ckeditor/ckeditor5-engine

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src/conversion/upcasthelpers.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
 */

import Matcher from '../view/matcher';
import ModelRange from '../model/range';
import ConversionHelpers from './conversionhelpers';

import { cloneDeep } from 'lodash-es';
import ModelSelection from '../model/selection';

/**
 * Contains {@link module:engine/view/view view} to {@link module:engine/model/model model} converters for
 * {@link module:engine/conversion/upcastdispatcher~UpcastDispatcher}.
 *
 * @module engine/conversion/upcasthelpers
 */

/**
 * Upcast conversion helper functions.
 *
 * @extends module:engine/conversion/conversionhelpers~ConversionHelpers
 */
export default class UpcastHelpers extends ConversionHelpers {
    /**
     * View element to model element conversion helper.
     *
     * This conversion results in creating a model element. For example,
     * view `<p>Foo</p>` becomes `<paragraph>Foo</paragraph>` in the model.
     *
     * Keep in mind that the element will be inserted only if it is allowed
     * by {@link module:engine/model/schema~Schema schema} configuration.
     *
     *        editor.conversion.for( 'upcast' ).elementToElement( {
     *            view: 'p',
     *            model: 'paragraph'
     *        } );
     *
     *        editor.conversion.for( 'upcast' ).elementToElement( {
     *            view: 'p',
     *            model: 'paragraph',
     *            converterPriority: 'high'
     *        } );
     *
     *        editor.conversion.for( 'upcast' ).elementToElement( {
     *            view: {
     *                name: 'p',
     *                classes: 'fancy'
     *            },
     *            model: 'fancyParagraph'
     *        } );
     *
     *        editor.conversion.for( 'upcast' ).elementToElement( {
     *             view: {
     *                name: 'p',
     *                classes: 'heading'
     *             },
     *             model: ( viewElement, modelWriter ) => {
     *                 return modelWriter.createElement( 'heading', { level: viewElement.getAttribute( 'data-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 {module:engine/view/matcher~MatcherPattern} [config.view] Pattern matching all view elements which should be converted. If not
     * set, the converter will fire for every view element.
     * @param {String|module:engine/model/element~Element|Function} config.model Name of the model element, a model element
     * instance or a function that takes a view element and returns a model element. The model element will be inserted in the model.
     * @param {module:utils/priorities~PriorityString} [config.converterPriority='normal'] Converter priority.
     * @returns {module:engine/conversion/upcasthelpers~UpcastHelpers}
     */
    elementToElement( config ) {
        return this.add( upcastElementToElement( config ) );
    }

    /**
     * View element to model attribute conversion helper.
     *
     * This conversion results in setting an attribute on a model node. For example, view `<strong>Foo</strong>` becomes
     * `Foo` {@link module:engine/model/text~Text model text node} with `bold` attribute set to `true`.
     *
     * This helper is meant to set a model attribute on all the elements that are inside the converted element:
     *
     *        <strong>Foo</strong>   -->   <strong><p>Foo</p></strong>   -->   <paragraph><$text bold="true">Foo</$text></paragraph>
     *
     * Above is a sample of HTML code, that goes through autoparagraphing (first step) and then is converted (second step).
     * Even though `<strong>` is over `<p>` element, `bold="true"` was added to the text. See
     * {@link module:engine/conversion/upcasthelpers~UpcastHelpers#attributeToAttribute} for comparison.
     *
     * Keep in mind that the attribute will be set only if it is allowed by {@link module:engine/model/schema~Schema schema} configuration.
     *
     *        editor.conversion.for( 'upcast' ).elementToAttribute( {
     *            view: 'strong',
     *            model: 'bold'
     *        } );
     *
     *        editor.conversion.for( 'upcast' ).elementToAttribute( {
     *            view: 'strong',
     *            model: 'bold',
     *            converterPriority: 'high'
     *        } );
     *
     *        editor.conversion.for( 'upcast' ).elementToAttribute( {
     *            view: {
     *                name: 'span',
     *                classes: 'bold'
     *            },
     *            model: 'bold'
     *        } );
     *
     *        editor.conversion.for( 'upcast' ).elementToAttribute( {
     *            view: {
     *                name: 'span',
     *                classes: [ 'styled', 'styled-dark' ]
     *            },
     *            model: {
     *                key: 'styled',
     *                value: 'dark'
     *            }
     *        } );
     *
     *         editor.conversion.for( 'upcast' ).elementToAttribute( {
     *            view: {
     *                name: 'span',
     *                styles: {
     *                    'font-size': /[\s\S]+/
     *                }
     *            },
     *            model: {
     *                key: 'fontSize',
     *                value: viewElement => {
     *                    const fontSize = viewElement.getStyle( 'font-size' );
     *                    const value = fontSize.substr( 0, fontSize.length - 2 );
     *
     *                    if ( value <= 10 ) {
     *                        return 'small';
     *                    } else if ( value > 12 ) {
     *                        return 'big';
     *                    }
     *
     *                    return null;
     *                }
     *            }
     *        } );
     *
     * See {@link module:engine/conversion/conversion~Conversion#for `conversion.for()`} to learn how to add a converter
     * to the conversion process.
     *
     * @method #elementToAttribute
     * @param {Object} config Conversion configuration.
     * @param {module:engine/view/matcher~MatcherPattern} config.view Pattern matching all view elements which should be converted.
     * @param {String|Object} config.model Model attribute key or an object with `key` and `value` properties, describing
     * the model attribute. `value` property may be set as a function that takes a view element and returns the value.
     * If `String` is given, the model attribute value will be set to `true`.
     * @param {module:utils/priorities~PriorityString} [config.converterPriority='low'] Converter priority.
     * @returns {module:engine/conversion/upcasthelpers~UpcastHelpers}
     */
    elementToAttribute( config ) {
        return this.add( upcastElementToAttribute( config ) );
    }

    /**
     * View attribute to model attribute conversion helper.
     *
     * This conversion results in setting an attribute on a model node. For example, view `<img src="foo.jpg"></img>` becomes
     * `<image source="foo.jpg"></image>` in the model.
     *
     * This helper is meant to convert view attributes from view elements which got converted to the model, so the view attribute
     * is set only on the corresponding model node:
     *
     *        <div class="dark"><div>foo</div></div>    -->    <div dark="true"><div>foo</div></div>
     *
     * Above, `class="dark"` attribute is added only to the `<div>` elements that has it. This is in contrary to
     * {@link module:engine/conversion/upcasthelpers~UpcastHelpers#elementToAttribute} which sets attributes for
     * all the children in the model:
     *
     *        <strong>Foo</strong>   -->   <strong><p>Foo</p></strong>   -->   <paragraph><$text bold="true">Foo</$text></paragraph>
     *
     * Above is a sample of HTML code, that goes through autoparagraphing (first step) and then is converted (second step).
     * Even though `<strong>` is over `<p>` element, `bold="true"` was added to the text.
     *
     * Keep in mind that the attribute will be set only if it is allowed by {@link module:engine/model/schema~Schema schema} configuration.
     *
     *        editor.conversion.for( 'upcast' ).attributeToAttribute( {
     *            view: 'src',
     *            model: 'source'
     *        } );
     *
     *        editor.conversion.for( 'upcast' ).attributeToAttribute( {
     *            view: { key: 'src' },
     *            model: 'source'
     *        } );
     *
     *        editor.conversion.for( 'upcast' ).attributeToAttribute( {
     *            view: { key: 'src' },
     *            model: 'source',
     *            converterPriority: 'normal'
     *        } );
     *
     *        editor.conversion.for( 'upcast' ).attributeToAttribute( {
     *            view: {
     *                key: 'data-style',
     *                value: /[\s\S]+/
     *            },
     *            model: 'styled'
     *        } );
     *
     *        editor.conversion.for( 'upcast' ).attributeToAttribute( {
     *            view: {
     *                name: 'img',
     *                key: 'class',
     *                value: 'styled-dark'
     *            },
     *            model: {
     *                key: 'styled',
     *                value: 'dark'
     *            }
     *        } );
     *
     *        editor.conversion.for( 'upcast' ).attributeToAttribute( {
     *            view: {
     *                key: 'class',
     *                value: /styled-[\S]+/
     *            },
     *            model: {
     *                key: 'styled'
     *                value: viewElement => {
     *                    const regexp = /styled-([\S]+)/;
     *                    const match = viewElement.getAttribute( 'class' ).match( regexp );
     *
     *                    return match[ 1 ];
     *                }
     *            }
     *        } );
     *
     * Converting styles works a bit differently as it requires `view.styles` to be an object and by default
     * a model attribute will be set to `true` by such a converter. You can set the model attribute to any value by providing the `value`
     * callback that returns the desired value.
     *
     *        // Default conversion of font-weight style will result in setting bold attribute to true.
     *        editor.conversion.for( 'upcast' ).attributeToAttribute( {
     *            view: {
     *                styles: {
     *                    'font-weight': 'bold'
     *                }
     *            },
     *            model: 'bold'
     *        } );
     *
     *        // This converter will pass any style value to the `lineHeight` model attribute.
     *        editor.conversion.for( 'upcast' ).attributeToAttribute( {
     *            view: {
     *                styles: {
     *                    'line-height': /[\s\S]+/
     *                }
     *            },
     *            model: {
     *                key: 'lineHeight',
     *                value: viewElement => viewElement.getStyle( 'line-height' )
     *            }
     *        } );
     *
     * 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.view Specifies which view attribute will be converted. If a `String` is passed,
     * attributes with given key will be converted. If an `Object` is passed, it must have a required `key` property,
     * specifying view attribute key, and may have an optional `value` property, specifying view attribute value and optional `name`
     * property specifying a view element name from/on which the attribute should be converted. `value` can be given as a `String`,
     * a `RegExp` or a function callback, that takes view attribute value as the only parameter and returns `Boolean`.
     * @param {String|Object} config.model Model attribute key or an object with `key` and `value` properties, describing
     * the model attribute. `value` property may be set as a function that takes a view element and returns the value.
     * If `String` is given, the model attribute value will be same as view attribute value.
     * @param {module:utils/priorities~PriorityString} [config.converterPriority='low'] Converter priority.
     * @returns {module:engine/conversion/upcasthelpers~UpcastHelpers}
     */
    attributeToAttribute( config ) {
        return this.add( upcastAttributeToAttribute( config ) );
    }

    /**
     * View element to model marker conversion helper.
     *
     * This conversion results in creating a model marker. For example, if the marker was stored in a view as an element:
     * `<p>Fo<span data-marker="comment" data-comment-id="7"></span>o</p><p>B<span data-marker="comment" data-comment-id="7"></span>ar</p>`,
     * after the conversion is done, the marker will be available in
     * {@link module:engine/model/model~Model#markers model document markers}.
     *
     *        editor.conversion.for( 'upcast' ).elementToMarker( {
     *            view: 'marker-search',
     *            model: 'search'
     *        } );
     *
     *        editor.conversion.for( 'upcast' ).elementToMarker( {
     *            view: 'marker-search',
     *            model: 'search',
     *            converterPriority: 'high'
     *        } );
     *
     *        editor.conversion.for( 'upcast' ).elementToMarker( {
     *            view: 'marker-search',
     *            model: viewElement => 'comment:' + viewElement.getAttribute( 'data-comment-id' )
     *        } );
     *
     *        editor.conversion.for( 'upcast' ).elementToMarker( {
     *            view: {
     *                name: 'span',
     *                attributes: {
     *                    'data-marker': 'search'
     *                }
     *            },
     *            model: 'search'
     *        } );
     *
     * See {@link module:engine/conversion/conversion~Conversion#for `conversion.for()`} to learn how to add a converter
     * to the conversion process.
     *
     * @method #elementToMarker
     * @param {Object} config Conversion configuration.
     * @param {module:engine/view/matcher~MatcherPattern} config.view Pattern matching all view elements which should be converted.
     * @param {String|Function} config.model Name of the model marker, or a function that takes a view element and returns
     * a model marker name.
     * @param {module:utils/priorities~PriorityString} [config.converterPriority='normal'] Converter priority.
     * @returns {module:engine/conversion/upcasthelpers~UpcastHelpers}
     */
    elementToMarker( config ) {
        return this.add( upcastElementToMarker( config ) );
    }
}

/**
 * Function factory, creates a converter that converts {@link module:engine/view/documentfragment~DocumentFragment view document fragment}
 * or all children of {@link module:engine/view/element~Element} into
 * {@link module:engine/model/documentfragment~DocumentFragment model document fragment}.
 * This is the "entry-point" converter for upcast (view to model conversion). This converter starts the conversion of all children
 * of passed view document fragment. Those children {@link module:engine/view/node~Node view nodes} are then handled by other converters.
 *
 * This also a "default", last resort converter for all view elements that has not been converted by other converters.
 * When a view element is being converted to the model but it does not have converter specified, that view element
 * will be converted to {@link module:engine/model/documentfragment~DocumentFragment model document fragment} and returned.
 *
 * @returns {Function} Universal converter for view {@link module:engine/view/documentfragment~DocumentFragment fragments} and
 * {@link module:engine/view/element~Element elements} that returns
 * {@link module:engine/model/documentfragment~DocumentFragment model fragment} with children of converted view item.
 */
export function convertToModelFragment() {
    return ( evt, data, conversionApi ) => {
        // Second argument in `consumable.consume` is discarded for ViewDocumentFragment but is needed for ViewElement.
        if ( !data.modelRange && conversionApi.consumable.consume( data.viewItem, { name: true } ) ) {
            const { modelRange, modelCursor } = conversionApi.convertChildren( data.viewItem, data.modelCursor );

            data.modelRange = modelRange;
            data.modelCursor = modelCursor;
        }
    };
}

/**
 * Function factory, creates a converter that converts {@link module:engine/view/text~Text} to {@link module:engine/model/text~Text}.
 *
 * @returns {Function} {@link module:engine/view/text~Text View text} converter.
 */
export function convertText() {
    return ( evt, data, conversionApi ) => {
        if ( conversionApi.schema.checkChild( data.modelCursor, '$text' ) ) {
            if ( conversionApi.consumable.consume( data.viewItem ) ) {
                const text = conversionApi.writer.createText( data.viewItem.data );

                conversionApi.writer.insert( text, data.modelCursor );

                data.modelRange = ModelRange._createFromPositionAndShift( data.modelCursor, text.offsetSize );
                data.modelCursor = data.modelRange.end;
            }
        }
    };
}

/**
 * Function factory, creates a callback function which converts a {@link module:engine/view/selection~Selection
 * view selection} taken from the {@link module:engine/view/document~Document#event:selectionChange} event
 * and sets in on the {@link module:engine/model/document~Document#selection model}.
 *
 * **Note**: because there is no view selection change dispatcher nor any other advanced view selection to model
 * conversion mechanism, the callback should be set directly on view document.
 *
 *        view.document.on( 'selectionChange', convertSelectionChange( modelDocument, mapper ) );
 *
 * @param {module:engine/model/model~Model} model Data model.
 * @param {module:engine/conversion/mapper~Mapper} mapper Conversion mapper.
 * @returns {Function} {@link module:engine/view/document~Document#event:selectionChange} callback function.
 */
export function convertSelectionChange( model, mapper ) {
    return ( evt, data ) => {
        const viewSelection = data.newSelection;
        const modelSelection = new ModelSelection();

        const ranges = [];

        for ( const viewRange of viewSelection.getRanges() ) {
            ranges.push( mapper.toModelRange( viewRange ) );
        }

        modelSelection.setTo( ranges, { backward: viewSelection.isBackward } );

        if ( !modelSelection.isEqual( model.document.selection ) ) {
            model.change( writer => {
                writer.setSelection( modelSelection );
            } );
        }
    };
}

// View element to model element conversion helper.
//
// See {@link ~UpcastHelpers#elementToElement `.elementToElement()` upcast helper} for examples.
//
// @param {Object} config Conversion configuration.
// @param {module:engine/view/matcher~MatcherPattern} [config.view] Pattern matching all view elements which should be converted. If not
// set, the converter will fire for every view element.
// @param {String|module:engine/model/element~Element|Function} config.model Name of the model element, a model element
// instance or a function that takes a view element and returns a model element. The model element will be inserted in the model.
// @param {module:utils/priorities~PriorityString} [config.converterPriority='normal'] Converter priority.
// @returns {Function} Conversion helper.
function upcastElementToElement( config ) {
    config = cloneDeep( config );

    const converter = prepareToElementConverter( config );

    const elementName = getViewElementNameFromConfig( config.view );
    const eventName = elementName ? 'element:' + elementName : 'element';

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

// View element to model attribute conversion helper.
//
// See {@link ~UpcastHelpers#elementToAttribute `.elementToAttribute()` upcast helper} for examples.
//
// @param {Object} config Conversion configuration.
// @param {module:engine/view/matcher~MatcherPattern} config.view Pattern matching all view elements which should be converted.
// @param {String|Object} config.model Model attribute key or an object with `key` and `value` properties, describing
// the model attribute. `value` property may be set as a function that takes a view element and returns the value.
// If `String` is given, the model attribute value will be set to `true`.
// @param {module:utils/priorities~PriorityString} [config.converterPriority='low'] Converter priority.
// @returns {Function} Conversion helper.
function upcastElementToAttribute( config ) {
    config = cloneDeep( config );

    normalizeModelAttributeConfig( config );

    const converter = prepareToAttributeConverter( config, false );

    const elementName = getViewElementNameFromConfig( config.view );
    const eventName = elementName ? 'element:' + elementName : 'element';

    return dispatcher => {
        dispatcher.on( eventName, converter, { priority: config.converterPriority || 'low' } );
    };
}

// View attribute to model attribute conversion helper.
//
// See {@link ~UpcastHelpers#attributeToAttribute `.attributeToAttribute()` upcast helper} for examples.
//
// @param {Object} config Conversion configuration.
// @param {String|Object} config.view Specifies which view attribute will be converted. If a `String` is passed,
// attributes with given key will be converted. If an `Object` is passed, it must have a required `key` property,
// specifying view attribute key, and may have an optional `value` property, specifying view attribute value and optional `name`
// property specifying a view element name from/on which the attribute should be converted. `value` can be given as a `String`,
// a `RegExp` or a function callback, that takes view attribute value as the only parameter and returns `Boolean`.
// @param {String|Object} config.model Model attribute key or an object with `key` and `value` properties, describing
// the model attribute. `value` property may be set as a function that takes a view element and returns the value.
// If `String` is given, the model attribute value will be same as view attribute value.
// @param {module:utils/priorities~PriorityString} [config.converterPriority='low'] Converter priority.
// @returns {Function} Conversion helper.
function upcastAttributeToAttribute( config ) {
    config = cloneDeep( config );

    let viewKey = null;

    if ( typeof config.view == 'string' || config.view.key ) {
        viewKey = normalizeViewAttributeKeyValueConfig( config );
    }

    normalizeModelAttributeConfig( config, viewKey );

    const converter = prepareToAttributeConverter( config, true );

    return dispatcher => {
        dispatcher.on( 'element', converter, { priority: config.converterPriority || 'low' } );
    };
}

// View element to model marker conversion helper.
//
// See {@link ~UpcastHelpers#elementToMarker `.elementToMarker()` upcast helper} for examples.
//
// @param {Object} config Conversion configuration.
// @param {module:engine/view/matcher~MatcherPattern} config.view Pattern matching all view elements which should be converted.
// @param {String|Function} config.model Name of the model marker, or a function that takes a view element and returns
// a model marker name.
// @param {module:utils/priorities~PriorityString} [config.converterPriority='normal'] Converter priority.
// @returns {Function} Conversion helper.
function upcastElementToMarker( config ) {
    config = cloneDeep( config );

    normalizeToMarkerConfig( config );

    return upcastElementToElement( config );
}

// Helper function for from-view-element conversion. Checks if `config.view` directly specifies converted view element's name
// and if so, returns it.
//
// @param {Object} config Conversion view config.
// @returns {String|null} View element name or `null` if name is not directly set.
function getViewElementNameFromConfig( viewConfig ) {
    if ( typeof viewConfig == 'string' ) {
        return viewConfig;
    }

    if ( typeof viewConfig == 'object' && typeof viewConfig.name == 'string' ) {
        return viewConfig.name;
    }

    return null;
}

// Helper for to-model-element conversion. Takes a config object and returns a proper converter function.
//
// @param {Object} config Conversion configuration.
// @returns {Function} View to model converter.
function prepareToElementConverter( config ) {
    const matcher = config.view ? new Matcher( config.view ) : null;

    return ( evt, data, conversionApi ) => {
        let match = {};

        // If `config.view` has not been passed do not try matching. In this case, the converter should fire for all elements.
        if ( matcher ) {
            // This will be usually just one pattern but we support matchers with many patterns too.
            const matcherResult = matcher.match( data.viewItem );

            // If there is no match, this callback should not do anything.
            if ( !matcherResult ) {
                return;
            }

            match = matcherResult.match;
        }

        // Force consuming element's name.
        match.name = true;

        // Create model element basing on config.
        const modelElement = getModelElement( config.model, data.viewItem, conversionApi.writer );

        // Do not convert if element building function returned falsy value.
        if ( !modelElement ) {
            return;
        }

        // When element was already consumed then skip it.
        if ( !conversionApi.consumable.test( data.viewItem, match ) ) {
            return;
        }

        // Find allowed parent for element that we are going to insert.
        // If current parent does not allow to insert element but one of the ancestors does
        // then split nodes to allowed parent.
        const splitResult = conversionApi.splitToAllowedParent( modelElement, data.modelCursor );

        // When there is no split result it means that we can't insert element to model tree, so let's skip it.
        if ( !splitResult ) {
            return;
        }

        // Insert element on allowed position.
        conversionApi.writer.insert( modelElement, splitResult.position );

        // Convert children and insert to element.
        conversionApi.convertChildren( data.viewItem, conversionApi.writer.createPositionAt( modelElement, 0 ) );

        // Consume appropriate value from consumable values list.
        conversionApi.consumable.consume( data.viewItem, match );

        const parts = conversionApi.getSplitParts( modelElement );

        // Set conversion result range.
        data.modelRange = new ModelRange(
            conversionApi.writer.createPositionBefore( modelElement ),
            conversionApi.writer.createPositionAfter( parts[ parts.length - 1 ] )
        );

        // Now we need to check where the `modelCursor` should be.
        if ( splitResult.cursorParent ) {
            // If we split parent to insert our element then we want to continue conversion in the new part of the split parent.
            //
            // before: <allowed><notAllowed>foo[]</notAllowed></allowed>
            // after:  <allowed><notAllowed>foo</notAllowed><converted></converted><notAllowed>[]</notAllowed></allowed>

            data.modelCursor = conversionApi.writer.createPositionAt( splitResult.cursorParent, 0 );
        } else {
            // Otherwise just continue after inserted element.

            data.modelCursor = data.modelRange.end;
        }
    };
}

// Helper function for upcasting-to-element converter. Takes the model configuration, the converted view element
// and a writer instance and returns a model element instance to be inserted in the model.
//
// @param {String|Function|module:engine/model/element~Element} model Model conversion configuration.
// @param {module:engine/view/node~Node} input The converted view node.
// @param {module:engine/model/writer~Writer} writer A writer instance to use to create the model element.
function getModelElement( model, input, writer ) {
    if ( model instanceof Function ) {
        return model( input, writer );
    } else {
        return writer.createElement( model );
    }
}

// Helper function view-attribute-to-model-attribute helper. Normalizes `config.view` which was set as `String` or
// as an `Object` with `key`, `value` and `name` properties. Normalized `config.view` has is compatible with
// {@link module:engine/view/matcher~MatcherPattern}.
//
// @param {Object} config Conversion config.
// @returns {String} Key of the converted view attribute.
function normalizeViewAttributeKeyValueConfig( config ) {
    if ( typeof config.view == 'string' ) {
        config.view = { key: config.view };
    }

    const key = config.view.key;
    let normalized;

    if ( key == 'class' || key == 'style' ) {
        const keyName = key == 'class' ? 'classes' : 'styles';

        normalized = {
            [ keyName ]: config.view.value
        };
    } else {
        const value = typeof config.view.value == 'undefined' ? /[\s\S]*/ : config.view.value;

        normalized = {
            attributes: {
                [ key ]: value
            }
        };
    }

    if ( config.view.name ) {
        normalized.name = config.view.name;
    }

    config.view = normalized;

    return key;
}

// Helper function that normalizes `config.model` in from-model-attribute conversion. `config.model` can be set
// as a `String`, an `Object` with only `key` property or an `Object` with `key` and `value` properties. Normalized
// `config.model` is an `Object` with `key` and `value` properties.
//
// @param {Object} config Conversion config.
// @param {String} viewAttributeKeyToCopy Key of the converted view attribute. If it is set, model attribute value
// will be equal to view attribute value.
function normalizeModelAttributeConfig( config, viewAttributeKeyToCopy = null ) {
    const defaultModelValue = viewAttributeKeyToCopy === null ? true : viewElement => viewElement.getAttribute( viewAttributeKeyToCopy );

    const key = typeof config.model != 'object' ? config.model : config.model.key;
    const value = typeof config.model != 'object' || typeof config.model.value == 'undefined' ? defaultModelValue : config.model.value;

    config.model = { key, value };
}

// Helper for to-model-attribute conversion. Takes the model attribute name and conversion configuration and returns
// a proper converter function.
//
// @param {String} modelAttributeKey The key of the model attribute to set on a model node.
// @param {Object|Array.<Object>} config Conversion configuration. It is possible to provide multiple configurations in an array.
// @param {Boolean} shallow If set to `true` the attribute will be set only on top-level nodes. Otherwise, it will be set
// on all elements in the range.
function prepareToAttributeConverter( config, shallow ) {
    const matcher = new Matcher( config.view );

    return ( evt, data, conversionApi ) => {
        const match = matcher.match( data.viewItem );

        // If there is no match, this callback should not do anything.
        if ( !match ) {
            return;
        }

        const modelKey = config.model.key;
        const modelValue = typeof config.model.value == 'function' ? config.model.value( data.viewItem ) : config.model.value;

        // Do not convert if attribute building function returned falsy value.
        if ( modelValue === null ) {
            return;
        }

        if ( onlyViewNameIsDefined( config.view, data.viewItem ) ) {
            match.match.name = true;
        } else {
            // Do not test or consume `name` consumable.
            delete match.match.name;
        }

        // Try to consume appropriate values from consumable values list.
        if ( !conversionApi.consumable.test( data.viewItem, match.match ) ) {
            return;
        }

        // Since we are converting to attribute we need an range on which we will set the attribute.
        // If the range is not created yet, we will create it.
        if ( !data.modelRange ) {
            // Convert children and set conversion result as a current data.
            data = Object.assign( data, conversionApi.convertChildren( data.viewItem, data.modelCursor ) );
        }

        // Set attribute on current `output`. `Schema` is checked inside this helper function.
        const attributeWasSet = setAttributeOn( data.modelRange, { key: modelKey, value: modelValue }, shallow, conversionApi );

        if ( attributeWasSet ) {
            conversionApi.consumable.consume( data.viewItem, match.match );
        }
    };
}

// Helper function that checks if element name should be consumed in attribute converters.
//
// @param {Object} config Conversion view config.
// @returns {Boolean}
function onlyViewNameIsDefined( viewConfig, viewItem ) {
    // https://github.com/ckeditor/ckeditor5-engine/issues/1786
    const configToTest = typeof viewConfig == 'function' ? viewConfig( viewItem ) : viewConfig;

    if ( typeof configToTest == 'object' && !getViewElementNameFromConfig( configToTest ) ) {
        return false;
    }

    return !configToTest.classes && !configToTest.attributes && !configToTest.styles;
}

// Helper function for to-model-attribute converter. Sets model attribute on given range. Checks {@link module:engine/model/schema~Schema}
// to ensure proper model structure.
//
// @param {module:engine/model/range~Range} modelRange Model range on which attribute should be set.
// @param {Object} modelAttribute Model attribute to set.
// @param {module:engine/conversion/upcastdispatcher~UpcastConversionApi} conversionApi Conversion API.
// @param {Boolean} shallow If set to `true` the attribute will be set only on top-level nodes. Otherwise, it will be set
// on all elements in the range.
// @returns {Boolean} `true` if attribute was set on at least one node from given `modelRange`.
function setAttributeOn( modelRange, modelAttribute, shallow, conversionApi ) {
    let result = false;

    // Set attribute on each item in range according to Schema.
    for ( const node of Array.from( modelRange.getItems( { shallow } ) ) ) {
        if ( conversionApi.schema.checkAttribute( node, modelAttribute.key ) ) {
            conversionApi.writer.setAttribute( modelAttribute.key, modelAttribute.value, node );

            result = true;
        }
    }

    return result;
}

// Helper function for upcasting-to-marker conversion. Takes the config in a format requested by `upcastElementToMarker()`
// function and converts it to a format that is supported by `_upcastElementToElement()` function.
//
// @param {Object} config Conversion configuration.
function normalizeToMarkerConfig( config ) {
    const oldModel = config.model;

    config.model = ( viewElement, modelWriter ) => {
        const markerName = typeof oldModel == 'string' ? oldModel : oldModel( viewElement );

        return modelWriter.createElement( '$marker', { 'data-name': markerName } );
    };
}