Schema (engine/model)

@ckeditor/ckeditor5-engine/src/model/schema

class

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 editor.model.schema.

Filtering

Public
Inherited
Mixed
Protected
Private
Deprecated

Properties

_attributeProperties : Object.<String, String>

private

A dictionary containing attribute properties.

See source

Methods

constructor()

Creates schema instance.

See source
addAttributeCheck( callback )
Allows registering a callback to the checkAttribute method calls.

See source
Callbacks allow you to implement rules which are not otherwise possible to achieve by using the declarative API of 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 event-checkAttribute event. For even better control, you can use that event instead.

Example:
```
// Disallow bold on $text inside heading1.
schema.addAttributeCheck( ( 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' } );
```
Parameters

callback : function

The callback to be called. It is called with two parameters: 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.
addChildCheck( callback )
Allows registering a callback to the checkChild method calls.

See source
Callbacks allow you to implement rules which are not otherwise possible to achieve by using the declarative API of SchemaItemDefinition. For example, by using this method you can disallow elements in specific contexts.

This method is a shorthand for using the 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' } );
```
Parameters

callback : function

The callback to be called. It is called with two parameters: SchemaContext (context) instance and 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.
bind( bindProperties ) → Object

mixed
Binds observable properties to other objects implementing the Observable interface.

See source
Read more in the dedicated guide covering the topic of property bindings with some additional examples.

Consider two objects: a button and an associated command (both Observable).

A simple property binding could be as follows:
```
button.bind( 'isEnabled' ).to( command, 'isEnabled' );
```
or even shorter:
```
button.bind( 'isEnabled' ).to( command );
```
which works in the following way:
- button.isEnabled instantly equals command.isEnabled,
- whenever command.isEnabled changes, button.isEnabled will immediately reflect its value.
Note: To release the binding, use unbind.

You can also "rename" the property in the binding by specifying the new name in the to() chain:
```
button.bind( 'isEnabled' ).to( command, 'isWorking' );
```
It is possible to bind more than one property at a time to shorten the code:
```
button.bind( 'isEnabled', 'value' ).to( command );
```
which corresponds to:
```
button.bind( 'isEnabled' ).to( command );
button.bind( 'value' ).to( command );
```
The binding can include more than one observable, combining multiple data sources in a custom callback:
```
button.bind( 'isEnabled' ).to( command, 'isEnabled', ui, 'isVisible',
    ( isCommandEnabled, isUIVisible ) => isCommandEnabled && isUIVisible );
```
It is also possible to bind to the same property in an array of observables. To bind a button to multiple commands (also Observables) so that each and every one of them must be enabled for the button to become enabled, use the following code:
```
button.bind( 'isEnabled' ).toMany( [ commandA, commandB, commandC ], 'isEnabled',
    ( isAEnabled, isBEnabled, isCEnabled ) => isAEnabled && isBEnabled && isCEnabled );
```
Parameters

bindProperties : String

Observable properties that will be bound to other observable(s).

Returns

Object

The bind chain with the to() and toMany() methods.
checkAttribute( context, attributeName )
Checks whether the given attribute can be applied in the given context (on the last item of the context).

See source
```
schema.checkAttribute( textNode, 'bold' ); // -> false

schema.extend( '$text', {
    allowAttributes: 'bold'
} );
schema.checkAttribute( textNode, 'bold' ); // -> true
```
Parameters

context : SchemaContextDefinition

The context in which the attribute will be checked.

attributeName : String

Fires
- checkAttribute
checkAttributeInSelection( selection, attribute ) → Boolean
Checks whether the attribute is allowed in selection:

See source
- 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.
Parameters

selection : Selection | DocumentSelection

Selection which will be checked.

attribute : String

The name of the attribute to check.

Returns

Boolean
checkChild( context, def )
Checks whether the given node (child) can be a child of the given context.

See source
```
schema.checkChild( model.document.getRoot(), paragraph ); // -> false

schema.register( 'paragraph', {
    allowIn: '$root'
} );
schema.checkChild( model.document.getRoot(), paragraph ); // -> true
```
Note: When verifying whether the given node can be a child of the given context, the schema also verifies the entire context — from its root to its last element. Therefore, it is possible for checkChild() to return false even though the context's last element can contain the checked child. It happens if one of the context's elements does not allow its child.

Parameters

context : SchemaContextDefinition

The context in which the child will be checked.

def : Node | String

The child to check.

Fires
- checkChild
checkMerge( positionOrBaseElement, elementToMerge ) → Boolean

Checks whether the given element (elementToMerge) can be merged with the specified base element (positionOrBaseElement).

See source

In other words — whether elementToMerge's children are allowed in the positionOrBaseElement.

This check ensures that elements merged with Writer#merge() will be valid.

Instead of elements, you can pass the instance of the Position class as the positionOrBaseElement. It means that the elements before and after the position will be checked whether they can be merged.

Parameters

positionOrBaseElement : Position | Element

The position or base element to which the elementToMerge will be merged.

elementToMerge : Element

The element to merge. Required if positionOrBaseElement is an element.

Returns

Boolean
createContext( context ) → SchemaContext

Creates an instance of the schema context.

See source

Parameters

context : SchemaContextDefinition

Returns

SchemaContext
decorate( methodName )

mixed
Turns the given methods of this object into event-based ones. This means that the new method will fire an event (named after the method) and the original action will be plugged as a listener to that event.

See source
Read more in the dedicated guide covering the topic of decorating methods with some additional examples.

Decorating the method does not change its behavior (it only adds an event), but it allows to modify it later on by listening to the method's event.

For example, to cancel the method execution the event can be stopped:
```
class Foo {
    constructor() {
        this.decorate( 'method' );
    }

    method() {
        console.log( 'called!' );
    }
}

const foo = new Foo();
foo.on( 'method', ( evt ) => {
    evt.stop();
}, { priority: 'high' } );

foo.method(); // Nothing is logged.
```
Note: The high priority listener has been used to execute this particular callback before the one which calls the original method (which uses the "normal" priority).

It is also possible to change the returned value:
```
foo.on( 'method', ( evt ) => {
    evt.return = 'Foo!';
} );

foo.method(); // -> 'Foo'
```
Finally, it is possible to access and modify the arguments the method is called with:
```
method( a, b ) {
    console.log( `${ a }, ${ b }`  );
}

// ...

foo.on( 'method', ( evt, args ) => {
    args[ 0 ] = 3;

    console.log( args[ 1 ] ); // -> 2
}, { priority: 'high' } );

foo.method( 1, 2 ); // -> '3, 2'
```
Parameters

methodName : String

Name of the method to decorate.
delegate( events ) → EmitterMixinDelegateChain

mixed
Delegates selected events to another Emitter. For instance:

See source
```
emitterA.delegate( 'eventX' ).to( emitterB );
emitterA.delegate( 'eventX', 'eventY' ).to( emitterC );
```
then eventX is delegated (fired by) emitterB and emitterC along with data:
```
emitterA.fire( 'eventX', data );
```
and eventY is delegated (fired by) emitterC along with data:
```
emitterA.fire( 'eventY', data );
```
Parameters

events : String

Event names that will be delegated to another emitter.

Returns

EmitterMixinDelegateChain
extend( itemName, definition )
Extends a registered item's definition.

See source
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
//    }
```
Parameters

itemName : String

definition : SchemaItemDefinition
findAllowedParent() → Element | null

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 limit element, object element or top-most ancestor won't be reached.

See source

Returns

Element | null

element Allowed parent or null if nothing was found.
fire( eventOrInfo, [ args ] ) → *

mixed

Fires an event, executing all callbacks registered for it.

See source

The first parameter passed to callbacks is an EventInfo object, followed by the optional args provided in the fire() method call.

Parameters

eventOrInfo : String | EventInfo

The name of the event or EventInfo object if event is delegated.

[ args ] : *

Additional arguments to be passed to the callbacks.

Returns

*

By default the method returns undefined. However, the return value can be changed by listeners through modification of the evt.return's property (the event info is the first param of every callback).
getAttributeProperties( attributeName ) → AttributeProperties

Returns properties associated with a given model attribute. See setAttributeProperties().

See source

Parameters

attributeName : String

A name of the attribute.

Returns

AttributeProperties
getDefinition( item ) → SchemaCompiledItemDefinition

Returns a definition of the given item or undefined if item is not registered.

See source

Parameters

item : Item | SchemaContextItem | String

Returns

SchemaCompiledItemDefinition
getDefinitions() → Object.<String, SchemaCompiledItemDefinition>

Returns all registered items.

See source

Returns

Object.<String, SchemaCompiledItemDefinition>
getLimitElement( selectionOrRangeOrPosition ) → Element

Returns the lowest limit element containing the entire selection/range/position or the root otherwise.

See source

Parameters

selectionOrRangeOrPosition : Selection | DocumentSelection | Range | Position

The selection/range/position to check.

Returns

Element

The lowest limit element containing the entire selectionOrRangeOrPosition.
getNearestSelectionRange( position, [ direction ] ) → Range | null
Basing on given position, finds and returns a range which is nearest to that position and is a correct range for selection.

See source
The correct selection range might be collapsed when it is located in a position where the text node can be placed. Non-collapsed range is returned when selection can be placed around element marked as an "object" in the schema.

Direction of searching for the nearest correct selection range can be specified as:
- both - searching will be performed in both ways,
- forward - searching will be performed only forward,
- backward - searching will be performed only backward.
When valid selection range cannot be found, null is returned.

Parameters

position : Position

Reference position where new selection range should be looked for.

[ direction ] : 'both' | 'forward' | 'backward'

Search direction.

Defaults to 'both'

Returns

Range | null

Nearest selection range or null if one cannot be found.
getValidRanges( ranges, attribute ) → Iterable.<Range>

Transforms the given set of ranges into a set of ranges where the given attribute is allowed (and can be applied).

See source

Parameters

ranges : Array.<Range>

Ranges to be validated.

attribute : String

The name of the attribute to check.

Returns

Iterable.<Range>

Ranges in which the attribute is allowed.
isBlock( item )
Returns true if the given item is defined to be a block by SchemaItemDefinition's isBlock property.

See source
```
schema.isBlock( 'paragraph' ); // -> true
schema.isBlock( '$root' ); // -> false

const paragraphElement = writer.createElement( 'paragraph' );
schema.isBlock( paragraphElement ); // -> true
```
Parameters

item : Item | SchemaContextItem | String
isInline( item )
Returns true if the given item is defined to be an inline element by SchemaItemDefinition's isInline property.

See source
```
schema.isInline( 'paragraph' ); // -> false
schema.isInline( 'softBreak' ); // -> true

const text = writer.createText('foo' );
schema.isInline( text ); // -> true
```
Parameters

item : Item | SchemaContextItem | String
isLimit( item )
Returns true if the given item is defined to be a limit element by SchemaItemDefinition's isLimit or isObject property (all objects are also limits).

See source
```
schema.isLimit( 'paragraph' ); // -> false
schema.isLimit( '$root' ); // -> true
schema.isLimit( editor.model.document.getRoot() ); // -> true
schema.isLimit( 'image' ); // -> true
```
Parameters

item : Item | SchemaContextItem | String
isObject( item )
Returns true if the given item is defined to be an object element by SchemaItemDefinition's isObject property.

See source
```
schema.isObject( 'paragraph' ); // -> false
schema.isObject( 'image' ); // -> true

const imageElement = writer.createElement( 'image' );
schema.isObject( imageElement ); // -> true
```
Parameters

item : Item | SchemaContextItem | String
isRegistered( item )
Returns true if the given item is registered in the schema.

See source
```
schema.isRegistered( 'paragraph' ); // -> true
schema.isRegistered( editor.model.document.getRoot() ); // -> true
schema.isRegistered( 'foo' ); // -> false
```
Parameters

item : Item | SchemaContextItem | String
listenTo( emitter, event, callback, [ options ] = { [options.priority] } )

mixed
Registers a callback function to be executed when an event is fired in a specific (emitter) object.

See source
Events can be grouped in namespaces using :. When namespaced event is fired, it additionally fires all callbacks for that namespace.
```
// myEmitter.on( ... ) is a shorthand for myEmitter.listenTo( myEmitter, ... ).
myEmitter.on( 'myGroup', genericCallback );
myEmitter.on( 'myGroup:myEvent', specificCallback );

// genericCallback is fired.
myEmitter.fire( 'myGroup' );
// both genericCallback and specificCallback are fired.
myEmitter.fire( 'myGroup:myEvent' );
// genericCallback is fired even though there are no callbacks for "foo".
myEmitter.fire( 'myGroup:foo' );
```
An event callback can stop the event and set the return value of the fire method.

Parameters

emitter : Emitter

The object that fires the event.

event : String

The name of the event.

callback : function

The function to be called on event.

[ options ] : Object

Additional options.

Properties

[ options.priority ] : PriorityString | Number

The priority of this event callback. The higher the priority value the sooner the callback will be fired. Events having the same priority are called in the order they were added.

Defaults to 'normal'

Defaults to {}
off( event, callback )

mixed

Stops executing the callback on the given event. Shorthand for this.stopListening( this, event, callback ).

See source

Parameters

event : String

The name of the event.

callback : function

The function to stop being called.
on( event, callback, [ options ] = { [options.priority] } )

mixed

Registers a callback function to be executed when an event is fired.

See source

Shorthand for this.listenTo( this, event, callback, options ) (it makes the emitter listen on itself).

Parameters

event : String

The name of the event.

callback : function

The function to be called on event.

[ options ] : Object

Additional options.

Properties

[ options.priority ] : PriorityString | Number

The priority of this event callback. The higher the priority value the sooner the callback will be fired. Events having the same priority are called in the order they were added.

Defaults to 'normal'

Defaults to {}
once( event, callback, [ options ] = { [options.priority] } )

mixed

Registers a callback function to be executed on the next time the event is fired only. This is similar to calling on followed by off in the callback.

See source

Parameters

event : String

The name of the event.

callback : function

The function to be called on event.

[ options ] : Object

Additional options.

Properties

[ options.priority ] : PriorityString | Number

The priority of this event callback. The higher the priority value the sooner the callback will be fired. Events having the same priority are called in the order they were added.

Defaults to 'normal'

Defaults to {}
register( itemName, definition )
Registers schema item. Can only be called once for every item name.

See source
```
schema.register( 'paragraph', {
    inheritAllFrom: '$block'
} );
```
Parameters

itemName : String

definition : SchemaItemDefinition
removeDisallowedAttributes( nodes, writer )

Removes attributes disallowed by the schema.

See source

Parameters

nodes : Iterable.<Node>

Nodes that will be filtered.

writer : Writer
set( name, [ value ] )

mixed

Creates and sets the value of an observable property of this object. Such an property becomes a part of the state and is be observable.

See source

It accepts also a single object literal containing key/value pairs with properties to be set.

This method throws the observable-set-cannot-override error if the observable instance already have a property with the given property name. This prevents from mistakenly overriding existing properties and methods, but means that foo.set( 'bar', 1 ) may be slightly slower than foo.bar = 1.

Parameters

name : String | Object

The property's name or object with name=>value pairs.

[ value ] : *

The property's value (if name was passed in the first parameter).
setAttributeProperties( attributeName, properties )
This method allows assigning additional metadata to the model attributes. For example, AttributeProperties#isFormatting property is used to mark formatting attributes (like bold or italic).

See source
```
// Mark bold as a formatting attribute.
schema.setAttributeProperties( 'bold', {
    isFormatting: true
} );

// Override code not to be considered a formatting markup.
schema.setAttributeProperties( 'code', {
    isFormatting: false
} );
```
Properties are not limited to members defined in the AttributeProperties type and you can also use custom properties:
```
schema.setAttributeProperties( 'blockQuote', {
    customProperty: 'value'
} );
```
Subsequent calls with the same attribute will extend its custom properties:
```
schema.setAttributeProperties( 'blockQuote', {
    one: 1
} );

schema.setAttributeProperties( 'blockQuote', {
    two: 2
} );

console.log( schema.getAttributeProperties( 'blockQuote' ) );
// Logs: { one: 1, two: 2 }
```
Parameters

attributeName : String

A name of the attribute to receive the properties.

properties : AttributeProperties

A dictionary of properties.
stopDelegating( [ event ], [ emitter ] )

mixed
Stops delegating events. It can be used at different levels:

See source
- To stop delegating all events.
- To stop delegating a specific event to all emitters.
- To stop delegating a specific event to a specific emitter.
Parameters

[ event ] : String

The name of the event to stop delegating. If omitted, stops it all delegations.

[ emitter ] : Emitter

(requires event) The object to stop delegating a particular event to. If omitted, stops delegation of event to all emitters.
stopListening( [ emitter ], [ event ], [ callback ] )

mixed
Stops listening for events. It can be used at different levels:

See source
- To stop listening to a specific callback.
- To stop listening to a specific event.
- To stop listening to all events fired by a specific object.
- To stop listening to all events fired by all objects.
Parameters

[ emitter ] : Emitter

The object to stop listening to. If omitted, stops it for all objects.

[ event ] : String

(Requires the emitter) The name of the event to stop listening to. If omitted, stops it for all events from emitter.

[ callback ] : function

(Requires the event) The function to be removed from the call list for the given event.
unbind( [ unbindProperties ] )

mixed
Removes the binding created with bind.

See source
```
// Removes the binding for the 'a' property.
A.unbind( 'a' );

// Removes bindings for all properties.
A.unbind();
```
Parameters

[ unbindProperties ] : String

Observable properties to be unbound. All the bindings will be released if no properties are provided.
_checkContextMatch( def, context, contextItemIndex )

private

Parameters

def : SchemaCompiledItemDefinition

context : SchemaContext

contextItemIndex : Number
_clearCache()

private
_compile()

private
_getValidRangesForRange( range, attribute ) → Iterable.<Range>

private

Takes a flat range and an attribute name. Traverses the range recursively and deeply to find and return all ranges inside the given range on which the attribute can be applied.

See source

This is a helper function for getValidRanges.

Parameters

range : Range

Range to process.

attribute : String

The name of the attribute to check.

Returns

Iterable.<Range>

Ranges in which the attribute is allowed.

Events

change:{property}( eventInfo, name, value, oldValue )

mixed
Fired when a property changed value.

See source
```
observable.set( 'prop', 1 );

observable.on( 'change:prop', ( evt, propertyName, newValue, oldValue ) => {
    console.log( `${ propertyName } has changed from ${ oldValue } to ${ newValue }` );
} );

observable.prop = 2; // -> 'prop has changed from 1 to 2'
```
Parameters

eventInfo : EventInfo

An object containing information about the fired event.

name : String

The property name.

value : *

The new property value.

oldValue : *

The previous property value.
checkAttribute( eventInfo, args )
Event fired when the checkAttribute method is called. It allows plugging in additional behavior – e.g. implementing rules which cannot be defined using the declarative SchemaItemDefinition interface.

See source
Note: The 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 checkAttribute method fires an event because it's 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 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 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' } );
```
Parameters

eventInfo : EventInfo

An object containing information about the fired event.

args : Array

The checkAttribute()'s arguments.
checkChild( eventInfo, args )
Event fired when the checkChild method is called. It allows plugging in additional behavior – e.g. implementing rules which cannot be defined using the declarative SchemaItemDefinition interface.

See source
Note: The 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 checkChild method fires an event because it is 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 SchemaContext instance and child to a 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 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' } );
```
Parameters

eventInfo : EventInfo

An object containing information about the fired event.

args : Array

The checkChild()'s arguments.
set:{property}( eventInfo, name, value, oldValue )

mixed
Fired when a property value is going to be set but is not set yet (before the change event is fired).

See source
You can control the final value of the property by using the event's return property.
```
observable.set( 'prop', 1 );

observable.on( 'set:prop', ( evt, propertyName, newValue, oldValue ) => {
    console.log( `Value is going to be changed from ${ oldValue } to ${ newValue }` );
    console.log( `Current property value is ${ observable[ propertyName ] }` );

    // Let's override the value.
    evt.return = 3;
} );

observable.on( 'change:prop', ( evt, propertyName, newValue, oldValue ) => {
    console.log( `Value has changed from ${ oldValue } to ${ newValue }` );
} );

observable.prop = 2; // -> 'Value is going to be changed from 1 to 2'
                     // -> 'Current property value is 1'
                     // -> 'Value has changed from 1 to 3'
```
Note: Event is fired even when the new value is the same as the old value.

Parameters

eventInfo : EventInfo

An object containing information about the fired event.

name : String

The property name.

value : *

The new property value.

oldValue : *

The previous property value.

Schema (engine/model)

Filtering

Properties

_attributeProperties : Object.<String, String>module:engine/model/schema~Schema#_attributeProperties

Methods

constructor()module:engine/model/schema~Schema#constructor

addAttributeCheck( callback )module:engine/model/schema~Schema#addAttributeCheck

Parameters

addChildCheck( callback )module:engine/model/schema~Schema#addChildCheck

Parameters

bind( bindProperties ) → Objectmodule:engine/model/schema~Schema#bind

Parameters

Returns

checkAttribute( context, attributeName )module:engine/model/schema~Schema#checkAttribute

Parameters

Fires

checkAttributeInSelection( selection, attribute ) → Booleanmodule:engine/model/schema~Schema#checkAttributeInSelection

Parameters

Returns

checkChild( context, def )module:engine/model/schema~Schema#checkChild

Parameters

Fires

checkMerge( positionOrBaseElement, elementToMerge ) → Booleanmodule:engine/model/schema~Schema#checkMerge

Parameters

Returns

createContext( context ) → SchemaContextmodule:engine/model/schema~Schema#createContext

Parameters

Returns

decorate( methodName )module:engine/model/schema~Schema#decorate

Parameters

delegate( events ) → EmitterMixinDelegateChainmodule:engine/model/schema~Schema#delegate

Parameters

Returns

extend( itemName, definition )module:engine/model/schema~Schema#extend

Parameters

findAllowedParent() → Element | nullmodule:engine/model/schema~Schema#findAllowedParent

Returns

fire( eventOrInfo, [ args ] ) → *module:engine/model/schema~Schema#fire

Parameters

Returns

getAttributeProperties( attributeName ) → AttributePropertiesmodule:engine/model/schema~Schema#getAttributeProperties

Parameters

Returns

getDefinition( item ) → SchemaCompiledItemDefinitionmodule:engine/model/schema~Schema#getDefinition

Parameters

Returns

getDefinitions() → Object.<String, SchemaCompiledItemDefinition>module:engine/model/schema~Schema#getDefinitions

Returns

getLimitElement( selectionOrRangeOrPosition ) → Elementmodule:engine/model/schema~Schema#getLimitElement

Parameters

Returns

getNearestSelectionRange( position, [ direction ] ) → Range | nullmodule:engine/model/schema~Schema#getNearestSelectionRange

Parameters

Returns

getValidRanges( ranges, attribute ) → Iterable.<Range>module:engine/model/schema~Schema#getValidRanges

Parameters

Returns

isBlock( item )module:engine/model/schema~Schema#isBlock

Parameters

isInline( item )module:engine/model/schema~Schema#isInline

Parameters

isLimit( item )module:engine/model/schema~Schema#isLimit

Parameters

isObject( item )module:engine/model/schema~Schema#isObject

Parameters

isRegistered( item )module:engine/model/schema~Schema#isRegistered

Parameters

listenTo( emitter, event, callback, [ options ] = { [options.priority] } )module:engine/model/schema~Schema#listenTo

Parameters

off( event, callback )module:engine/model/schema~Schema#off

Parameters

on( event, callback, [ options ] = { [options.priority] } )module:engine/model/schema~Schema#on

Parameters

once( event, callback, [ options ] = { [options.priority] } )module:engine/model/schema~Schema#once

Parameters

register( itemName, definition )module:engine/model/schema~Schema#register

Parameters

removeDisallowedAttributes( nodes, writer )module:engine/model/schema~Schema#removeDisallowedAttributes

Parameters

set( name, [ value ] )module:engine/model/schema~Schema#set

`_attributeProperties : Object.<String, String>`

`constructor()`

`addAttributeCheck( callback )`

`addChildCheck( callback )`

`bind( bindProperties ) → Object`

`checkAttribute( context, attributeName )`

`checkAttributeInSelection( selection, attribute ) → Boolean`

`checkChild( context, def )`

`checkMerge( positionOrBaseElement, elementToMerge ) → Boolean`

`createContext( context ) → SchemaContext`

`decorate( methodName )`

`delegate( events ) → EmitterMixinDelegateChain`

`extend( itemName, definition )`

`findAllowedParent() → Element | null`

`fire( eventOrInfo, [ args ] ) → *`

`getAttributeProperties( attributeName ) → AttributeProperties`

`getDefinition( item ) → SchemaCompiledItemDefinition`

`getDefinitions() → Object.<String, SchemaCompiledItemDefinition>`

`getLimitElement( selectionOrRangeOrPosition ) → Element`

`getNearestSelectionRange( position, [ direction ] ) → Range | null`

`getValidRanges( ranges, attribute ) → Iterable.<Range>`

`isBlock( item )`

`isInline( item )`

`isLimit( item )`

`isObject( item )`

`isRegistered( item )`

`listenTo( emitter, event, callback, [ options ] = { [options.priority] } )`

`off( event, callback )`

`on( event, callback, [ options ] = { [options.priority] } )`

`once( event, callback, [ options ] = { [options.priority] } )`

`register( itemName, definition )`

`removeDisallowedAttributes( nodes, writer )`

`set( name, [ value ] )`

`setAttributeProperties( attributeName, properties )`

`stopDelegating( [ event ], [ emitter ] )`

`stopListening( [ emitter ], [ event ], [ callback ] )`

`unbind( [ unbindProperties ] )`

`_checkContextMatch( def, context, contextItemIndex )`

`_clearCache()`

`_compile()`

`_getValidRangesForRange( range, attribute ) → Iterable.<Range>`

`change:{property}( eventInfo, name, value, oldValue )`

`checkAttribute( eventInfo, args )`

`checkChild( eventInfo, args )`

`set:{property}( eventInfo, name, value, oldValue )`