/conversion/conversionhelpers~ConversionHelpers
 */
export default class DowncastHelpers extends ConversionHelpers {
    /**
     * Model element to view element conversion helper.
     *
     * This conversion results in creating a view element. For example, model `<paragraph>Foo</paragraph>` becomes `<p>Foo</p>` in the view.
     *
     *        editor.conversion.for( 'downcast' ).elementToElement( {
     *            model: 'paragraph',
     *            view: 'p'
     *        } );
     *
     *        editor.conversion.for( 'downcast' ).elementToElement( {
     *            model: 'paragraph',
     *            view: 'div',
     *            converterPriority: 'high'
     *        } );
     *
     *        editor.conversion.for( 'downcast' ).elementToElement( {
     *            model: 'fancyParagraph',
     *            view: {
     *                name: 'p',
     *                classes: 'fancy'
     *            }
     *        } );
     *
     *        editor.conversion.for( 'downcast' ).elementToElement( {
     *            model: 'heading',
     *            view: ( modelElement, viewWriter ) => {
     *                return viewWriter.createContainerElement( 'h' + modelElement.getAttribute( 'level' ) )
     *            }
     *        } );
     *
     * See {@link module:engine/conversion/conversion~Conversion#for `conversion.for()`} to learn how to add a converter
     * to the conversion process.
     *
     * @method #elementToElement
     * @param {Object} config Conversion configuration.
     * @param {String} config.model The name of the model element to convert.
     * @param {module:engine/view/elementdefinition~ElementDefinition|Function} config.view A view element definition or a function
     * that takes the model element and {@link module:engine/view/downcastwriter~DowncastWriter view downcast writer}
     * as parameters and returns a view container element.
     * @returns {module:engine/conversion/downcasthelpers~DowncastHelpers}
     */
    elementToElement( config ) {
        return this.add( downcastElementToElement( config ) );
    }

    /**
     * Model attribute to view element conversion helper.
     *
     * This conversion results in wrapping view nodes with a view attribute element. For example, a model text node with
     * `"Foo"` as data and the `bold` attribute becomes `<strong>Foo</strong>` in the view.
     *
     *        editor.conversion.for( 'downcast' ).attributeToElement( {
     *            model: 'bold',
     *            view: 'strong'
     *        } );
     *
     *        editor.conversion.for( 'downcast' ).attributeToElement( {
     *            model: 'bold',
     *            view: 'b',
     *            converterPriority: 'high'
     *        } );
     *
     *        editor.conversion.for( 'downcast' ).attributeToElement( {
     *            model: 'invert',
     *            view: {
     *                name: 'span',
     *                classes: [ 'font-light', 'bg-dark' ]
     *            }
     *        } );
     *
     *        editor.conversion.for( 'downcast' ).attributeToElement( {
     *            model: {
     *                key: 'fontSize',
     *                values: [ 'big', 'small' ]
     *            },
     *            view: {
     *                big: {
     *                    name: 'span',
     *                    styles: {
     *                        'font-size': '1.2em'
     *                    }
     *                },
     *                small: {
     *                    name: 'span',
     *                    styles: {
     *                        'font-size': '0.8em'
     *                    }
     *                }
     *            }
     *        } );
     *
     *        editor.conversion.for( 'downcast' ).attributeToElement( {
     *            model: 'bold',
     *            view: ( modelAttributeValue, viewWriter ) => {
     *                return viewWriter.createAttributeElement( 'span', {
     *                    style: 'font-weight:' + modelAttributeValue
     *                } );
     *            }
     *        } );
     *
     *        editor.conversion.for( 'downcast' ).attributeToElement( {
     *            model: {
     *                key: 'color',
     *                name: '$text'
     *            },
     *            view: ( modelAttributeValue, viewWriter ) => {
     *                return viewWriter.createAttributeElement( 'span', {
     *                    style: 'color:' + modelAttributeValue
     *                } );
     *            }
     *        } );
     *
     * See {@link module:engine/conversion/conversion~Conversion#for `conversion.for()`} to learn how to add a converter
     * to the conversion process.
     *
     * @method #attributeToElement
     * @param {Object} config Conversion configuration.
     * @param {String|Object} config.model The key of the attribute to convert from or a `{ key, values }` object. `values` is an array
     * of `String`s with possible values if the model attribute is an enumerable.
     * @param {module:engine/view/elementdefinition~ElementDefinition|Function|Object} config.view A view element definition or a function
     * that takes the model attribute value and {@link module:engine/view/downcastwriter~DowncastWriter view downcast writer}
     * as parameters and returns a view attribute element. If `config.model.values` is
     * given, `config.view` should be an object assigning values from `config.model.values` to view element definitions or functions.
     * @param {module:utils/priorities~PriorityString} [config.converterPriority='normal'] Converter priority.
     * @returns {module:engine/conversion/downcasthelpers~DowncastHelpers}
     */
    attributeToElement( config ) {
        return this.add( downcastAttributeToElement( config ) );
    }

    /**
     * Model attribute to view attribute conversion helper.
     *
     * This conversion results in adding an attribute to a view node, basing on an attribute from a model node. For example,
     * `<image src='foo.jpg'></image>` is converted to `<img src='foo.jpg'></img>`.
     *
     *        editor.conversion.for( 'downcast' ).attributeToAttribute( {
     *            model: 'source',
     *            view: 'src'
     *        } );
     *
     *        editor.conversion.for( 'downcast' ).attributeToAttribute( {
     *            model: 'source',
     *            view: 'href',
     *            converterPriority: 'high'
     *        } );
     *
     *        editor.conversion.for( 'downcast' ).attributeToAttribute( {
     *            model: {
     *                name: 'image',
     *                key: 'source'
     *            },
     *            view: 'src'
     *        } );
     *
     *        editor.conversion.for( 'downcast' ).attributeToAttribute( {
     *            model: {
     *                name: 'styled',
     *                values: [ 'dark', 'light' ]
     *            },
     *            view: {
     *                dark: {
     *                    key: 'class',
     *                    value: [ 'styled', 'styled-dark' ]
     *                },
     *                light: {
     *                    key: 'class',
     *                    value: [ 'styled', 'styled-light' ]
     *                }
     *            }
     *        } );
     *
     *        editor.conversion.for( 'downcast' ).attributeToAttribute( {
     *            model: 'styled',
     *            view: modelAttributeValue => ( { key: 'class', value: 'styled-' + modelAttributeValue } )
     *        } );
     *
     * **Note**: Downcasting to a style property requires providing `value` as an object:
     *
     *        editor.conversion.for( 'downcast' ).attributeToAttribute( {
     *            model: 'lineHeight',
     *            view: modelAttributeValue => ( {
     *                key: 'style',
     *                value: {
     *                    'line-height': modelAttributeValue,
     *                    'border-bottom': '1px dotted #ba2'
     *                }
     *            } )
     *        } );
     *
     * See {@link module:engine/conversion/conversion~Conversion#for `conversion.for()`} to learn how to add a converter
     * to the conversion process.
     *
     * @method #attributeToAttribute
     * @param {Object} config Conversion configuration.
     * @param {String|Object} config.model The key of the attribute to convert from or a `{ key, values, [ name ] }` object describing
     * the attribute key, possible values and, optionally, an element name to convert from.
     * @param {String|Object|Function} config.view A view attribute key, or a `{ key, value }` object or a function that takes
     * the model attribute value and returns a `{ key, value }` object. If `key` is `'class'`, `value` can be a `String` or an
     * array of `String`s. If `key` is `'style'`, `value` is an object with key-value pairs. In other cases, `value` is a `String`.
     * If `config.model.values` is set, `config.view` should be an object assigning values from `config.model.values` to
     * `{ key, value }` objects or a functions.
     * @param {module:utils/priorities~PriorityString} [config.converterPriority='normal'] Converter priority.
     * @returns {module:engine/conversion/downcasthelpers~DowncastHelpers}
     */
    attributeToAttribute( config ) {
        return this.add( downcastAttributeToAttribute( config ) );
    }

    /**
     * Model marker to view element conversion helper.
     *
     * This conversion results in creating a view element on the boundaries of the converted marker. If the converted marker
     * is collapsed, only one element is created. For example, model marker set like this: `<paragraph>F[oo b]ar</paragraph>`
     * becomes `<p>F<span data-marker="search"></span>oo b<span data-marker="search"></span>ar</p>` in the view.
     *
     *        editor.conversion.for( 'downcast' ).markerToElement( {
     *            model: 'search',
     *            view: 'marker-search'
     *        } );
     *
     *        editor.conversion.for( 'downcast' ).markerToElement( {
     *            model: 'search',
     *            view: 'search-result',
     *            converterPriority: 'high'
     *        } );
     *
     *        editor.conversion.for( 'downcast' ).markerToElement( {
     *            model: 'search',
     *            view: {
     *                name: 'span',
     *                attributes: {
     *                    'data-marker': 'search'
     *                }
     *            }
     *        } );
     *
     *        editor.conversion.for( 'downcast' ).markerToElement( {
     *            model: 'search',
     *            view: ( markerData, viewWriter ) => {
     *                return viewWriter.createUIElement( 'span', {
     *                    'data-marker': 'search',
     *                    'data-start': markerData.isOpening
     *                } );
     *            }
     *        } );
     *
     * If a function is passed as the `config.view` parameter, it will be used to generate both boundary elements. The function
     * receives the `data` object as a parameter and should return an instance of the
     * {@link module:engine/view/uielement~UIElement view UI element}. The `data` object and
     * {@link module:engine/conversion/downcastdispatcher~DowncastConversionApi `conversionApi`} are passed from
     * {@link module:engine/conversion/downcastdispatcher~DowncastDispatcher#event:addMarker}. Additionally,
     * the `data.isOpening` parameter is passed, which is set to `true` for the marker start boundary element, and `false` to
     * the marker end boundary element.
     *
     * This kind of conversion is useful for saving data into the database, so it should be used in the data conversion pipeline.
     *
     * See {@link module:engine/conversion/conversion~Conversion#for `conversion.for()`} to learn how to add a converter
     * to the conversion process.
     *
     * @method #markerToElement
     * @param {Object} config Conversion configuration.
     * @param {String} config.model The name of the model marker (or model marker group) to convert.
     * @param {module:engine/view/elementdefinition~ElementDefinition|Function} config.view A view element definition or a function
     * that takes the model marker data as a parameter and returns a view UI element.
     * @param {module:utils/priorities~PriorityString} [config.converterPriority='normal'] Converter priority.
     * @returns {module:engine/conversion/downcasthelpers~DowncastHelpers}
     */
    markerToElement( config ) {
        return this.add( downcastMarkerToElement( config ) );
    }

    /**
     * Model marker to highlight conversion helper.
     *
     * This conversion results in creating a highlight on view nodes. For this kind of conversion,
     * {@link module:engine/conversion/downcasthelpers~HighlightDescriptor} should be provided.
     *
     * For text nodes, a `<span>` {@link module:engine/view/attributeelement~AttributeElement} is created and it wraps all text nodes
     * in the converted marker range. For example, a model marker set like this: `<paragraph>F[oo b]ar</paragraph>` becomes
     * `<p>F<span class="comment">oo b</span>ar</p>` in the view.
     *
     * {@link module:engine/view/containerelement~ContainerElement} may provide a custom way of handling highlight. Most often,
     * the element itself is given classes and attributes described in the highlight descriptor (instead of being wrapped in `<span>`).
     * For example, a model marker set like this: `[<image src="foo.jpg"></image>]` becomes `<img src="foo.jpg" class="comment"></img>`
     * in the view.
     *
     * For container elements, the conversion is two-step. While the converter processes the highlight descriptor and passes it
     * to a container element, it is the container element instance itself that applies values from the highlight descriptor.
     * So, in a sense, the converter takes care of stating what should be applied on what, while the element decides how to apply that.
     *
     *        editor.conversion.for( 'downcast' ).markerToHighlight( { model: 'comment', view: { classes: 'comment' } } );
     *
     *        editor.conversion.for( 'downcast' ).markerToHighlight( {
     *            model: 'comment',
     *            view: { classes: 'new-comment' },
     *            converterPriority: 'high'
     *        } );
     *
     *        editor.conversion.for( 'downcast' ).markerToHighlight( {
     *            model: 'comment',
     *            view: data => {
     *                // Assuming that the marker name is in a form of comment:commentType.
     *                const commentType = data.markerName.split( ':' )[ 1 ];
     *
     *                return {
     *                    classes: [ 'comment', 'comment-' + commentType ]
     *                };
     *            }
     *        } );
     *
     * If a function is passed as the `config.view` parameter, it will be used to generate the highlight descriptor. The function
     * receives the `data` object as a parameter and should return a
     * {@link module:engine/conversion/downcasthelpers~HighlightDescriptor highlight descriptor}.
     * The `data` object properties are passed from {@link module:engine/conversion/downcastdispatcher~DowncastDispatcher#event:addMarker}.
     *
     * See {@link module:engine/conversion/conversion~Conversion#for `conversion.for()`} to learn how to add a converter
     * to the conversion process.
     *
     * @method #markerToHighlight
     * @param {Object} config Conversion configuration.
     * @param {String} config.model The name of the model marker (or model marker group) to convert.
     * @param {module:engine/conversion/downcasthelpers~HighlightDescriptor|Function} config.view A highlight descriptor
     * that will be used for highlighting or a function that takes the model marker data as a parameter and returns a highlight descriptor.
     * @param {module:utils/priorities~PriorityString} [config.converterPriority='normal'] Converter priority.
     * @returns {module:engine/conversion/downcasthelpers~DowncastHelpers}
     */
    markerToHighlight( config ) {
        return this.add( downcastMarkerToHighlight( config ) );
    }
}

/**
 * Function factory that creates a default downcast converter for text insertion changes.
 *
 * The converter automatically consumes the corresponding value from the consumables list and stops the event (see
 * {@link module:engine/conversion/downcastdispatcher~DowncastDispatcher}).
 *
 *        modelDispatcher.on( 'insert:$text', insertText() );
 *
 * @returns {Function} Insert text event converter.
 */
export function insertText() {
    return ( evt, data, conversionApi ) => {
        if ( !conversionApi.consumable.consume( data.item, 'insert' ) ) {
            return;
        }

        const viewWriter = conversionApi.writer;
        const viewPosition = conversionApi.mapper.toViewPosition( data.range.start );
        const viewText = viewWriter.createText( data.item.data );

        viewWriter.insert( viewPosition, viewText );
    };
}

/**
 * Function factory that creates a default downcast converter for node remove changes.
 *
 *        modelDispatcher.on( 'remove', remove() );
 *
 * @returns {Function} Remove event converter.
 */
export function remove() {
    return ( evt, data, conversionApi ) => {
        // Find view range start position by mapping model position at which the remove happened.
        const viewStart = conversionApi.mapper.toViewPosition( data.position );

        const modelEnd = data.position.getShiftedBy( data.length );
        const viewEnd = conversionApi.mapper.toViewPosition( modelEnd, { isPhantom: true } );

        const viewRange = conversionApi.writer.createRange( viewStart, viewEnd );

        // Trim the range to remove in case some UI elements are on the view range boundaries.
        const removed = conversionApi.writer.remove( viewRange.getTrimmed() );

        // After the range is removed, unbind all view elements from the model.
        // Range inside view document fragment is used to unbind deeply.
        for ( const child of conversionApi.writer.createRangeIn( removed ).getItems() ) {
            conversionApi.mapper.unbindViewElement( child );
        }
    };
}

/**
 * Creates a `<span>` {@link module:engine/view/attributeelement~AttributeElement view attribute element} from the information
 * provided by the {@link module:engine/conversion/downcasthelpers~HighlightDescriptor highlight descriptor} object. If a priority
 * is not provided in the descriptor, the default priority will be used.
 *
 * @param {module:engine/view/downcastwriter~DowncastWriter} writer
 * @param {module:engine/conversion/downcasthelpers~HighlightDescriptor} descriptor
 * @returns {module:engine/view/attributeelement~AttributeElement}
 */
export function createViewElementFromHighlightDescriptor( writer, descriptor ) {
    const viewElement = writer.createAttributeElement( 'span', descriptor.attributes );

    if ( descriptor.classes ) {
        viewElement._addClass( descriptor.classes );
    }

    if ( descriptor.priority ) {
        viewElement._priority = descriptor.priority;
    }

    viewElement._id = descriptor.id;

    return viewElement;
}

/**
 * Function factory that creates a converter which converts a non-collapsed {@link module:engine/model/selection~Selection model selection}
 * to a {@link module:engine/view/documentselection~DocumentSelection view selection}. The converter consumes appropriate
 * value from the `consumable` object and maps model positions from the selection to view positions.
 *
 *        modelDispatcher.on( 'selection', convertRangeSelection() );
 *
 * @returns {Function} Selection converter.
 */
export function convertRangeSelection() {
    return ( evt, data, conversionApi ) => {
        const selection = data.selection;

        if ( selection.isCollapsed ) {
            return;
        }

        if ( !conversionApi.consumable.consume( selection, 'selection' ) ) {
            return;
        }

        const viewRanges = [];

        for ( const range of selection.getRanges() ) {
            const viewRange = conversionApi.mapper.toViewRange( range );
            viewRanges.push( viewRange );
        }

        conversionApi.writer.setSelection( viewRanges, { backward: selection.isBackward } );
    };
}

/**
 * Function factory that creates a converter which converts a collapsed {@link module:engine/model/selection~Selection model selection} to
 * a {@link module:engine/view/documentselection~DocumentSelection view selection}. The converter consumes appropriate
 * value from the `consumable` object, maps the model selection position to the view position and breaks
 * {@link module:engine/view/attributeelement~AttributeElement attribute elements} at the selection position.
 *
 *        modelDispatcher.on( 'selection', convertCollapsedSelection() );
 *
 * An example of the view state before and after converting the collapsed selection:
 *
 *           <p><strong>f^oo<strong>bar</p>
 *        -> <p><strong>f</strong>^<strong>oo</strong>bar</p>
 *
 * By breaking attribute elements like `<strong>`, the selection is in a correct element. Then, when the selection attribute is
 * converted, broken attributes might be merged again, or the position where the selection is may be wrapped
 * with different, appropriate attribute elements.
 *
 * See also {@link module:engine/conversion/downcasthelpers~clearAttributes} which does a clean-up
 * by merging attributes.
 *
 * @returns {Function} Selection converter.
 */
export function convertCollapsedSelection() {
    return ( evt, data, conversionApi ) => {
        const selection = data.selection;

        if ( !selection.isCollapsed ) {
            return;
        }

        if ( !conversionApi.consumable.consume( selection, 'selection' ) ) {
            return;
        }

        const viewWriter = conversionApi.writer;
        const modelPosition = selection.getFirstPosition();
        const viewPosition = conversionApi.mapper.toViewPosition( modelPosition );
        const brokenPosition = viewWriter.breakAttributes( viewPosition );

        viewWriter.setSelection( brokenPosition );
    };
}

/**
 * Function factory that creates a converter which clears artifacts after the previous
 * {@link module:engine/model/selection~Selection model selection} conversion. It removes all empty
 * {@link module:engine/view/attributeelement~AttributeElement view attribute elements} and merges sibling attributes at all start and end
 * positions of all ranges.
 *
 *           <p><strong>^</strong></p>
 *        -> <p>^</p>
 *
 *           <p><strong>foo</strong>^<strong>bar</strong>bar</p>
 *        -> <p><strong>foo^bar<strong>bar</p>
 *
 *           <p><strong>foo</strong><em>^</em><strong>bar</strong>bar</p>
 *        -> <p><strong>foo^bar<strong>bar</p>
 *
 * This listener should be assigned before any converter for the new selection:
 *
 *        modelDispatcher.on( 'selection', clearAttributes() );
 *
 * See {@link module:engine/conversion/downcasthelpers~convertCollapsedSelection}
 * which does the opposite by breaking attributes in the selection position.
 *
 * @returns {Function} Selection converter.
 */
export function clearAttributes() {
    return ( evt, data, conversionApi ) => {
        const viewWriter = conversionApi.writer;
        const viewSelection = viewWriter.document.selection;

        for ( const range of viewSelection.getRanges() ) {
            // Not collapsed selection should not have artifacts.
            if ( range.isCollapsed ) {
                // Position might be in the node removed by the view writer.
                if ( range.end.parent.isAttached() ) {
                    conversionApi.writer.mergeAttributes( range.start );
                }
            }
        }
        viewWriter.setSelection( null );
    };
}

/**
 * Function factory that creates a converter which converts set/change/remove attribute changes from the model to the view.
 * It can also be used to convert selection attributes. In that case, an empty attribute element will be created and the
 * selection will be put inside it.
 *
 * Attributes from the model are converted to a view element that will be wrapping these view nodes that are bound to
 * model elements having the given attribute. This is useful for attributes like `bold` that may be set on text nodes in the model
 * but are represented as an element in the view:
 *
 *        [paragraph]              MODEL ====> VIEW        <p>
 *            |- a {bold: true}                             |- <b>
 *            |- b {bold: true}                             |   |- ab
 *            |- c                                          |- c
 *
 * Passed `Function` will be provided with the attribute value and then all the parameters of the
 * {@link module:engine/conversion/downcastdispatcher~DowncastDispatcher#event:attribute `attribute` event}.
 * It is expected that the function returns an {@link module:engine/view/element~Element}.
 * The result of the function will be the wrapping element.
 * When the provided `Function` does not return any element, no conversion will take place.
 *
 * The converter automatically consumes the corresponding value from the consumables list and stops the event (see
 * {@link module:engine/conversion/downcastdispatcher~DowncastDispatcher}).
 *
 *        modelDispatcher.on( 'attribute:bold', wrap( ( modelAttributeValue, viewWriter ) => {
 *            return viewWriter.createAttributeElement( 'strong' );
 *        } );
 *
 * @protected
 * @param {Function} elementCreator Function returning a view element that will be used for wrapping.
 * @returns {Function} Set/change attribute converter.
 */
export function wrap( elementCreator ) {
    return ( evt, data, conversionApi ) => {
        // Recreate current wrapping node. It will be used to unwrap view range if the attribute value has changed
        // or the attribute was removed.
        const oldViewElement = elementCreator( data.attributeOldValue, conversionApi.writer );

        // Create node to wrap with.
        const newViewElement = elementCreator( data.attributeNewValue, conversionApi.writer );

        if ( !oldViewElement && !newViewElement ) {
            return;
        }

        if ( !conversionApi.consumable.consume( data.item, evt.name ) ) {
            return;
        }

        const viewWriter = conversionApi.writer;
        const viewSelection = viewWriter.document.selection;

        if ( data.item instanceof ModelSelection || data.item instanceof DocumentSelection ) {
            // Selection attribute conversion.
            viewWriter.wrap( viewSelection.getFirstRange(), newViewElement );
        } else {
            // Node attribute conversion.
            let viewRange = conversionApi.mapper.toViewRange( data.range );

            // First, unwrap the range from current wrapper.
            if ( data.attributeOldValue !== null && oldViewElement ) {
                viewRange = viewWriter.unwrap( viewRange, oldViewElement );
            }

            if ( data.attributeNewValue !== null && newViewElement ) {
                viewWriter.wrap( viewRange, newViewElement );
            }
        }
    };
}

/**
 * Function factory that creates a converter which converts node insertion changes from the model to the view.
 * The function passed will be provided with all the parameters of the dispatcher's
 * {@link module:engine/conversion/downcastdispatcher~DowncastDispatcher#event:insert `insert` event}.
 * It is expected that the function returns an {@link module:engine/view/element~Element}.
 * The result of the function will be inserted into the view.
 *
 * The converter automatically consumes the corresponding value from the consumables list, stops the event (see
 * {@link module:engine/conversion/downcastdispatcher~DowncastDispatcher}) and binds the model and view elements.
 *
 *        downcastDispatcher.on(
 *            'insert:myElem',
 *            insertElement( ( modelItem, viewWriter ) => {
 *                const text = viewWriter.createText( 'myText' );
 *                const myElem = viewWriter.createElement( 'myElem', { myAttr: 'my-' + modelItem.getAttribute( 'myAttr' ) }, text );
 *
 *                // Do something fancy with `myElem` using `modelItem` or other parameters.
 *
 *                return myElem;
 *            }
 *        ) );
 *
 * @protected
 * @param {Function} elementCreator Function returning a view element, which will be inserted.
 * @returns {Function} Insert element event converter.
 */
export function insertElement( elementCreator ) {
    return ( evt, data, conversionApi ) => {
        const viewElement = elementCreator( data.item, conversionApi.writer );

        if ( !viewElement ) {
            return;
        }

        if ( !conversionApi.consumable.consume( data.item, 'insert' ) ) {
            return;
        }

        const viewPosition = conversionApi.mapper.toViewPosition( data.range.start );

        conversionApi.mapper.bindElements( data.item, viewElement );
        conversionApi.writer.insert( viewPosition, viewElement );
    };
}

/**
 * Function factory that creates a converter which converts marker adding change to the
 * {@link module:engine/view/uielement~UIElement view UI element}.
 *
 * The view UI element that will be added to the view depends on the passed parameter. See {@link ~insertElement}.
 * In case of a non-collapsed range, the UI element will not wrap nodes but separate elements will be placed at the beginning
 * and at the end of the range.
 *
 * This converter binds created UI elements with the marker name using {@link module:engine/conversion/mapper~Mapper#bindElementToMarker}.
 *
 * @protected
 * @param {module:engine/view/uielement~UIElement|Function} elementCreator A view UI element or a function returning the view element
 * that will be inserted.
 * @returns {Function} Insert element event converter.
 */
export function insertUIElement( elementCreator ) {
    return ( evt, data, conversionApi ) => {
        // Create two view elements. One will be inserted at the beginning of marker, one at the end.
        // If marker is collapsed, only "opening" element will be inserted.
        data.isOpening = true;
        const viewStartElement = elementCreator( data, conversionApi.writer );

        data.isOpening = false;
        const viewEndElement = elementCreator( data, conversionApi.writer );

        if ( !viewStartElement || !viewEndElement ) {
            return;
        }

        const markerRange = data.markerRange;

        // Marker that is collapsed has consumable build differently that non-collapsed one.
        // For more information see `addMarker` event description.
        // If marker's range is collapsed - check if it can be consumed.
        if ( markerRange.isCollapsed && !conversionApi.consumable.consume( markerRange, evt.name ) ) {
            return;
        }

        // If marker's range is not collapsed - consume all items inside.
        for ( const value of markerRange ) {
            if ( !conversionApi.consumable.consume( value.item, evt.name ) ) {
                return;
            }
        }

        const mapper = conversionApi.mapper;
        const viewWriter = conversionApi.writer;

        // Add "opening" element.
        viewWriter.insert( mapper.toViewPosition( markerRange.start ), viewStartElement );
        conversionApi.mapper.bindElementToMarker( viewStartElement, data.markerName );

        // Add "closing" element only if range is not collapsed.
        if ( !markerRange.isCollapsed ) {
            viewWriter.insert( mapper.toViewPosition( markerRange.end ), viewEndElement );
            conversionApi.mapper.bindElementToMarker( viewEndElement, data.markerName );
        }

        evt.stop();
    };
}

// Function factory that returns a default downcast converter for removing a {@link module:engine/view/uielement~UIElement UI element}
// basing on marker remove change.
//
// This converter unbinds elements from the marker name.
//
// @returns {Function} Removed UI element converter.
function removeUIElement() {
    return ( evt, data, conversionApi ) => {
        const elements = conversionApi.mapper.markerNameToElements( data.markerName );

        if ( !elements ) {
            return;
        }

        for ( const element of elements ) {
            conversionApi.mapper.unbindElementFromMarkerName( element, data.markerName );
            conversionApi.writer.clear( conversionApi.writer.createRangeOn( element ), element );
        }

        conversionApi.writer.clearClonedElementsGroup( data.markerName );

        evt.stop();
    };
}

// Function factory that creates a converter which converts set/change/remove attribute changes from the model to the view.
//
// Attributes from the model are converted to the view element attributes in the view. You may provide a custom function to generate
// a key-value attribute pair to add/change/remove. If not provided, model attributes will be converted to view element
// attributes on a one-to-one basis.
//
// *Note:** The provided attribute creator should always return the same `key` for a given attribute from the model.
//
// The converter automatically consumes the corresponding value from the consumables list and stops the event (see
// {@link module:engine/conversion/downcastdispatcher~DowncastDispatcher}).
//
//        modelDispatcher.on( 'attribute:customAttr:myElem', changeAttribute( ( value, data ) => {
//            // Change attribute key from `customAttr` to `class` in the view.
//            const key = 'class';
//            let value = data.attributeNewValue;
//
//            // Force attribute value to 'empty' if the model element is empty.
//            if ( data.item.childCount === 0 ) {
//                value = 'empty';
//            }
//
//            // Return the key-value pair.
//            return { key, value };
//        } ) );
//
// @param {Function} [attributeCreator] Function returning an object with two properties: `key` and `value`, which
// represent the attribute key and attribute value to be set on a {@link module:engine/view/element~Element view element}.
// The function is passed the model attribute value as the first parameter and additional data about the change as the second parameter.
// @returns {Function} Set/change attribute converter.
function changeAttribute( attributeCreator ) {
    return ( evt, data, conversionApi ) => {
        const oldAttribute = attributeCreator( data.attributeOldValue, data );
        const newAttribute = attributeCreator( data.attributeNewValue, data );

        if ( !oldAttribute && !newAttribute ) {
            return;
        }

        if ( !conversionApi.consumable.consume( data.item, evt.name ) ) {
            return;
        }

        const viewElement = conversionApi.mapper.toViewElement( data.item );
        const viewWriter = conversionApi.writer;

        // If model item cannot be mapped to a view element, it means item is not an `Element` instance but a `TextProxy` node.
        // Only elements can have attributes in a view so do not proceed for anything else (#1587).
        if ( !viewElement ) {
            /**
             * This error occurs when a {@link module:engine/model/textproxy~TextProxy text node's} attribute is to be downcasted
             * by {@link module:engine/conversion/conversion~Conversion#attributeToAttribute `Attribute to Attribute converter`}.
             * In most cases it is caused by converters misconfiguration when only "generic" converter is defined:
             *
             *        editor.conversion.for( 'downcast' ).attributeToAttribute( {
             *            model: 'attribute-name',
             *            view: 'attribute-name'
             *        } ) );
             *
             * and given attribute is used on text node, for example:
             *
             *        model.change( writer => {
             *            writer.insertText( 'Foo', { 'attribute-name': 'bar' }, parent, 0 );
             *        } );
             *
             * In such cases, to convert the same attribute for both {@link module:engine/model/element~Element}
             * and {@link module:engine/model/textproxy~TextProxy `Text`} nodes, text specific
             * {@link module:engine/conversion/conversion~Conversion#attributeToElement `Attribute to Element converter`}
             * with higher {@link module:utils/priorities~PriorityString priority} must also be defined:
             *
             *        editor.conversion.for( 'downcast' ).attributeToElement( {
             *            model: {
             *                key: 'attribute-name',
             *                name: '$text'
             *            },
             *            view: ( value, writer ) => {
             *                return writer.createAttributeElement( 'span', { 'attribute-name': value } );
             *            },
             *            converterPriority: 'high'
             *        } ) );
             *
             * @error conversion-attribute-to-attribute-on-text
             */
            throw new CKEditorError(
                'conversion-attribute-to-attribute-on-text: ' +
                'Trying to convert text node\'s attribute with attribute-to-attribute converter.',
                [ data, conversionApi ]
            );
        }

        // First remove the old attribute if there was one.
        if ( data.attributeOldValue !== null && oldAttribute ) {
            if ( oldAttribute.key == 'class' ) {
                const classes = Array.isArray( oldAttribute.value ) ? oldAttribute.value : [ oldAttribute.value ];

                for ( const className of classes ) {
                    viewWriter.removeClass( className, viewElement );
                }
            } else if ( oldAttribute.key == 'style' ) {
                const keys = Object.keys( oldAttribute.value );

                for ( const key of keys ) {
                    viewWriter.removeStyle( key, viewElement );
                }
            } else {
                viewWriter.removeAttribute( oldAttribute.key, viewElement );
            }
        }

        // Then set the new attribute.
        if ( data.attributeNewValue !== null && newAttribute ) {
            if ( newAttribute.key == 'class' ) {
                const classes = Array.isArray( newAttribute.value ) ? newAttribute.value : [ newAttribute.value ];

                for ( const className of classes ) {
                    viewWriter.addClass( className, viewElement );
                }
            } else if ( newAttribute.key == 'style' ) {
                const keys = Object.keys( newAttribute.value );

                for ( const key of keys ) {
                    viewWriter.setStyle( key, newAttribute.value[ key ], viewElement );
                }
            } else {
                viewWriter.setAttribute( newAttribute.key, newAttribute.value, viewElement );
            }
        }
    };
}

// Function factory that creates a converter which converts the text inside marker's range. The converter wraps the text with
// {@link module:engine/view/attributeelement~AttributeElement} created from the provided descriptor.
// See {link module:engine/conversion/downcasthelpers~createViewElementFromHighlightDescriptor}.
//
// It can also be used to convert the selection that is inside a marker. In that case, an empty attribute element will be
// created and the selection will be put inside it.
//
// If the highlight descriptor does not provide the `priority` property, `10` will be used.
//
// If the highlight descriptor does not provide the `id` property, the name of the marker will be used.
//
// This converter binds the created {@link module:engine/view/attributeelement~AttributeElement attribute elemens} with the marker name
// using the {@link module:engine/conversion/mapper~Mapper#bindElementToMarker} method.
//
// @param {module:engine/conversion/downcasthelpers~HighlightDescriptor|Function} highlightDescriptor
// @returns {Function}
function highlightText( highlightDescriptor ) {
    return ( evt, data, conversionApi ) => {
        if ( !data.item ) {
            return;
        }

        if ( !( data.item instanceof ModelSelection || data.item instanceof DocumentSelection ) && !data.item.is( 'textProxy' ) ) {
            return;
        }

        const descriptor = prepareDescriptor( highlightDescriptor, data, conversionApi );

        if ( !descriptor ) {
            return;
        }

        if ( !conversionApi.consumable.consume( data.item, evt.name ) ) {
            return;
        }

        const viewWriter = conversionApi.writer;
        const viewElement = createViewElementFromHighlightDescriptor( viewWriter, descriptor );
        const viewSelection = viewWriter.document.selection;

        if ( data.item instanceof ModelSelection || data.item instanceof DocumentSelection ) {
            viewWriter.wrap( viewSelection.getFirstRange(), viewElement, viewSelection );
        } else {
            const viewRange = conversionApi.mapper.toViewRange( data.range );
            const rangeAfterWrap = viewWriter.wrap( viewRange, viewElement );

            for ( const element of rangeAfterWrap.getItems() ) {
                if ( element.is( 'attributeElement' ) && element.isSimilar( viewElement ) ) {
                    conversionApi.mapper.bindElementToMarker( element, data.markerName );

                    // One attribute element is enough, because all of them are bound together by the view writer.
                    // Mapper uses this binding to get all the elements no matter how many of them are registered in the mapper.
                    break;
                }
            }
        }
    };
}

// Converter function factory. It creates a function which applies the marker's highlight to an element inside the marker's range.
//
// The converter checks if an element has the `addHighlight` function stored as a
// {@link module:engine/view/element~Element#_setCustomProperty custom property} and, if so, uses it to apply the highlight.
// In such case the converter will consume all element's children, assuming that they were handled by the element itself.
//
// When the `addHighlight` custom property is not present, the element is not converted in any special way.
// This means that converters will proceed to convert the element's child nodes.
//
// If the highlight descriptor does not provide the `priority` property, `10` will be used.
//
// If the highlight descriptor does not provide the `id` property, the name of the marker will be used.
//
// This converter binds altered {@link module:engine/view/containerelement~ContainerElement container elements} with the marker name using
// the {@link module:engine/conversion/mapper~Mapper#bindElementToMarker} method.
//
// @param {module:engine/conversion/downcasthelpers~HighlightDescriptor|Function} highlightDescriptor
// @returns {Function}
function highlightElement( highlightDescriptor ) {
    return ( evt, data, conversionApi ) => {
        if ( !data.item ) {
            return;
        }

        if ( !( data.item instanceof ModelElement ) ) {
            return;
        }

        const descriptor = prepareDescriptor( highlightDescriptor, data, conversionApi );

        if ( !descriptor ) {
            return;
        }

        if ( !conversionApi.consumable.test( data.item, evt.name ) ) {
            return;
        }

        const viewElement = conversionApi.mapper.toViewElement( data.item );

        if ( viewElement && viewElement.getCustomProperty( 'addHighlight' ) ) {
            // Consume element itself.
            conversionApi.consumable.consume( data.item, evt.name );

            // Consume all children nodes.
            for ( const value of ModelRange._createIn( data.item ) ) {
                conversionApi.consumable.consume( value.item, evt.name );
            }

            viewElement.getCustomProperty( 'addHighlight' )( viewElement, descriptor, conversionApi.writer );

            conversionApi.mapper.bindElementToMarker( viewElement, data.markerName );
        }
    };
}

// Function factory that creates a converter which converts the removing model marker to the view.
//
// Both text nodes and elements are handled by this converter but they are handled a bit differently.
//
// Text nodes are unwrapped using the {@link module:engine/view/attributeelement~AttributeElement attribute element} created from the
// provided highlight descriptor. See {link module:engine/conversion/downcasthelpers~HighlightDescriptor}.
//
// For elements, the converter checks if an element has the `removeHighlight` function stored as a
// {@link module:engine/view/element~Element#_setCustomProperty custom property}. If so, it uses it to remove the highlight.
// In such case, the children of that element will not be converted.
//
// When `removeHighlight` is not present, the element is not converted in any special way.
// The converter will proceed to convert the element's child nodes instead.
//
// If the highlight descriptor does not provide the `priority` property, `10` will be used.
//
// If the highlight descriptor does not provide the `id` property, the name of the marker will be used.
//
// This converter unbinds elements from the marker name.
//
// @param {module:engine/conversion/downcasthelpers~HighlightDescriptor|Function} highlightDescriptor
// @returns {Function}
function removeHighlight( highlightDescriptor ) {
    return ( evt, data, conversionApi ) => {
        // This conversion makes sense only for non-collapsed range.
        if ( data.markerRange.isCollapsed ) {
            return;
        }

        const descriptor = prepareDescriptor( highlightDescriptor, data, conversionApi );

        if ( !descriptor ) {
            return;
        }

        // View element that will be used to unwrap `AttributeElement`s.
        const viewHighlightElement = createViewElementFromHighlightDescriptor( conversionApi.writer, descriptor );

        // Get all elements bound with given marker name.
        const elements = conversionApi.mapper.markerNameToElements( data.markerName );

        if ( !elements ) {
            return;
        }

        for ( const element of elements ) {
            conversionApi.mapper.unbindElementFromMarkerName( element, data.markerName );

            if ( element.is( 'attributeElement' ) ) {
                conversionApi.writer.unwrap( conversionApi.writer.createRangeOn( element ), viewHighlightElement );
            } else {
                // if element.is( 'containerElement' ).
                element.getCustomProperty( 'removeHighlight' )( element, descriptor.id, conversionApi.writer );
            }
        }

        conversionApi.writer.clearClonedElementsGroup( data.markerName );

        evt.stop();
    };
}

// Model element to view element conversion helper.
//
// See {@link ~DowncastHelpers#elementToElement `.elementToElement()` downcast helper} for examples.
//
// @param {Object} config Conversion configuration.
// @param {String} config.model The name of the model element to convert.
// @param {module:engine/view/elementdefinition~ElementDefinition|Function} config.view A view element definition or a function
// that takes the model element and {@link module:engine/view/downcastwriter~DowncastWriter view downcast writer}
// as parameters and returns a view container element.
// @returns {Function} Conversion helper.
function downcastElementToElement( config ) {
    config = cloneDeep( config );

    config.view = normalizeToElementConfig( config.view, 'container' );

    return dispatcher => {
        dispatcher.on( 'insert:' + config.model, insertElement( config.view ), { priority: config.converterPriority || 'normal' } );
    };
}

// Model attribute to view element conversion helper.
//
// See {@link ~DowncastHelpers#attributeToElement `.attributeToElement()` downcast helper} for examples.
//
// @param {Object} config Conversion configuration.
// @param {String|Object} config.model The key of the attribute to convert from or a `{ key, values }` object. `values` is an array
// of `String`s with possible values if the model attribute is an enumerable.
// @param {module:engine/view/elementdefinition~ElementDefinition|Function|Object} config.view A view element definition or a function
// that takes the model attribute value and {@link module:engine/view/downcastwriter~DowncastWriter view downcast writer}
// as parameters and returns a view attribute element. If `config.model.values` is
// given, `config.view` should be an object assigning values from `config.model.values` to view element definitions or functions.
// @param {module:utils/priorities~PriorityString} [config.converterPriority='normal'] Converter priority.
// @returns {Function} Conversion helper.
function downcastAttributeToElement( config ) {
    config = cloneDeep( config );

    const modelKey = config.model.key ? config.model.key : config.model;
    let eventName = 'attribute:' + modelKey;

    if ( config.model.name ) {
        eventName += ':' + config.model.name;
    }

    if ( config.model.values ) {
        for ( const modelValue of config.model.values ) {
            config.view[ modelValue ] = normalizeToElementConfig( config.view[ modelValue ], 'attribute' );
        }
    } else {
        config.view = normalizeToElementConfig( config.view, 'attribute' );
    }

    const elementCreator = getFromAttributeCreator( config );

    return dispatcher => {
        dispatcher.on( eventName, wrap( elementCreator ), { priority: config.converterPriority || 'normal' } );
    };
}

// Model attribute to view attribute conversion helper.
//
// See {@link ~DowncastHelpers#attributeToAttribute `.attributeToAttribute()` downcast helper} for examples.
//
// @param {Object} config Conversion configuration.
// @param {String|Object} config.model The key of the attribute to convert from or a `{ key, values, [ name ] }` object describing
// the attribute key, possible values and, optionally, an element name to convert from.
// @param {String|Object|Function} config.view A view attribute key, or a `{ key, value }` object or a function that takes
// the model attribute value and returns a `{ key, value }` object. If `key` is `'class'`, `value` can be a `String` or an
// array of `String`s. If `key` is `'style'`, `value` is an object with key-value pairs. In other cases, `value` is a `String`.
// If `config.model.values` is set, `config.view` should be an object assigning values from `config.model.values` to
// `{ key, value }` objects or a functions.
// @param {module:utils/priorities~PriorityString} [config.converterPriority='normal'] Converter priority.
// @returns {Function} Conversion helper.
function downcastAttributeToAttribute( config ) {
    config = cloneDeep( config );

    const modelKey = config.model.key ? config.model.key : config.model;
    let eventName = 'attribute:' + modelKey;

    if ( config.model.name ) {
        eventName += ':' + config.model.name;
    }

    if ( config.model.values ) {
        for ( const modelValue of config.model.values ) {
            config.view[ modelValue ] = normalizeToAttributeConfig( config.view[ modelValue ] );
        }
    } else {
        config.view = normalizeToAttributeConfig( config.view );
    }

    const elementCreator = getFromAttributeCreator( config );

    return dispatcher => {
        dispatcher.on( eventName, changeAttribute( elementCreator ), { priority: config.converterPriority || 'normal' } );
    };
}

// Model marker to view element conversion helper.
//
// See {@link ~DowncastHelpers#markerToElement `.markerToElement()` downcast helper} for examples.
//
// @param {Object} config Conversion configuration.
// @param {String} config.model The name of the model marker (or model marker group) to convert.
// @param {module:engine/view/elementdefinition~ElementDefinition|Function} config.view A view element definition or a function
// that takes the model marker data as a parameter and returns a view UI element.
// @param {module:utils/priorities~PriorityString} [config.converterPriority='normal'] Converter priority.
// @returns {Function} Conversion helper.
function downcastMarkerToElement( config ) {
    config = cloneDeep( config );

    config.view = normalizeToElementConfig( config.view, 'ui' );

    return dispatcher => {
        dispatcher.on( 'addMarker:' + config.model, insertUIElement( config.view ), { priority: config.converterPriority || 'normal' } );
        dispatcher.on( 'removeMarker:' + config.model, removeUIElement( config.view ), { priority: config.converterPriority || 'normal' } );
    };
}

// Model marker to highlight conversion helper.
//
// See {@link ~DowncastHelpers#markerToElement `.markerToElement()` downcast helper} for examples.
//
// @param {Object} config Conversion configuration.
// @param {String} config.model The name of the model marker (or model marker group) to convert.
// @param {module:engine/conversion/downcasthelpers~HighlightDescriptor|Function} config.view A highlight descriptor
// that will be used for highlighting or a function that takes the model marker data as a parameter and returns a highlight descriptor.
// @param {module:utils/priorities~PriorityString} [config.converterPriority='normal'] Converter priority.
// @returns {Function} Conversion helper.
function downcastMarkerToHighlight( config ) {
    return dispatcher => {
        dispatcher.on( 'addMarker:' + config.model, highlightText( config.view ), { priority: config.converterPriority || 'normal' } );
        dispatcher.on( 'addMarker:' + config.model, highlightElement( config.view ), { priority: config.converterPriority || 'normal' } );
        dispatcher.on( 'removeMarker:' + config.model, removeHighlight( config.view ), { priority: config.converterPriority || 'normal' } );
    };
}

// Takes `config.view`, and if it is an {@link module:engine/view/elementdefinition~ElementDefinition}, converts it
// to a function (because lower level converters accept only element creator functions).
//
// @param {module:engine/view/elementdefinition~ElementDefinition|Function} view View configuration.
// @param {'container'|'attribute'|'ui'} viewElementType View element type to create.
// @returns {Function} Element creator function to use in lower level converters.
function normalizeToElementConfig( view, viewElementType ) {
    if ( typeof view == 'function' ) {
        // If `view` is already a function, don't do anything.
        return view;
    }

    return ( modelData, viewWriter ) => createViewElementFromDefinition( view, viewWriter, viewElementType );
}

// Creates a view element instance from the provided {@link module:engine/view/elementdefinition~ElementDefinition} and class.
//
// @param {module:engine/view/elementdefinition~ElementDefinition} viewElementDefinition
// @param {module:engine/view/downcastwriter~DowncastWriter} viewWriter
// @param {'container'|'attribute'|'ui'} viewElementType
// @returns {module:engine/view/element~Element}
function createViewElementFromDefinition( viewElementDefinition, viewWriter, viewElementType ) {
    if ( typeof viewElementDefinition == 'string' ) {
        // If `viewElementDefinition` is given as a `String`, normalize it to an object with `name` property.
        viewElementDefinition = { name: viewElementDefinition };
    }

    let element;
    const attributes = Object.assign( {}, viewElementDefinition.attributes );

    if ( viewElementType == 'container' ) {
        element = viewWriter.createContainerElement( viewElementDefinition.name, attributes );
    } else if ( viewElementType == 'attribute' ) {
        const options = {
            priority: viewElementDefinition.priority || ViewAttributeElement.DEFAULT_PRIORITY
        };

        element = viewWriter.createAttributeElement( viewElementDefinition.name, attributes, options );
    } else {
        // 'ui'.
        element = viewWriter.createUIElement( viewElementDefinition.name, attributes );
    }

    if ( viewElementDefinition.styles ) {
        const keys = Object.keys( viewElementDefinition.styles );

        for ( const key of keys ) {
            viewWriter.setStyle( key, viewElementDefinition.styles[ key ], element );
        }
    }

    if ( viewElementDefinition.classes ) {
        const classes = viewElementDefinition.classes;

        if ( typeof classes == 'string' ) {
            viewWriter.addClass( classes, element );
        } else {
            for ( const className of classes ) {
                viewWriter.addClass( className, element );
            }
        }
    }

    return element;
}

function getFromAttributeCreator( config ) {
    if ( config.model.values ) {
        return ( modelAttributeValue, viewWriter ) => {
            const view = config.view[ modelAttributeValue ];

            if ( view ) {
                return view( modelAttributeValue, viewWriter );
            }

            return null;
        };
    } else {
        return config.view;
    }
}

// Takes the configuration, adds default parameters if they do not exist and normalizes other parameters to be used in downcast converters
// for generating a view attribute.
//
// @param {Object} view View configuration.
function normalizeToAttributeConfig( view ) {
    if ( typeof view == 'string' ) {
        return modelAttributeValue => ( { key: view, value: modelAttributeValue } );
    } else if ( typeof view == 'object' ) {
        // { key, value, ... }
        if ( view.value ) {
            return () => view;
        }
        // { key, ... }
        else {
            return modelAttributeValue => ( { key: view.key, value: modelAttributeValue } );
        }
    } else {
        // function.
        return view;
    }
}

// Helper function for `highlight`. Prepares the actual descriptor object using value passed to the converter.
function prepareDescriptor( highlightDescriptor, data, conversionApi ) {
    // If passed descriptor is a creator function, call it. If not, just use passed value.
    const descriptor = typeof highlightDescriptor == 'function' ?
        highlightDescriptor( data, conversionApi ) :
        highlightDescriptor;

    if ( !descriptor ) {
        return null;
    }

    // Apply default descriptor priority.
    if ( !descriptor.priority ) {
        descriptor.priority = 10;
    }

    // Default descriptor id is marker name.
    if ( !descriptor.id ) {
        descriptor.id = data.markerName;
    }

    return descriptor;
}

/**
 * An object describing how the marker highlight should be represented in the view.
 *
 * Each text node contained in a highlighted range will be wrapped in a `<span>`
 * {@link module:engine/view/attributeelement~AttributeElement view attribute element} with CSS class(es), attributes and a priority
 * described by this object.
 *
 * Additionally, each {@link module:engine/view/containerelement~ContainerElement container element} can handle displaying the highlight
 * separately by providing the `addHighlight` and `removeHighlight` custom properties. In this case:
 *
 *  * The `HighlightDescriptor` object is passed to the `addHighlight` function upon conversion and should be used to apply the highlight to
 *  the element.
 *  * The descriptor `id` is passed to the `removeHighlight` function upon conversion and should be used to remove the highlight with the
 *  given ID from the element.
 *
 * @typedef {Object} module:engine/conversion/downcasthelpers~HighlightDescriptor
 *
 * @property {String|Array.<String>} classes A CSS class or an array of classes to set. If the descriptor is used to
 * create an {@link module:engine/view/attributeelement~AttributeElement attribute element} over text nodes, these classes will be set
 * on that attribute element. If the descriptor is applied to an element, usually these classes will be set on that element, however,
 * this depends on how the element converts the descriptor.
 *
 * @property {String} [id] Descriptor identifier. If not provided, it defaults to the converted marker's name.
 *
 * @property {Number} [priority] Descriptor priority. If not provided, it defaults to `10`. If the descriptor is used to create
 * an {@link module:engine/view/attributeelement~AttributeElement attribute element}, it will be that element's
 * {@link module:engine/view/attributeelement~AttributeElement#priority priority}. If the descriptor is applied to an element,
 * the priority will be used to determine which descriptor is more important.
 *
 * @property {Object} [attributes] Attributes to set. If the descriptor is used to create
 * an {@link module:engine/view/attributeelement~AttributeElement attribute element} over text nodes, these attributes will be set on that
 * attribute element. If the descriptor is applied to an element, usually these attributes will be set on that element, however,
 * this depends on how the element converts the descriptor.
 */