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schema.js
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schema.js
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/**
* @license Copyright (c) 2003-2018, CKSource - Frederico Knabben. All rights reserved.
* For licensing, see LICENSE.md.
*/
/**
* @module engine/model/schema
*/
import CKEditorError from '@ckeditor/ckeditor5-utils/src/ckeditorerror';
import ObservableMixin from '@ckeditor/ckeditor5-utils/src/observablemixin';
import mix from '@ckeditor/ckeditor5-utils/src/mix';
import Range from './range';
import Position from './position';
import Element from './element';
/**
* The model's schema. It defines allowed and disallowed structures of nodes as well as nodes' attributes.
* The schema is usually defined by features and based on them the editing framework and features
* make decisions how to change and process the model.
*
* The instance of schema is available in {@link module:engine/model/model~Model#schema `editor.model.schema`}.
*
* # Schema definitions
*
* Schema defines allowed model structures and allowed attributes separately. They are also checked separately
* by using the {@link ~Schema#checkChild} and {@link ~Schema#checkAttribute} methods.
*
* ## Defining allowed structures
*
* When a feature introduces a model element it should register it in the schema. Besides
* defining that such an element may exist in the model, the feature also needs to define where
* this element may be placed:
*
* schema.register( 'myElement', {
* allowIn: '$root'
* } );
*
* This lets the schema know that `<myElement>` may be a child of the `<$root>` element. `$root` is one of generic
* nodes defined by the editing framework. By default, the editor names the main root element a `<$root>`,
* so the above definition allows `<myElement>` in the main editor element.
*
* In other words, this would be correct:
*
* <$root><myElement></myElement></$root>
*
* While this would not be correct:
*
* <$root><foo><myElement></myElement></foo></$root>
*
* ## Generic items
*
* There are three basic generic items: `$root`, `$block` and `$text`.
* They are defined as follows:
*
* this.schema.register( '$root', {
* isLimit: true
* } );
* this.schema.register( '$block', {
* allowIn: '$root',
* isBlock: true
* } );
* this.schema.register( '$text', {
* allowIn: '$block'
* } );
*
* These definitions can then be reused by features to create their own definitions in a more extensible way.
* For example, the {@link module:paragraph/paragraph~Paragraph} feature will define its item as:
*
* schema.register( 'paragraph', {
* inheritAllFrom: '$block'
* } );
*
* Which translates to:
*
* schema.register( 'paragraph', {
* allowWhere: '$block',
* allowContentOf: '$block',
* allowAttributesOf: '$block',
* inheritTypesFrom: '$block'
* } );
*
* Which can be read as:
*
* * The `<paragraph>` element will be allowed in elements in which `<$block>` is allowed (e.g. in `<$root>`).
* * The `<paragraph>` element will allow all nodes which are allowed in `<$block>` (e.g. `$text`).
* * The `<paragraph>` element will allow all attributes allowed on `<$block>`.
* * The `<paragraph>` element will inherit all `is*` properties of `<$block>` (e.g. `isBlock`).
*
* Thanks to the fact that `<paragraph>`'s definition is inherited from `<$block>` other features can use the `<$block>`
* type to indirectly extend `<paragraph>`'s definition. For example, the {@link module:block-quote/blockquote~BlockQuote}
* feature does this:
*
* schema.register( 'blockQuote', {
* allowWhere: '$block',
* allowContentOf: '$root'
* } );
*
* Thanks to that, despite the fact that block quote and paragraph features know nothing about themselves, paragraphs
* will be allowed in block quotes and block quotes will be allowed in all places where blocks are. So if anyone will
* register a `<section>` element (with `allowContentOf: '$root'` rule), that `<section>` elements will allow
* block quotes too.
*
* The side effect of such a definition inheritance is that now `<blockQuote>` is allowed in `<blockQuote>` which needs to be
* resolved by a callback which will disallow this specific structure.
*
* You can read more about the format of an item definition in {@link module:engine/model/schema~SchemaItemDefinition}.
*
* ## Defining advanced rules in `checkChild()`'s callbacks
*
* The {@link ~Schema#checkChild} method which is the base method used to check whether some element is allowed in a given structure
* is {@link module:utils/observablemixin~ObservableMixin#decorate a decorated method}.
* It means that you can add listeners to implement your specific rules which are not limited by the declarative
* {@link module:engine/model/schema~SchemaItemDefinition API}.
*
* Those listeners can be added either by listening directly to the {@link ~Schema#event:checkChild} event or
* by using the handy {@link ~Schema#addChildCheck} method.
*
* For instance, the block quote feature defines such a listener to disallow nested `<blockQuote>` structures:
*
* schema.addChildCheck( context, childDefinition ) => {
* // Note that context is automatically normalized to SchemaContext instance and
* // child to its definition (SchemaCompiledItemDefinition).
*
* // If checkChild() is called with a context that ends with blockQuote and blockQuote as a child
* // to check, make the checkChild() method return false.
* if ( context.endsWith( 'blockQuote' ) && childDefinition.name == 'blockQuote' ) {
* return false;
* }
* } );
*
* ## Defining attributes
*
* TODO
*
* ## Implementing additional constraints
*
* Schema's capabilities were limited to simple (and atomic) {@link ~Schema#checkChild} and
* {@link ~Schema#checkAttribute} checks on purpose.
* One may imagine that schema should support defining more complex rules such as
* "element `<x>` must be always followed by `<y>`".
* While it is feasible to create an API which would enable feeding the schema with such definitions,
* it is unfortunately unrealistic to then expect that every editing feature will consider those rules when processing the model.
* It is also unrealistic to expect that it will be done automatically by the schema and the editing engine themselves.
*
* For instance, let's get back to the "element `<x>` must be always followed by `<y>`" rule and this initial content:
*
* <$root>
* <x>foo</x>
* <y>bar[bom</y>
* <z>bom]bar</z>
* </$root>
*
* Now, imagine that the user presses the "block quote" button. Usually it would wrap the two selected blocks
* (`<y>` and `<z>`) with a `<blockQuote>` element:
*
* <$root>
* <x>foo</x>
* <blockQuote>
* <y>bar[bom</y>
* <z>bom]bar</z>
* </blockQuote>
* </$root>
*
* But it turns out that this creates an incorrect structure – `<x>` is not followed by `<y>` anymore.
*
* What should happen instead? There are at least 4 possible solutions: the block quote feature should not be
* applicable in such a context, someone should create a new `<y>` right after `<x>`, `<x>` should be moved
* inside `<blockQuote>` together with `<y>` or vice versa.
*
* While this is a relatively simple scenario (unlike most real-time collaboration scenarios),
* it turns out that it's already hard to say what should happen and who should react to fix this content.
*
* Therefore, if your editor needs to implement such rules, you should do that through model's post-fixers
* fixing incorrect content or actively prevent such situations (e.g. by disabling certain features).
* It means that those constraints will be defined specifically for your scenario by your code which
* makes their implementation much easier.
*
* So the answer for who and how should implement additional constraints is your features or your editor
* through CKEditor 5's rich and open API.
*
* @mixes module:utils/observablemixin~ObservableMixin
*/
export default class Schema {
/**
* Creates schema instance.
*/
constructor() {
this._sourceDefinitions = {};
this.decorate( 'checkChild' );
this.decorate( 'checkAttribute' );
this.on( 'checkAttribute', ( evt, args ) => {
args[ 0 ] = new SchemaContext( args[ 0 ] );
}, { priority: 'highest' } );
this.on( 'checkChild', ( evt, args ) => {
args[ 0 ] = new SchemaContext( args[ 0 ] );
args[ 1 ] = this.getDefinition( args[ 1 ] );
}, { priority: 'highest' } );
}
/**
* Registers schema item. Can only be called once for every item name.
*
* schema.register( 'paragraph', {
* inheritAllFrom: '$block'
* } );
*
* @param {String} itemName
* @param {module:engine/model/schema~SchemaItemDefinition} definition
*/
register( itemName, definition ) {
if ( this._sourceDefinitions[ itemName ] ) {
// TODO docs
throw new CKEditorError( 'schema-cannot-register-item-twice: A single item cannot be registered twice in the schema.', {
itemName
} );
}
this._sourceDefinitions[ itemName ] = [
Object.assign( {}, definition )
];
this._clearCache();
}
/**
* Extends a {@link #register registered} item's definition.
*
* Extending properties such as `allowIn` will add more items to the existing properties,
* while redefining properties such as `isBlock` will override the previously defined ones.
*
* schema.register( 'foo', {
* allowIn: '$root',
* isBlock: true;
* } );
* schema.extend( 'foo', {
* allowIn: 'blockQuote',
* isBlock: false
* } );
*
* schema.getDefinition( 'foo' );
* // {
* // allowIn: [ '$root', 'blockQuote' ],
* // isBlock: false
* // }
*
* @param {String} itemName
* @param {module:engine/model/schema~SchemaItemDefinition} definition
*/
extend( itemName, definition ) {
if ( !this._sourceDefinitions[ itemName ] ) {
// TODO docs
throw new CKEditorError( 'schema-cannot-extend-missing-item: Cannot extend an item which was not registered yet.', {
itemName
} );
}
this._sourceDefinitions[ itemName ].push( Object.assign( {}, definition ) );
this._clearCache();
}
/**
* Returns all registered items.
*
* @returns {Object.<String,module:engine/model/schema~SchemaCompiledItemDefinition>}
*/
getDefinitions() {
if ( !this._compiledDefinitions ) {
this._compile();
}
return this._compiledDefinitions;
}
/**
* Returns a definition of the given item or `undefined` if item is not registered.
*
* @param {module:engine/model/item~Item|module:engine/model/schema~SchemaContextItem|String} item
* @returns {module:engine/model/schema~SchemaCompiledItemDefinition}
*/
getDefinition( item ) {
let itemName;
if ( typeof item == 'string' ) {
itemName = item;
} else if ( item.is && ( item.is( 'text' ) || item.is( 'textProxy' ) ) ) {
itemName = '$text';
}
// Element or module:engine/model/schema~SchemaContextItem.
else {
itemName = item.name;
}
return this.getDefinitions()[ itemName ];
}
/**
* Returns `true` if the given item is registered in the schema.
*
* schema.isRegistered( 'paragraph' ); // -> true
* schema.isRegistered( editor.model.document.getRoot() ); // -> true
* schema.isRegistered( 'foo' ); // -> false
*
* @param {module:engine/model/item~Item|module:engine/model/schema~SchemaContextItem|String} item
*/
isRegistered( item ) {
return !!this.getDefinition( item );
}
/**
* Returns `true` if the given item is defined to be
* a block by {@link module:engine/model/schema~SchemaItemDefinition}'s `isBlock` property.
*
* schema.isBlock( 'paragraph' ); // -> true
* schema.isBlock( '$root' ); // -> false
*
* const paragraphElement = writer.createElement( 'paragraph' );
* schema.isBlock( paragraphElement ); // -> true
*
* @param {module:engine/model/item~Item|module:engine/model/schema~SchemaContextItem|String} item
*/
isBlock( item ) {
const def = this.getDefinition( item );
return !!( def && def.isBlock );
}
/**
* Returns `true` if the given item is defined to be
* a limit element by {@link module:engine/model/schema~SchemaItemDefinition}'s `isLimit` property.
*
* schema.isLimit( 'paragraph' ); // -> false
* schema.isLimit( '$root' ); // -> true
* schema.isLimit( editor.model.document.getRoot() ); // -> true
*
* @param {module:engine/model/item~Item|module:engine/model/schema~SchemaContextItem|String} item
*/
isLimit( item ) {
const def = this.getDefinition( item );
return !!( def && def.isLimit );
}
/**
* Returns `true` if the given item is defined to be
* a object element by {@link module:engine/model/schema~SchemaItemDefinition}'s `isObject` property.
*
* schema.isObject( 'paragraph' ); // -> false
* schema.isObject( 'image' ); // -> true
*
* const imageElement = writer.createElement( 'image' );
* schema.isObject( imageElement ); // -> true
*
* @param {module:engine/model/item~Item|module:engine/model/schema~SchemaContextItem|String} item
*/
isObject( item ) {
const def = this.getDefinition( item );
return !!( def && def.isObject );
}
/**
* Checks whether the given node (`child`) can be a child of the given context.
*
* schema.checkChild( model.document.getRoot(), paragraph ); // -> false
*
* schema.register( 'paragraph', {
* allowIn: '$root'
* } );
* schema.checkChild( model.document.getRoot(), paragraph ); // -> true
*
* @fires checkChild
* @param {module:engine/model/schema~SchemaContextDefinition|module:engine/model/schema~SchemaContext} context
* Context in which the child will be checked.
* @param {module:engine/model/node~Node|String} def The child to check.
*/
checkChild( context, def ) {
// Note: context and child are already normalized here to a SchemaContext and SchemaCompiledItemDefinition.
if ( !def ) {
return false;
}
return this._checkContextMatch( def, context );
}
/**
* Checks whether the given attribute can be applied in the given context (on the last
* item of the context).
*
* schema.checkAttribute( textNode, 'bold' ); // -> false
*
* schema.extend( '$text', {
* allowAttributes: 'bold'
* } );
* schema.checkAttribute( textNode, 'bold' ); // -> true
*
* @fires checkAttribute
* @param {module:engine/model/schema~SchemaContextDefinition|module:engine/model/schema~SchemaContext} context
* Context in which the attribute will be checked.
* @param {String} attributeName
*/
checkAttribute( context, attributeName ) {
const def = this.getDefinition( context.last );
if ( !def ) {
return false;
}
return def.allowAttributes.includes( attributeName );
}
/**
* Checks whether the given element (`elementToMerge`) can be merged with the specified base element (`positionOrBaseElement`).
*
* In other words – whether `elementToMerge`'s children {@link #checkChild are allowed} in the `positionOrBaseElement`.
*
* This check ensures that elements merged with {@link module:engine/model/writer~Writer#merge `Writer#merge()`}
* will be valid.
*
* Instead of elements, you can pass the instance of {@link module:engine/model/position~Position} class as the `positionOrBaseElement`.
* It means that the elements before and after the position will be checked whether they can be merged.
*
* @param {module:engine/model/position~Position|module:engine/model/element~Element} positionOrBaseElement The position or base
* element to which the `elementToMerge` will be merged.
* @param {module:engine/model/element~Element} elementToMerge The element to merge. Required if `positionOrBaseElement` is a element.
* @returns {Boolean}
*/
checkMerge( positionOrBaseElement, elementToMerge = null ) {
if ( positionOrBaseElement instanceof Position ) {
const nodeBefore = positionOrBaseElement.nodeBefore;
const nodeAfter = positionOrBaseElement.nodeAfter;
if ( !( nodeBefore instanceof Element ) ) {
/**
* The node before the merge position must be an element.
*
* @error schema-check-merge-no-element-before
*/
throw new CKEditorError( 'schema-check-merge-no-element-before: The node before the merge position must be an element.' );
}
if ( !( nodeAfter instanceof Element ) ) {
/**
* The node after the merge position must be an element.
*
* @error schema-check-merge-no-element-after
*/
throw new CKEditorError( 'schema-check-merge-no-element-after: The node after the merge position must be an element.' );
}
return this.checkMerge( nodeBefore, nodeAfter );
}
for ( const child of elementToMerge.getChildren() ) {
if ( !this.checkChild( positionOrBaseElement, child ) ) {
return false;
}
}
return true;
}
/**
* Allows registering a callback to the {@link #checkChild} method calls.
*
* Callbacks allow you to implement rules which are not otherwise possible to achieve
* by using the declarative API of {@link module:engine/model/schema~SchemaItemDefinition}.
* For example, by using this method you can disallow elements in specific contexts.
*
* This method is a shorthand for using the {@link #event:checkChild} event. For even better control,
* you can use that event instead.
*
* Example:
*
* // Disallow heading1 directly inside a blockQuote.
* schema.addChildCheck( ( context, childDefinition ) => {
* if ( context.endsWith( 'blockQuote' ) && childDefinition.name == 'heading1' ) {
* return false;
* }
* } );
*
* Which translates to:
*
* schema.on( 'checkChild', ( evt, args ) => {
* const context = args[ 0 ];
* const childDefinition = args[ 1 ];
*
* if ( context.endsWith( 'blockQuote' ) && childDefinition && childDefinition.name == 'heading1' ) {
* // Prevent next listeners from being called.
* evt.stop();
* // Set the checkChild()'s return value.
* evt.return = false;
* }
* }, { priority: 'high' } );
*
* @param {Function} callback The callback to be called. It is called with two parameters:
* {@link module:engine/model/schema~SchemaContext} (context) instance and
* {@link module:engine/model/schema~SchemaCompiledItemDefinition} (child-to-check definition).
* The callback may return `true/false` to override `checkChild()`'s return value. If it does not return
* a boolean value, the default algorithm (or other callbacks) will define `checkChild()`'s return value.
*/
addChildCheck( callback ) {
this.on( 'checkChild', ( evt, [ ctx, childDef ] ) => {
// checkChild() was called with a non-registered child.
// In 99% cases such check should return false, so not to overcomplicate all callbacks
// don't even execute them.
if ( !childDef ) {
return;
}
const retValue = callback( ctx, childDef );
if ( typeof retValue == 'boolean' ) {
evt.stop();
evt.return = retValue;
}
}, { priority: 'high' } );
}
/**
* Allows registering a callback to the {@link #checkAttribute} method calls.
*
* Callbacks allow you to implement rules which are not otherwise possible to achieve
* by using the declarative API of {@link module:engine/model/schema~SchemaItemDefinition}.
* For example, by using this method you can disallow attribute if node to which it is applied
* is contained within some other element (e.g. you want to disallow `bold` on `$text` within `heading1`).
*
* This method is a shorthand for using the {@link #event:checkAttribute} event. For even better control,
* you can use that event instead.
*
* Example:
*
* // Disallow bold on $text inside heading1.
* schema.addChildCheck( ( context, attributeName ) => {
* if ( context.endsWith( 'heading1 $text' ) && attributeName == 'bold' ) {
* return false;
* }
* } );
*
* Which translates to:
*
* schema.on( 'checkAttribute', ( evt, args ) => {
* const context = args[ 0 ];
* const attributeName = args[ 1 ];
*
* if ( context.endsWith( 'heading1 $text' ) && attributeName == 'bold' ) {
* // Prevent next listeners from being called.
* evt.stop();
* // Set the checkAttribute()'s return value.
* evt.return = false;
* }
* }, { priority: 'high' } );
*
* @param {Function} callback The callback to be called. It is called with two parameters:
* {@link module:engine/model/schema~SchemaContext} (context) instance and attribute name.
* The callback may return `true/false` to override `checkAttribute()`'s return value. If it does not return
* a boolean value, the default algorithm (or other callbacks) will define `checkAttribute()`'s return value.
*/
addAttributeCheck( callback ) {
this.on( 'checkAttribute', ( evt, [ ctx, attributeName ] ) => {
const retValue = callback( ctx, attributeName );
if ( typeof retValue == 'boolean' ) {
evt.stop();
evt.return = retValue;
}
}, { priority: 'high' } );
}
/**
* Returns the lowest {@link module:engine/model/schema~Schema#isLimit limit element} containing the entire
* selection or the root otherwise.
*
* @param {module:engine/model/selection~Selection} selection Selection which returns the common ancestor.
* @returns {module:engine/model/element~Element}
*/
getLimitElement( selection ) {
// Find the common ancestor for all selection's ranges.
let element = Array.from( selection.getRanges() )
.reduce( ( node, range ) => {
if ( !node ) {
return range.getCommonAncestor();
}
return node.getCommonAncestor( range.getCommonAncestor() );
}, null );
while ( !this.isLimit( element ) ) {
if ( element.parent ) {
element = element.parent;
} else {
break;
}
}
return element;
}
/**
* Checks whether the attribute is allowed in selection:
*
* * if the selection is not collapsed, then checks if the attribute is allowed on any of nodes in that range,
* * if the selection is collapsed, then checks if on the selection position there's a text with the
* specified attribute allowed.
*
* @param {module:engine/model/selection~Selection} selection Selection which will be checked.
* @param {String} attribute The name of the attribute to check.
* @returns {Boolean}
*/
checkAttributeInSelection( selection, attribute ) {
if ( selection.isCollapsed ) {
// Check whether schema allows for a text with the attribute in the selection.
return this.checkAttribute( [ ...selection.getFirstPosition().getAncestors(), '$text' ], attribute );
} else {
const ranges = selection.getRanges();
// For all ranges, check nodes in them until you find a node that is allowed to have the attribute.
for ( const range of ranges ) {
for ( const value of range ) {
if ( this.checkAttribute( value.item, attribute ) ) {
// If we found a node that is allowed to have the attribute, return true.
return true;
}
}
}
}
// If we haven't found such node, return false.
return false;
}
/**
* Transforms the given set of ranges into a set of ranges where the given attribute is allowed (and can be applied).
*
* @param {Array.<module:engine/model/range~Range>} ranges Ranges to be validated.
* @param {String} attribute The name of the attribute to check.
* @returns {Array.<module:engine/model/range~Range>} Ranges in which the attribute is allowed.
*/
getValidRanges( ranges, attribute ) {
const validRanges = [];
for ( const range of ranges ) {
let last = range.start;
let from = range.start;
const to = range.end;
for ( const value of range.getWalker() ) {
if ( !this.checkAttribute( value.item, attribute ) ) {
if ( !from.isEqual( last ) ) {
validRanges.push( new Range( from, last ) );
}
from = value.nextPosition;
}
last = value.nextPosition;
}
if ( from && !from.isEqual( to ) ) {
validRanges.push( new Range( from, to ) );
}
}
return validRanges;
}
/**
* Tries to find position ancestors that allows to insert given node.
* It starts searching from the given position and goes node by node to the top of the model tree
* as long as {@link module:engine/model/schema~Schema#isLimit limit element} or top-most ancestor won't be reached.
*
* @params {module:engine/model/node~Node} node Node for which allowed parent should be found.
* @params {module:engine/model/position~Position} position Position from searching will start.
* @params {module:engine/model/element~Element} [limitElement] Custom limit element.
* @returns {module:engine/model/element~Element|null} element Allowed parent or null if nothing was found.
*/
findAllowedParent( node, position, limitElement ) {
let parent = position.parent;
while ( parent ) {
if ( this.checkChild( parent, node ) ) {
return parent;
}
if ( this.isLimit( parent ) ) {
return null;
}
if ( parent === limitElement ) {
return null;
}
parent = parent.parent;
}
return null;
}
/**
* Removes attributes disallowed by the schema.
*
* @param {Iterable.<module:engine/model/node~Node>} nodes Nodes that will be filtered.
* @param {module:engine/model/writer~Writer} writer
*/
removeDisallowedAttributes( nodes, writer ) {
for ( const node of nodes ) {
for ( const attribute of node.getAttributeKeys() ) {
if ( !this.checkAttribute( node, attribute ) ) {
writer.removeAttribute( attribute, node );
}
}
if ( node.is( 'element' ) ) {
this.removeDisallowedAttributes( node.getChildren(), writer );
}
}
}
/**
* @private
*/
_clearCache() {
this._compiledDefinitions = null;
}
/**
* @private
*/
_compile() {
const compiledDefinitions = {};
const sourceRules = this._sourceDefinitions;
const itemNames = Object.keys( sourceRules );
for ( const itemName of itemNames ) {
compiledDefinitions[ itemName ] = compileBaseItemRule( sourceRules[ itemName ], itemName );
}
for ( const itemName of itemNames ) {
compileAllowContentOf( compiledDefinitions, itemName );
}
for ( const itemName of itemNames ) {
compileAllowWhere( compiledDefinitions, itemName );
}
for ( const itemName of itemNames ) {
compileAllowAttributesOf( compiledDefinitions, itemName );
compileInheritPropertiesFrom( compiledDefinitions, itemName );
}
for ( const itemName of itemNames ) {
cleanUpAllowIn( compiledDefinitions, itemName );
cleanUpAllowAttributes( compiledDefinitions, itemName );
}
this._compiledDefinitions = compiledDefinitions;
}
/**
* @private
* @param {module:engine/model/schema~SchemaCompiledItemDefinition} def
* @param {module:engine/model/schema~SchemaContext} context
* @param {Number} contextItemIndex
*/
_checkContextMatch( def, context, contextItemIndex = context.length - 1 ) {
const contextItem = context.getItem( contextItemIndex );
if ( def.allowIn.includes( contextItem.name ) ) {
if ( contextItemIndex == 0 ) {
return true;
} else {
const parentRule = this.getDefinition( contextItem );
return this._checkContextMatch( parentRule, context, contextItemIndex - 1 );
}
} else {
return false;
}
}
}
mix( Schema, ObservableMixin );
/**
* Event fired when the {@link #checkChild} method is called. It allows plugging in
* additional behavior – e.g. implementing rules which cannot be defined using the declarative
* {@link module:engine/model/schema~SchemaItemDefinition} interface.
*
* **Note:** The {@link #addChildCheck} method is a more handy way to register callbacks. Internally,
* it registers a listener to this event but comes with a simpler API and it is the recommended choice
* in most of the cases.
*
* The {@link #checkChild} method fires an event because it is
* {@link module:utils/observablemixin~ObservableMixin#decorate decorated} with it. Thanks to that you can
* use this event in a various way, but the most important use case is overriding standard behaviour of the
* `checkChild()` method. Let's see a typical listener template:
*
* schema.on( 'checkChild', ( evt, args ) => {
* const context = args[ 0 ];
* const childDefinition = args[ 1 ];
* }, { priority: 'high' } );
*
* The listener is added with a `high` priority to be executed before the default method is really called. The `args` callback
* parameter contains arguments passed to `checkChild( context, child )`. However, the `context` parameter is already
* normalized to a {@link module:engine/model/schema~SchemaContext} instance and `child` to a
* {@link module:engine/model/schema~SchemaCompiledItemDefinition} instance, so you don't have to worry about
* the various ways how `context` and `child` may be passed to `checkChild()`.
*
* **Note:** `childDefinition` may be `undefined` if `checkChild()` was called with a non-registered element.
*
* So, in order to implement a rule "disallow `heading1` in `blockQuote`" you can add such a listener:
*
* schema.on( 'checkChild', ( evt, args ) => {
* const context = args[ 0 ];
* const childDefinition = args[ 1 ];
*
* if ( context.endsWith( 'blockQuote' ) && childDefinition && childDefinition.name == 'heading1' ) {
* // Prevent next listeners from being called.
* evt.stop();
* // Set the checkChild()'s return value.
* evt.return = false;
* }
* }, { priority: 'high' } );
*
* Allowing elements in specific contexts will be a far less common use case, because it's normally handled by
* `allowIn` rule from {@link module:engine/model/schema~SchemaItemDefinition} but if you have a complex scenario
* where `listItem` should be allowed only in element `foo` which must be in element `bar`, then this would be the way:
*
* schema.on( 'checkChild', ( evt, args ) => {
* const context = args[ 0 ];
* const childDefinition = args[ 1 ];
*
* if ( context.endsWith( 'bar foo' ) && childDefinition.name == 'listItem' ) {
* // Prevent next listeners from being called.
* evt.stop();
* // Set the checkChild()'s return value.
* evt.return = true;
* }
* }, { priority: 'high' } );
*
* @event checkChild
* @param {Array} args The `checkChild()`'s arguments.
*/
/**
* Event fired when the {@link #checkAttribute} method is called. It allows plugging in
* additional behavior – e.g. implementing rules which cannot be defined using the declarative
* {@link module:engine/model/schema~SchemaItemDefinition} interface.
*
* **Note:** The {@link #addAttributeCheck} method is a more handy way to register callbacks. Internally,
* it registers a listener to this event but comes with a simpler API and it is the recommended choice
* in most of the cases.
*
* The {@link #checkAttribute} method fires an event because it's
* {@link module:utils/observablemixin~ObservableMixin#decorate decorated} with it. Thanks to that you can
* use this event in a various way, but the most important use case is overriding standard behaviour of the
* `checkAttribute()` method. Let's see a typical listener template:
*
* schema.on( 'checkAttribute', ( evt, args ) => {
* const context = args[ 0 ];
* const attributeName = args[ 1 ];
* }, { priority: 'high' } );
*
* The listener is added with a `high` priority to be executed before the default method is really called. The `args` callback
* parameter contains arguments passed to `checkAttribute( context, attributeName )`. However, the `context` parameter is already
* normalized to a {@link module:engine/model/schema~SchemaContext} instance, so you don't have to worry about
* the various ways how `context` may be passed to `checkAttribute()`.
*
* So, in order to implement a rule "disallow `bold` in a text which is in a `heading1` you can add such a listener:
*
* schema.on( 'checkAttribute', ( evt, args ) => {
* const context = args[ 0 ];
* const atributeName = args[ 1 ];
*
* if ( context.endsWith( 'heading1 $text' ) && attributeName == 'bold' ) {
* // Prevent next listeners from being called.
* evt.stop();
* // Set the checkAttribute()'s return value.
* evt.return = false;
* }
* }, { priority: 'high' } );
*
* Allowing attributes in specific contexts will be a far less common use case, because it's normally handled by
* `allowAttributes` rule from {@link module:engine/model/schema~SchemaItemDefinition} but if you have a complex scenario
* where `bold` should be allowed only in element `foo` which must be in element `bar`, then this would be the way:
*
* schema.on( 'checkAttribute', ( evt, args ) => {
* const context = args[ 0 ];
* const atributeName = args[ 1 ];
*
* if ( context.endsWith( 'bar foo $text' ) && attributeName == 'bold' ) {
* // Prevent next listeners from being called.
* evt.stop();
* // Set the checkAttribute()'s return value.
* evt.return = true;
* }
* }, { priority: 'high' } );
*
* @event checkAttribute
* @param {Array} args The `checkAttribute()`'s arguments.
*/
/**
* A definition of a {@link module:engine/model/schema~Schema schema} item.
*
* You can define the following rules:
*
* * `allowIn` – a string or an array of strings. Defines in which other items this item will be allowed.
* * `allowAttributes` – a string or an array of strings. Defines allowed attributes of the given item.
* * `allowContentOf` – a string or an array of strings. Inherit "allowed children" from other items.
* * `allowWhere` – a string or an array of strings. Inherit "allowed in" from other items.
* * `allowAttributesOf` – a string or an array of strings. Inherit attributes from other items.
* * `inheritTypesFrom` – a string or an array of strings. Inherit `is*` properties of other items.
* * `inheritAllFrom` – a string. A shorthand for `allowContentOf`, `allowWhere`, `allowAttributesOf`, `inheritTypesFrom`.
* * additionall, you can define the following `is*` properties: `isBlock`, `isLimit`, `isObject`. Read about them below.
*
* # The is* properties
*
* There are 3 commonly used `is*` properties. Their role is to assign additional semantics to schema items.
* You can define more properties but you will also need to implement support for them in the existing editor features.
*
* * `isBlock` – whether this item is paragraph-like. Generally speaking, a content is usually made out of blocks
* like paragraphs, list items, images, headings, etc. All these elements are marked as blocks. A block
* should not allow another block inside. Note: there's also the `$block` generic item which has `isBlock` set to `true`.
* Most block type items will inherit from `$block` (through `inheritAllFrom`).
* * `isLimit` – can be understood as whether this element should not be split by <kbd>Enter</kbd>.
* Examples of limit elements – `$root`, table cell, image caption, etc. In other words, all actions which happen inside
* a limit element are limitted to its content.
* * `isObject` – whether item is "self-contained" and should be treated as a whole. Examples of object elements –
* `image`, `table`, `video`, etc.
*
* # Generic items
*
* There are three basic generic items: `$root`, `$block` and `$text`.
* They are defined as follows:
*
* this.schema.register( '$root', {
* isLimit: true
* } );
* this.schema.register( '$block', {
* allowIn: '$root',
* isBlock: true
* } );
* this.schema.register( '$text', {
* allowIn: '$block'
* } );
*
* They reflect a typical editor content which is contained within one root, consists of several blocks
* (paragraphs, lists items, headings, images) which, in turn, may contain text inside.
*
* By inheriting from the generic items you can define new items which will get extended by other editor features.
* Read more about generic types in the {@linkTODO Defining schema} guide.
*
* # Example definitions
*
* Allow `paragraph` in roots and block quotes:
*
* schema.register( 'paragraph', {
* allowIn: [ '$root', 'blockQuote' ],
* isBlock: true
* } );
*
* Allow `paragraph` everywhere where `$block` is allowed (i.e. in `$root`):
*
* schema.register( 'paragraph', {
* allowWhere: '$block',
* isBlock: true
* } );
*
* Make `image` a block object, which is allowed everywhere where `$block` is.
* Also, allow `src` and `alt` attributes on it:
*
* schema.register( 'image', {
* allowWhere: '$block',
* allowAttributes: [ 'src', 'alt' ],
* isBlock: true,
* isObject: true
* } );
*
* Make `caption` allowed in `image` and make it allow all the content of `$block`s (usually, `$text`).
* Also, mark it as a limit element so it can't be split:
*
* schema.register( 'caption', {
* allowIn: 'image',
* allowContentOf: '$block',
* isLimit: true
* } );
*
* Make `listItem` inherit all from `$block` but also allow additional attributes:
*
* schema.register( 'listItem', {
* inheritAllFrom: '$block',
* allowAttributes: [ 'type', 'indent' ]
* } );
*