mapper.js 24 KB

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  1. /**
  2. * @license Copyright (c) 2003-2019, CKSource - Frederico Knabben. All rights reserved.
  3. * For licensing, see LICENSE.md.
  4. */
  5. /**
  6. * @module engine/conversion/mapper
  7. */
  8. import ModelPosition from '../model/position';
  9. import ModelRange from '../model/range';
  10. import ViewPosition from '../view/position';
  11. import ViewRange from '../view/range';
  12. import ViewText from '../view/text';
  13. import EmitterMixin from '@ckeditor/ckeditor5-utils/src/emittermixin';
  14. import mix from '@ckeditor/ckeditor5-utils/src/mix';
  15. /**
  16. * Maps elements, positions and markers between {@link module:engine/view/document~Document the view} and
  17. * {@link module:engine/model/model the model}.
  18. *
  19. * Mapper use bound elements to find corresponding elements and positions, so, to get proper results,
  20. * all model elements should be {@link module:engine/conversion/mapper~Mapper#bindElements bound}.
  21. *
  22. * To map complex model to/from view relations, you may provide custom callbacks for
  23. * {@link module:engine/conversion/mapper~Mapper#event:modelToViewPosition modelToViewPosition event} and
  24. * {@link module:engine/conversion/mapper~Mapper#event:viewToModelPosition viewToModelPosition event} that are fired whenever
  25. * a position mapping request occurs.
  26. * Those events are fired by {@link module:engine/conversion/mapper~Mapper#toViewPosition toViewPosition}
  27. * and {@link module:engine/conversion/mapper~Mapper#toModelPosition toModelPosition} methods. `Mapper` adds it's own default callbacks
  28. * with `'lowest'` priority. To override default `Mapper` mapping, add custom callback with higher priority and
  29. * stop the event.
  30. */
  31. export default class Mapper {
  32. /**
  33. * Creates an instance of the mapper.
  34. */
  35. constructor() {
  36. /**
  37. * Model element to view element mapping.
  38. *
  39. * @private
  40. * @member {WeakMap}
  41. */
  42. this._modelToViewMapping = new WeakMap();
  43. /**
  44. * View element to model element mapping.
  45. *
  46. * @private
  47. * @member {WeakMap}
  48. */
  49. this._viewToModelMapping = new WeakMap();
  50. /**
  51. * A map containing callbacks between view element names and functions evaluating length of view elements
  52. * in model.
  53. *
  54. * @private
  55. * @member {Map}
  56. */
  57. this._viewToModelLengthCallbacks = new Map();
  58. /**
  59. * Model marker name to view elements mapping.
  60. *
  61. * Keys are `String`s while values are `Set`s with {@link module:engine/view/element~Element view elements}.
  62. * One marker (name) can be mapped to multiple elements.
  63. *
  64. * @private
  65. * @member {Map}
  66. */
  67. this._markerNameToElements = new Map();
  68. // Default mapper algorithm for mapping model position to view position.
  69. this.on( 'modelToViewPosition', ( evt, data ) => {
  70. if ( data.viewPosition ) {
  71. return;
  72. }
  73. const viewContainer = this._modelToViewMapping.get( data.modelPosition.parent );
  74. data.viewPosition = this._findPositionIn( viewContainer, data.modelPosition.offset );
  75. }, { priority: 'low' } );
  76. // Default mapper algorithm for mapping view position to model position.
  77. this.on( 'viewToModelPosition', ( evt, data ) => {
  78. if ( data.modelPosition ) {
  79. return;
  80. }
  81. const viewBlock = this.findMappedViewAncestor( data.viewPosition );
  82. const modelParent = this._viewToModelMapping.get( viewBlock );
  83. const modelOffset = this._toModelOffset( data.viewPosition.parent, data.viewPosition.offset, viewBlock );
  84. data.modelPosition = ModelPosition._createAt( modelParent, modelOffset );
  85. }, { priority: 'low' } );
  86. }
  87. /**
  88. * Marks model and view elements as corresponding. Corresponding elements can be retrieved by using
  89. * the {@link module:engine/conversion/mapper~Mapper#toModelElement toModelElement} and
  90. * {@link module:engine/conversion/mapper~Mapper#toViewElement toViewElement} methods.
  91. * The information that elements are bound is also used to translate positions.
  92. *
  93. * @param {module:engine/model/element~Element} modelElement Model element.
  94. * @param {module:engine/view/element~Element} viewElement View element.
  95. */
  96. bindElements( modelElement, viewElement ) {
  97. this._modelToViewMapping.set( modelElement, viewElement );
  98. this._viewToModelMapping.set( viewElement, modelElement );
  99. }
  100. /**
  101. * Unbinds given {@link module:engine/view/element~Element view element} from the map.
  102. *
  103. * **Note:** view-to-model binding will be removed, if it existed. However, corresponding model-to-view binding
  104. * will be removed only if model element is still bound to passed `viewElement`.
  105. *
  106. * This behavior lets for re-binding model element to another view element without fear of losing the new binding
  107. * when the previously bound view element is unbound.
  108. *
  109. * @param {module:engine/view/element~Element} viewElement View element to unbind.
  110. */
  111. unbindViewElement( viewElement ) {
  112. const modelElement = this.toModelElement( viewElement );
  113. this._viewToModelMapping.delete( viewElement );
  114. if ( this._modelToViewMapping.get( modelElement ) == viewElement ) {
  115. this._modelToViewMapping.delete( modelElement );
  116. }
  117. }
  118. /**
  119. * Unbinds given {@link module:engine/model/element~Element model element} from the map.
  120. *
  121. * **Note:** model-to-view binding will be removed, if it existed. However, corresponding view-to-model binding
  122. * will be removed only if view element is still bound to passed `modelElement`.
  123. *
  124. * This behavior lets for re-binding view element to another model element without fear of losing the new binding
  125. * when the previously bound model element is unbound.
  126. *
  127. * @param {module:engine/model/element~Element} modelElement Model element to unbind.
  128. */
  129. unbindModelElement( modelElement ) {
  130. const viewElement = this.toViewElement( modelElement );
  131. this._modelToViewMapping.delete( modelElement );
  132. if ( this._viewToModelMapping.get( viewElement ) == modelElement ) {
  133. this._viewToModelMapping.delete( viewElement );
  134. }
  135. }
  136. /**
  137. * Binds given marker name with given {@link module:engine/view/element~Element view element}. The element
  138. * will be added to the current set of elements bound with given marker name.
  139. *
  140. * @param {module:engine/view/element~Element} element Element to bind.
  141. * @param {String} name Marker name.
  142. */
  143. bindElementToMarker( element, name ) {
  144. const elements = this._markerNameToElements.get( name ) || new Set();
  145. elements.add( element );
  146. this._markerNameToElements.set( name, elements );
  147. }
  148. /**
  149. * Unbinds all elements from given marker name.
  150. *
  151. * @param {String} name Marker name.
  152. */
  153. unbindElementsFromMarkerName( name ) {
  154. this._markerNameToElements.delete( name );
  155. }
  156. /**
  157. * Removes all model to view and view to model bindings.
  158. */
  159. clearBindings() {
  160. this._modelToViewMapping = new WeakMap();
  161. this._viewToModelMapping = new WeakMap();
  162. this._markerNameToElements = new Map();
  163. }
  164. /**
  165. * Gets the corresponding model element.
  166. *
  167. * **Note:** {@link module:engine/view/uielement~UIElement} does not have corresponding element in model.
  168. *
  169. * @param {module:engine/view/element~Element} viewElement View element.
  170. * @returns {module:engine/model/element~Element|undefined} Corresponding model element or `undefined` if not found.
  171. */
  172. toModelElement( viewElement ) {
  173. return this._viewToModelMapping.get( viewElement );
  174. }
  175. /**
  176. * Gets the corresponding view element.
  177. *
  178. * @param {module:engine/model/element~Element} modelElement Model element.
  179. * @returns {module:engine/view/element~Element|undefined} Corresponding view element or `undefined` if not found.
  180. */
  181. toViewElement( modelElement ) {
  182. return this._modelToViewMapping.get( modelElement );
  183. }
  184. /**
  185. * Gets the corresponding model range.
  186. *
  187. * @param {module:engine/view/range~Range} viewRange View range.
  188. * @returns {module:engine/model/range~Range} Corresponding model range.
  189. */
  190. toModelRange( viewRange ) {
  191. return new ModelRange( this.toModelPosition( viewRange.start ), this.toModelPosition( viewRange.end ) );
  192. }
  193. /**
  194. * Gets the corresponding view range.
  195. *
  196. * @param {module:engine/model/range~Range} modelRange Model range.
  197. * @returns {module:engine/view/range~Range} Corresponding view range.
  198. */
  199. toViewRange( modelRange ) {
  200. return new ViewRange( this.toViewPosition( modelRange.start ), this.toViewPosition( modelRange.end ) );
  201. }
  202. /**
  203. * Gets the corresponding model position.
  204. *
  205. * @fires viewToModelPosition
  206. * @param {module:engine/view/position~Position} viewPosition View position.
  207. * @returns {module:engine/model/position~Position} Corresponding model position.
  208. */
  209. toModelPosition( viewPosition ) {
  210. const data = {
  211. viewPosition,
  212. mapper: this
  213. };
  214. this.fire( 'viewToModelPosition', data );
  215. return data.modelPosition;
  216. }
  217. /**
  218. * Gets the corresponding view position.
  219. *
  220. * @fires modelToViewPosition
  221. * @param {module:engine/model/position~Position} modelPosition Model position.
  222. * @param {Object} [options] Additional options for position mapping process.
  223. * @param {Boolean} [options.isPhantom=false] Should be set to `true` if the model position to map is pointing to a place
  224. * in model tree which no longer exists. For example, it could be an end of a removed model range.
  225. * @returns {module:engine/view/position~Position} Corresponding view position.
  226. */
  227. toViewPosition( modelPosition, options = { isPhantom: false } ) {
  228. const data = {
  229. modelPosition,
  230. mapper: this,
  231. isPhantom: options.isPhantom
  232. };
  233. this.fire( 'modelToViewPosition', data );
  234. return data.viewPosition;
  235. }
  236. /**
  237. * Gets all view elements bound to the given marker name.
  238. *
  239. * @param {String} name Marker name.
  240. * @returns {Set.<module:engine/view/element~Element>|null} View elements bound with given marker name or `null`
  241. * if no elements are bound to given marker name.
  242. */
  243. markerNameToElements( name ) {
  244. const boundElements = this._markerNameToElements.get( name );
  245. if ( !boundElements ) {
  246. return null;
  247. }
  248. const elements = new Set();
  249. for ( const element of boundElements ) {
  250. if ( element.is( 'attributeElement' ) ) {
  251. for ( const clone of element.getElementsWithSameId() ) {
  252. elements.add( clone );
  253. }
  254. } else {
  255. elements.add( element );
  256. }
  257. }
  258. return elements;
  259. }
  260. /**
  261. * Registers a callback that evaluates the length in the model of a view element with given name.
  262. *
  263. * The callback is fired with one argument, which is a view element instance. The callback is expected to return
  264. * a number representing the length of view element in model.
  265. *
  266. * // List item in view may contain nested list, which have other list items. In model though,
  267. * // the lists are represented by flat structure. Because of those differences, length of list view element
  268. * // may be greater than one. In the callback it's checked how many nested list items are in evaluated list item.
  269. *
  270. * function getViewListItemLength( element ) {
  271. * let length = 1;
  272. *
  273. * for ( let child of element.getChildren() ) {
  274. * if ( child.name == 'ul' || child.name == 'ol' ) {
  275. * for ( let item of child.getChildren() ) {
  276. * length += getViewListItemLength( item );
  277. * }
  278. * }
  279. * }
  280. *
  281. * return length;
  282. * }
  283. *
  284. * mapper.registerViewToModelLength( 'li', getViewListItemLength );
  285. *
  286. * @param {String} viewElementName Name of view element for which callback is registered.
  287. * @param {Function} lengthCallback Function return a length of view element instance in model.
  288. */
  289. registerViewToModelLength( viewElementName, lengthCallback ) {
  290. this._viewToModelLengthCallbacks.set( viewElementName, lengthCallback );
  291. }
  292. /**
  293. * For given `viewPosition`, finds and returns the closest ancestor of this position that has a mapping to
  294. * the model.
  295. *
  296. * @param {module:engine/view/position~Position} viewPosition Position for which mapped ancestor should be found.
  297. * @returns {module:engine/view/element~Element}
  298. */
  299. findMappedViewAncestor( viewPosition ) {
  300. let parent = viewPosition.parent;
  301. while ( !this._viewToModelMapping.has( parent ) ) {
  302. parent = parent.parent;
  303. }
  304. return parent;
  305. }
  306. /**
  307. * Calculates model offset based on the view position and the block element.
  308. *
  309. * Example:
  310. *
  311. * <p>foo<b>ba|r</b></p> // _toModelOffset( b, 2, p ) -> 5
  312. *
  313. * Is a sum of:
  314. *
  315. * <p>foo|<b>bar</b></p> // _toModelOffset( p, 3, p ) -> 3
  316. * <p>foo<b>ba|r</b></p> // _toModelOffset( b, 2, b ) -> 2
  317. *
  318. * @private
  319. * @param {module:engine/view/element~Element} viewParent Position parent.
  320. * @param {Number} viewOffset Position offset.
  321. * @param {module:engine/view/element~Element} viewBlock Block used as a base to calculate offset.
  322. * @returns {Number} Offset in the model.
  323. */
  324. _toModelOffset( viewParent, viewOffset, viewBlock ) {
  325. if ( viewBlock != viewParent ) {
  326. // See example.
  327. const offsetToParentStart = this._toModelOffset( viewParent.parent, viewParent.index, viewBlock );
  328. const offsetInParent = this._toModelOffset( viewParent, viewOffset, viewParent );
  329. return offsetToParentStart + offsetInParent;
  330. }
  331. // viewBlock == viewParent, so we need to calculate the offset in the parent element.
  332. // If the position is a text it is simple ("ba|r" -> 2).
  333. if ( viewParent.is( 'text' ) ) {
  334. return viewOffset;
  335. }
  336. // If the position is in an element we need to sum lengths of siblings ( <b> bar </b> foo | -> 3 + 3 = 6 ).
  337. let modelOffset = 0;
  338. for ( let i = 0; i < viewOffset; i++ ) {
  339. modelOffset += this.getModelLength( viewParent.getChild( i ) );
  340. }
  341. return modelOffset;
  342. }
  343. /**
  344. * Gets the length of the view element in the model.
  345. *
  346. * The length is calculated as follows:
  347. * * if {@link #registerViewToModelLength length mapping callback} is provided for given `viewNode` it is used to
  348. * evaluate model length (`viewNode` is used as first and only parameter passed to the callback),
  349. * * length of a {@link module:engine/view/text~Text text node} is equal to the length of it's
  350. * {@link module:engine/view/text~Text#data data},
  351. * * length of a {@link module:engine/view/uielement~UIElement ui element} is equal to 0,
  352. * * length of a mapped {@link module:engine/view/element~Element element} is equal to 1,
  353. * * length of a not-mapped {@link module:engine/view/element~Element element} is equal to the length of it's children.
  354. *
  355. * Examples:
  356. *
  357. * foo -> 3 // Text length is equal to it's data length.
  358. * <p>foo</p> -> 1 // Length of an element which is mapped is by default equal to 1.
  359. * <b>foo</b> -> 3 // Length of an element which is not mapped is a length of its children.
  360. * <div><p>x</p><p>y</p></div> -> 2 // Assuming that <div> is not mapped and <p> are mapped.
  361. *
  362. * @param {module:engine/view/element~Element} viewNode View node.
  363. * @returns {Number} Length of the node in the tree model.
  364. */
  365. getModelLength( viewNode ) {
  366. if ( this._viewToModelLengthCallbacks.get( viewNode.name ) ) {
  367. const callback = this._viewToModelLengthCallbacks.get( viewNode.name );
  368. return callback( viewNode );
  369. } else if ( this._viewToModelMapping.has( viewNode ) ) {
  370. return 1;
  371. } else if ( viewNode.is( 'text' ) ) {
  372. return viewNode.data.length;
  373. } else if ( viewNode.is( 'uiElement' ) ) {
  374. return 0;
  375. } else {
  376. let len = 0;
  377. for ( const child of viewNode.getChildren() ) {
  378. len += this.getModelLength( child );
  379. }
  380. return len;
  381. }
  382. }
  383. /**
  384. * Finds the position in the view node (or its children) with the expected model offset.
  385. *
  386. * Example:
  387. *
  388. * <p>fo<b>bar</b>bom</p> -> expected offset: 4
  389. *
  390. * _findPositionIn( p, 4 ):
  391. * <p>|fo<b>bar</b>bom</p> -> expected offset: 4, actual offset: 0
  392. * <p>fo|<b>bar</b>bom</p> -> expected offset: 4, actual offset: 2
  393. * <p>fo<b>bar</b>|bom</p> -> expected offset: 4, actual offset: 5 -> we are too far
  394. *
  395. * _findPositionIn( b, 4 - ( 5 - 3 ) ):
  396. * <p>fo<b>|bar</b>bom</p> -> expected offset: 2, actual offset: 0
  397. * <p>fo<b>bar|</b>bom</p> -> expected offset: 2, actual offset: 3 -> we are too far
  398. *
  399. * _findPositionIn( bar, 2 - ( 3 - 3 ) ):
  400. * We are in the text node so we can simple find the offset.
  401. * <p>fo<b>ba|r</b>bom</p> -> expected offset: 2, actual offset: 2 -> position found
  402. *
  403. * @private
  404. * @param {module:engine/view/element~Element} viewParent Tree view element in which we are looking for the position.
  405. * @param {Number} expectedOffset Expected offset.
  406. * @returns {module:engine/view/position~Position} Found position.
  407. */
  408. _findPositionIn( viewParent, expectedOffset ) {
  409. // Last scanned view node.
  410. let viewNode;
  411. // Length of the last scanned view node.
  412. let lastLength = 0;
  413. let modelOffset = 0;
  414. let viewOffset = 0;
  415. // In the text node it is simple: offset in the model equals offset in the text.
  416. if ( viewParent.is( 'text' ) ) {
  417. return new ViewPosition( viewParent, expectedOffset );
  418. }
  419. // In other cases we add lengths of child nodes to find the proper offset.
  420. // If it is smaller we add the length.
  421. while ( modelOffset < expectedOffset ) {
  422. viewNode = viewParent.getChild( viewOffset );
  423. lastLength = this.getModelLength( viewNode );
  424. modelOffset += lastLength;
  425. viewOffset++;
  426. }
  427. // If it equals we found the position.
  428. if ( modelOffset == expectedOffset ) {
  429. return this._moveViewPositionToTextNode( new ViewPosition( viewParent, viewOffset ) );
  430. }
  431. // If it is higher we need to enter last child.
  432. else {
  433. // ( modelOffset - lastLength ) is the offset to the child we enter,
  434. // so we subtract it from the expected offset to fine the offset in the child.
  435. return this._findPositionIn( viewNode, expectedOffset - ( modelOffset - lastLength ) );
  436. }
  437. }
  438. /**
  439. * Because we prefer positions in text nodes over positions next to text node moves view position to the text node
  440. * if it was next to it.
  441. *
  442. * <p>[]<b>foo</b></p> -> <p>[]<b>foo</b></p> // do not touch if position is not directly next to text
  443. * <p>foo[]<b>foo</b></p> -> <p>foo{}<b>foo</b></p> // move to text node
  444. * <p><b>[]foo</b></p> -> <p><b>{}foo</b></p> // move to text node
  445. *
  446. * @private
  447. * @param {module:engine/view/position~Position} viewPosition Position potentially next to text node.
  448. * @returns {module:engine/view/position~Position} Position in text node if possible.
  449. */
  450. _moveViewPositionToTextNode( viewPosition ) {
  451. // If the position is just after text node, put it at the end of that text node.
  452. // If the position is just before text node, put it at the beginning of that text node.
  453. const nodeBefore = viewPosition.nodeBefore;
  454. const nodeAfter = viewPosition.nodeAfter;
  455. if ( nodeBefore instanceof ViewText ) {
  456. return new ViewPosition( nodeBefore, nodeBefore.data.length );
  457. } else if ( nodeAfter instanceof ViewText ) {
  458. return new ViewPosition( nodeAfter, 0 );
  459. }
  460. // Otherwise, just return the given position.
  461. return viewPosition;
  462. }
  463. /**
  464. * Fired for each model-to-view position mapping request. The purpose of this event is to enable custom model-to-view position
  465. * mapping. Callbacks added to this event take {@link module:engine/model/position~Position model position} and are expected to
  466. * calculate {@link module:engine/view/position~Position view position}. Calculated view position should be added as `viewPosition`
  467. * value in `data` object that is passed as one of parameters to the event callback.
  468. *
  469. * // Assume that "captionedImage" model element is converted to <img> and following <span> elements in view,
  470. * // and the model element is bound to <img> element. Force mapping model positions inside "captionedImage" to that
  471. * // <span> element.
  472. * mapper.on( 'modelToViewPosition', ( evt, data ) => {
  473. * const positionParent = modelPosition.parent;
  474. *
  475. * if ( positionParent.name == 'captionedImage' ) {
  476. * const viewImg = data.mapper.toViewElement( positionParent );
  477. * const viewCaption = viewImg.nextSibling; // The <span> element.
  478. *
  479. * data.viewPosition = new ViewPosition( viewCaption, modelPosition.offset );
  480. *
  481. * // Stop the event if other callbacks should not modify calculated value.
  482. * evt.stop();
  483. * }
  484. * } );
  485. *
  486. * **Note:** keep in mind that sometimes a "phantom" model position is being converted. "Phantom" model position is
  487. * a position that points to a non-existing place in model. Such position might still be valid for conversion, though
  488. * (it would point to a correct place in view when converted). One example of such situation is when a range is
  489. * removed from model, there may be a need to map the range's end (which is no longer valid model position). To
  490. * handle such situation, check `data.isPhantom` flag:
  491. *
  492. * // Assume that there is "customElement" model element and whenever position is before it, we want to move it
  493. * // to the inside of the view element bound to "customElement".
  494. * mapper.on( 'modelToViewPosition', ( evt, data ) => {
  495. * if ( data.isPhantom ) {
  496. * return;
  497. * }
  498. *
  499. * // Below line might crash for phantom position that does not exist in model.
  500. * const sibling = data.modelPosition.nodeBefore;
  501. *
  502. * // Check if this is the element we are interested in.
  503. * if ( !sibling.is( 'customElement' ) ) {
  504. * return;
  505. * }
  506. *
  507. * const viewElement = data.mapper.toViewElement( sibling );
  508. *
  509. * data.viewPosition = new ViewPosition( sibling, 0 );
  510. *
  511. * evt.stop();
  512. * } );
  513. *
  514. * **Note:** default mapping callback is provided with `low` priority setting and does not cancel the event, so it is possible to
  515. * attach a custom callback after default callback and also use `data.viewPosition` calculated by default callback
  516. * (for example to fix it).
  517. *
  518. * **Note:** default mapping callback will not fire if `data.viewPosition` is already set.
  519. *
  520. * **Note:** these callbacks are called **very often**. For efficiency reasons, it is advised to use them only when position
  521. * mapping between given model and view elements is unsolvable using just elements mapping and default algorithm. Also,
  522. * the condition that checks if special case scenario happened should be as simple as possible.
  523. *
  524. * @event modelToViewPosition
  525. * @param {Object} data Data pipeline object that can store and pass data between callbacks. The callback should add
  526. * `viewPosition` value to that object with calculated {@link module:engine/view/position~Position view position}.
  527. * @param {module:engine/conversion/mapper~Mapper} data.mapper Mapper instance that fired the event.
  528. */
  529. /**
  530. * Fired for each view-to-model position mapping request. See {@link module:engine/conversion/mapper~Mapper#event:modelToViewPosition}.
  531. *
  532. * // See example in `modelToViewPosition` event description.
  533. * // This custom mapping will map positions from <span> element next to <img> to the "captionedImage" element.
  534. * mapper.on( 'viewToModelPosition', ( evt, data ) => {
  535. * const positionParent = viewPosition.parent;
  536. *
  537. * if ( positionParent.hasClass( 'image-caption' ) ) {
  538. * const viewImg = positionParent.previousSibling;
  539. * const modelImg = data.mapper.toModelElement( viewImg );
  540. *
  541. * data.modelPosition = new ModelPosition( modelImg, viewPosition.offset );
  542. * evt.stop();
  543. * }
  544. * } );
  545. *
  546. * **Note:** default mapping callback is provided with `low` priority setting and does not cancel the event, so it is possible to
  547. * attach a custom callback after default callback and also use `data.modelPosition` calculated by default callback
  548. * (for example to fix it).
  549. *
  550. * **Note:** default mapping callback will not fire if `data.modelPosition` is already set.
  551. *
  552. * **Note:** these callbacks are called **very often**. For efficiency reasons, it is advised to use them only when position
  553. * mapping between given model and view elements is unsolvable using just elements mapping and default algorithm. Also,
  554. * the condition that checks if special case scenario happened should be as simple as possible.
  555. *
  556. * @event viewToModelPosition
  557. * @param {Object} data Data pipeline object that can store and pass data between callbacks. The callback should add
  558. * `modelPosition` value to that object with calculated {@link module:engine/model/position~Position model position}.
  559. * @param {module:engine/conversion/mapper~Mapper} data.mapper Mapper instance that fired the event.
  560. */
  561. }
  562. mix( Mapper, EmitterMixin );