text: /**
   * @license Copyright (c) 2003-2019, 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 } )
  	 *		} );
  	 *
  	 * 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/conversion/downcasthelpers~HighlightDescriptor} descriptor
   * @returns {module:engine/view/attributeelement~AttributeElement}
   */
  export function createViewElementFromHighlightDescriptor( descriptor ) {
  	const viewElement = new ViewAttributeElement( '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.document ) {
  					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 viewElement = createViewElementFromHighlightDescriptor( descriptor );
  		const viewWriter = conversionApi.writer;
  		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( 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.
   */
  