# FocusTracker

class

Allows observing a group of DOM `Element`s or [view instances](module_ui_view-View.md) whether at least one of them (or their child) is focused.

Used by the [`Editor`](module_core_editor_editor-Editor.md) in order to track whether the focus is still within the application, or were used outside of its UI.

**Note** `focus` and `blur` listeners use event capturing, so it is only needed to register wrapper `Element` which contain other `focusable` elements. But note that this wrapper element has to be focusable too (have e.g. `tabindex="-1"`).

Check out the ["Deep dive into focus tracking"](../framework/deep-dive/ui/focus-tracking.md) guide to learn more.

[See source](https://github.com/ckeditor/ckeditor5/blob/master/packages/ckeditor5-utils/src/focustracker.ts#L33)

<a id="properties">

## Properties

<a id="member-elements">

### `elements: Array<Element>` _(readonly)_

List of registered DOM elements.

**Note**: The list does do not include elements from [`externalViews`](#member-externalViews).

[See source](https://github.com/ckeditor/ckeditor5/blob/master/packages/ckeditor5-utils/src/focustracker.ts#L91)

<a id="member-externalViews">

### `externalViews: Array<ViewWithFocusTracker>` _(readonly)_

List of external focusable views that contribute to the state of this focus tracker. See [`add`](#function-add) to learn more.

[See source](https://github.com/ckeditor/ckeditor5/blob/master/packages/ckeditor5-utils/src/focustracker.ts#L98)

<a id="member-focusedElement">

### `focusedElement: Element | null` _(readonly)_

The currently focused element.

While [`isFocused`](#member-isFocused) remains `true`, the focus can move between different UI elements. This property tracks those elements and tells which one is currently focused.

**Note**: The values of this property are restricted to [`elements`](#member-elements) or [elements](module_ui_view-View.md#member-element) registered in [`externalViews`](#member-externalViews).

[See source](https://github.com/ckeditor/ckeditor5/blob/master/packages/ckeditor5-utils/src/focustracker.ts#L54)

<a id="member-isFocused">

### `isFocused: boolean` _(readonly)_

True when one of the registered [`elements`](#member-elements) or [`externalViews`](#member-externalViews) is focused.

[See source](https://github.com/ckeditor/ckeditor5/blob/master/packages/ckeditor5-utils/src/focustracker.ts#L40)

<a id="member-_elements">

### `_elements: Set<Element>` _(internal)_

List of registered DOM elements.

[See source](https://github.com/ckeditor/ckeditor5/blob/master/packages/ckeditor5-utils/src/focustracker.ts#L61)

<a id="member-_externalViews">

### `_externalViews: Set<ViewWithFocusTracker>` _(internal)_

List of views with external focus trackers that contribute to the state of this focus tracker.

[See source](https://github.com/ckeditor/ckeditor5/blob/master/packages/ckeditor5-utils/src/focustracker.ts#L68)

<a id="member-_blurTimeout">

### `_blurTimeout: Timeout | null` _(private)_

Asynchronous blur event timeout.

[See source](https://github.com/ckeditor/ckeditor5/blob/master/packages/ckeditor5-utils/src/focustracker.ts#L73)

<a id="methods">

## Methods

<a id="function-constructor">

### `constructor()`

[See source](https://github.com/ckeditor/ckeditor5/blob/master/packages/ckeditor5-utils/src/focustracker.ts#L77)

<a id="function-add">

### `add( elementOrView ) → void`

Starts tracking a specified DOM element or a [`View`](module_ui_view-View.md) instance.

* If a DOM element is passed, the focus tracker listens to the `focus` and `blur` events on this element. Tracked elements are listed in [`elements`](#member-elements).
* If a [`View`](module_ui_view-View.md) instance is passed that has a `FocusTracker` instance ([`ViewWithFocusTracker`](module_utils_focustracker-ViewWithFocusTracker.md)), the external focus tracker's state ([`isFocused`](#member-isFocused), [`focusedElement`](#member-focusedElement)) starts contributing to the current tracker instance. This allows for increasing the "reach" of a focus tracker instance, by connecting two or more focus trackers together when DOM elements they track are located in different subtrees in DOM. External focus trackers are listed in [`externalViews`](#member-externalViews).
* If a [`View`](module_ui_view-View.md) instance is passed that has no `FocusTracker` (**not** a [`ViewWithFocusTracker`](module_utils_focustracker-ViewWithFocusTracker.md)), its [`element`](module_ui_view-View.md#member-element) is used to track focus like any other DOM element.

[See source](https://github.com/ckeditor/ckeditor5/blob/master/packages/ckeditor5-utils/src/focustracker.ts#L114)

#### Parameters

* `elementOrView: Element | View<HTMLElement>`

#### Returns

* `void`

<a id="function-bind:DUAL_BIND">

### `bind( bindProperty1, bindProperty2 ) → ObservableDualBindChain<K1, FocusTracker[ K1 ], K2, FocusTracker[ K2 ]>` _(inherited)_

Binds [observable properties](#function-set:KEY_VALUE) to other objects implementing the [`Observable`](module_utils_observablemixin-Observable.md) interface.

Read more in the [dedicated](../framework/deep-dive/observables.md#property-bindings) 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:

```typescript
button.bind( 'isEnabled' ).to( command, 'isEnabled' );
```

or even shorter:

```typescript
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`](module_utils_observablemixin-Observable.md#function-unbind).

You can also "rename" the property in the binding by specifying the new name in the `to()` chain:

```typescript
button.bind( 'isEnabled' ).to( command, 'isWorking' );
```

It is possible to bind more than one property at a time to shorten the code:

```typescript
button.bind( 'isEnabled', 'value' ).to( command );
```

which corresponds to:

```typescript
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:

```typescript
button.bind( 'isEnabled' ).to( command, 'isEnabled', ui, 'isVisible',
	( isCommandEnabled, isUIVisible ) => isCommandEnabled && isUIVisible );
```

Using a custom callback allows processing the value before passing it to the target property:

```typescript
button.bind( 'isEnabled' ).to( command, 'value', value => value === 'heading1' );
```

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:

```typescript
button.bind( 'isEnabled' ).toMany( [ commandA, commandB, commandC ], 'isEnabled',
	( isAEnabled, isBEnabled, isCEnabled ) => isAEnabled && isBEnabled && isCEnabled );
```

[See source](https://github.com/ckeditor/ckeditor5/blob/master/packages/ckeditor5-utils/src/observablemixin.ts#L996)

#### Type parameters

* * `K1`
  * `K2`

#### Parameters

* `bindProperty1: K1`

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

* `bindProperty2: K2`

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

#### Returns

* `ObservableDualBindChain<K1, FocusTracker[ K1 ], K2, FocusTracker[ K2 ]>`

  The bind chain with the `to()` and `toMany()` methods.

<a id="function-bind:MANY_BIND">

### `bind( bindProperties ) → ObservableMultiBindChain` _(inherited)_

Binds [observable properties](#function-set:KEY_VALUE) to other objects implementing the [`Observable`](module_utils_observablemixin-Observable.md) interface.

Read more in the [dedicated](../framework/deep-dive/observables.md#property-bindings) 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:

```typescript
button.bind( 'isEnabled' ).to( command, 'isEnabled' );
```

or even shorter:

```typescript
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`](module_utils_observablemixin-Observable.md#function-unbind).

You can also "rename" the property in the binding by specifying the new name in the `to()` chain:

```typescript
button.bind( 'isEnabled' ).to( command, 'isWorking' );
```

It is possible to bind more than one property at a time to shorten the code:

```typescript
button.bind( 'isEnabled', 'value' ).to( command );
```

which corresponds to:

```typescript
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:

```typescript
button.bind( 'isEnabled' ).to( command, 'isEnabled', ui, 'isVisible',
	( isCommandEnabled, isUIVisible ) => isCommandEnabled && isUIVisible );
```

Using a custom callback allows processing the value before passing it to the target property:

```typescript
button.bind( 'isEnabled' ).to( command, 'value', value => value === 'heading1' );
```

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:

```typescript
button.bind( 'isEnabled' ).toMany( [ commandA, commandB, commandC ], 'isEnabled',
	( isAEnabled, isBEnabled, isCEnabled ) => isAEnabled && isBEnabled && isCEnabled );
```

[See source](https://github.com/ckeditor/ckeditor5/blob/master/packages/ckeditor5-utils/src/observablemixin.ts#L1074)

#### Parameters

* `bindProperties: Array<'off' | 'set' | 'bind' | 'unbind' | 'decorate' | 'stopListening' | 'on' | 'once' | 'listenTo' | 'fire' | 'delegate' | 'stopDelegating' | 'destroy' | 'remove' | 'add' | 'isFocused' | 'focusedElement' | '_elements' | '_externalViews' | 'elements' | 'externalViews'>`

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

#### Returns

* `ObservableMultiBindChain`

  The bind chain with the `to()` and `toMany()` methods.

<a id="function-bind:SINGLE_BIND">

### `bind( bindProperty ) → ObservableSingleBindChain<K, FocusTracker[ K ]>` _(inherited)_

Binds [observable properties](#function-set:KEY_VALUE) to other objects implementing the [`Observable`](module_utils_observablemixin-Observable.md) interface.

Read more in the [dedicated](../framework/deep-dive/observables.md#property-bindings) 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:

```typescript
button.bind( 'isEnabled' ).to( command, 'isEnabled' );
```

or even shorter:

```typescript
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`](module_utils_observablemixin-Observable.md#function-unbind).

You can also "rename" the property in the binding by specifying the new name in the `to()` chain:

```typescript
button.bind( 'isEnabled' ).to( command, 'isWorking' );
```

It is possible to bind more than one property at a time to shorten the code:

```typescript
button.bind( 'isEnabled', 'value' ).to( command );
```

which corresponds to:

```typescript
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:

```typescript
button.bind( 'isEnabled' ).to( command, 'isEnabled', ui, 'isVisible',
	( isCommandEnabled, isUIVisible ) => isCommandEnabled && isUIVisible );
```

Using a custom callback allows processing the value before passing it to the target property:

```typescript
button.bind( 'isEnabled' ).to( command, 'value', value => value === 'heading1' );
```

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:

```typescript
button.bind( 'isEnabled' ).toMany( [ commandA, commandB, commandC ], 'isEnabled',
	( isAEnabled, isBEnabled, isCEnabled ) => isAEnabled && isBEnabled && isCEnabled );
```

[See source](https://github.com/ckeditor/ckeditor5/blob/master/packages/ckeditor5-utils/src/observablemixin.ts#L918)

#### Type parameters

* `K`

#### Parameters

* `bindProperty: K`

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

#### Returns

* `ObservableSingleBindChain<K, FocusTracker[ K ]>`

  The bind chain with the `to()` and `toMany()` methods.

<a id="function-decorate">

### `decorate( methodName ) → void` _(inherited)_

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.

Read more in the [dedicated](../framework/deep-dive/observables.md#decorating-object-methods) 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](module_utils_eventinfo-EventInfo.md#member-stop):

```typescript
class Foo extends ObservableMixin() {
	constructor() {
		super();
		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](module_utils_priorities-PriorityString.md) 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:

```typescript
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:

```typescript
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'
```

[See source](https://github.com/ckeditor/ckeditor5/blob/master/packages/ckeditor5-utils/src/observablemixin.ts#L1159)

#### Parameters

* `methodName: 'off' | 'set' | 'bind' | 'unbind' | 'decorate' | 'stopListening' | 'on' | 'once' | 'listenTo' | 'fire' | 'delegate' | 'stopDelegating' | 'destroy' | 'remove' | 'add' | 'isFocused' | 'focusedElement' | '_elements' | '_externalViews' | 'elements' | 'externalViews'`

  Name of the method to decorate.

#### Returns

* `void`

<a id="function-delegate">

### `delegate( events ) → EmitterMixinDelegateChain` _(inherited)_

Delegates selected events to another [`Emitter`](module_utils_emittermixin-Emitter.md). For instance:

```typescript
emitterA.delegate( 'eventX' ).to( emitterB );
emitterA.delegate( 'eventX', 'eventY' ).to( emitterC );
```

then `eventX` is delegated (fired by) `emitterB` and `emitterC` along with `data`:

```typescript
emitterA.fire( 'eventX', data );
```

and `eventY` is delegated (fired by) `emitterC` along with `data`:

```typescript
emitterA.fire( 'eventY', data );
```

[See source](https://github.com/ckeditor/ckeditor5/blob/master/packages/ckeditor5-utils/src/emittermixin.ts#L539)

#### Parameters

* `events: Array<string>`

  Event names that will be delegated to another emitter.

#### Returns

* `EmitterMixinDelegateChain`

<a id="function-destroy">

### `destroy() → void`

Destroys the focus tracker by:

* Disabling all event listeners attached to tracked elements or external views.
* Removing all tracked elements and views that were previously added.

[See source](https://github.com/ckeditor/ckeditor5/blob/master/packages/ckeditor5-utils/src/focustracker.ts#L248)

#### Returns

* `void`

<a id="function-fire">

### `fire( eventOrInfo, args ) → GetEventInfo<TEvent>[ 'return' ]` _(inherited)_

Fires an event, executing all callbacks registered for it.

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

[See source](https://github.com/ckeditor/ckeditor5/blob/master/packages/ckeditor5-utils/src/emittermixin.ts#L512)

#### Type parameters

* `TEvent: extends BaseEvent`

  The type describing the event. See [`BaseEvent`](module_utils_emittermixin-BaseEvent.md).

#### Parameters

* `eventOrInfo: GetNameOrEventInfo<TEvent>`

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

* `args: TEvent[ 'args' ]`

  Additional arguments to be passed to the callbacks.

#### Returns

* `GetEventInfo<TEvent>[ 'return' ]`

  By default the method returns `undefined`. However, the return value can be changed by listeners through modification of the [`evt.return`](module_utils_eventinfo-EventInfo.md#member-return)'s property (the event info is the first param of every callback).

<a id="function-listenTo:DOM_EMITTER">

### `listenTo( emitter, event, callback, options? ) → void` _(inherited)_

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

Events can be grouped in namespaces using `:`. When namespaced event is fired, it additionally fires all callbacks for that namespace.

```typescript
// 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](module_utils_eventinfo-EventInfo.md#member-stop) and set the [return value](module_utils_eventinfo-EventInfo.md#member-return) of the [`fire`](#function-fire) method.

[See source](https://github.com/ckeditor/ckeditor5/blob/master/packages/ckeditor5-utils/src/dom/emittermixin.ts#L457)

#### Type parameters

* `TEvent: extends BaseEvent`

  The type describing the event. See [`BaseEvent`](module_utils_emittermixin-BaseEvent.md).

#### Parameters

* `emitter: Emitter`

  The object that fires the event.

* `event: TEvent[ 'name' ]`

  The name of the event.

* `callback: GetCallback<TEvent>`

  The function to be called on event.

* `options?: CallbackOptions`

  Additional options.

#### Returns

* `void`

<a id="function-listenTo:BASE_EMITTER">

### `listenTo( emitter, event, callback, options? ) → void` _(inherited)_

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

Events can be grouped in namespaces using `:`. When namespaced event is fired, it additionally fires all callbacks for that namespace.

```typescript
// 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](module_utils_eventinfo-EventInfo.md#member-stop) and set the [return value](module_utils_eventinfo-EventInfo.md#member-return) of the [`fire`](#function-fire) method.

[See source](https://github.com/ckeditor/ckeditor5/blob/master/packages/ckeditor5-utils/src/emittermixin.ts#L475)

#### Type parameters

* `TEvent: extends BaseEvent`

  The type describing the event. See [`BaseEvent`](module_utils_emittermixin-BaseEvent.md).

#### Parameters

* `emitter: Emitter`

  The object that fires the event.

* `event: TEvent[ 'name' ]`

  The name of the event.

* `callback: GetCallback<TEvent>`

  The function to be called on event.

* `options?: GetCallbackOptions<TEvent>`

  Additional options.

#### Returns

* `void`

<a id="function-listenTo:HTML_EMITTER">

### `listenTo( emitter, event, callback, options? ) → void` _(inherited)_

Registers a callback function to be executed when an event is fired in a specific Emitter or DOM Node. It is backwards compatible with [`listenTo`](module_utils_emittermixin-Emitter.md#function-listenTo:BASE_EMITTER).

[See source](https://github.com/ckeditor/ckeditor5/blob/master/packages/ckeditor5-utils/src/dom/emittermixin.ts#L421)

#### Type parameters

* `K: extends keyof DomEventMap`

#### Parameters

* `emitter: Window | EventTarget | Node`

  The object that fires the event.

* `event: K`

  The name of the event.

* `callback: ( this: this, ev: EventInfo, event: DomEventMap[ K ] ) => void`

  The function to be called on event.

* `options?: CallbackOptions & object`

  Additional options.

#### Returns

* `void`

<a id="function-off">

### `off( event, callback ) → void` _(inherited)_

Stops executing the callback on the given event. Shorthand for [`this.stopListening( this, event, callback )`](#function-stopListening:DOM_STOP).

[See source](https://github.com/ckeditor/ckeditor5/blob/master/packages/ckeditor5-utils/src/emittermixin.ts#L444)

#### Parameters

* `event: string`

  The name of the event.

* `callback: Function`

  The function to stop being called.

#### Returns

* `void`

<a id="function-on">

### `on( event, callback, options? ) → void` _(inherited)_

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

Shorthand for [`this.listenTo( this, event, callback, options )`](#function-listenTo:HTML_EMITTER) (it makes the emitter listen on itself).

[See source](https://github.com/ckeditor/ckeditor5/blob/master/packages/ckeditor5-utils/src/emittermixin.ts#L416)

#### Type parameters

* `TEvent: extends BaseEvent`

  The type descibing the event. See [`BaseEvent`](module_utils_emittermixin-BaseEvent.md).

#### Parameters

* `event: TEvent[ 'name' ]`

  The name of the event.

* `callback: GetCallback<TEvent>`

  The function to be called on event.

* `options?: GetCallbackOptions<TEvent>`

  Additional options.

#### Returns

* `void`

<a id="function-once">

### `once( event, callback, options? ) → void` _(inherited)_

Registers a callback function to be executed on the next time the event is fired only. This is similar to calling [`on`](#function-on) followed by [`off`](#function-off) in the callback.

[See source](https://github.com/ckeditor/ckeditor5/blob/master/packages/ckeditor5-utils/src/emittermixin.ts#L431)

#### Type parameters

* `TEvent: extends BaseEvent`

  The type descibing the event. See [`BaseEvent`](module_utils_emittermixin-BaseEvent.md).

#### Parameters

* `event: TEvent[ 'name' ]`

  The name of the event.

* `callback: GetCallback<TEvent>`

  The function to be called on event.

* `options?: GetCallbackOptions<TEvent>`

  Additional options.

#### Returns

* `void`

<a id="function-remove">

### `remove( elementOrView ) → void`

Stops tracking focus in the specified DOM element or a [view instance](module_ui_view-View.md). See [`add`](#function-add) to learn more.

[See source](https://github.com/ckeditor/ckeditor5/blob/master/packages/ckeditor5-utils/src/focustracker.ts#L143)

#### Parameters

* `elementOrView: Element | View<HTMLElement>`

#### Returns

* `void`

<a id="function-set:OBJECT">

### `set( values ) → void` _(inherited)_

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

It accepts 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 has 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`.

In TypeScript, those properties should be declared in class using `declare` keyword. In example:

```typescript
public declare myProp1: number;
public declare myProp2: string;

constructor() {
	this.set( {
		'myProp1: 2,
		'myProp2: 'foo'
	} );
}
```

[See source](https://github.com/ckeditor/ckeditor5/blob/master/packages/ckeditor5-utils/src/observablemixin.ts#L843)

#### Parameters

* `values: object & object`

  An object with `name=>value` pairs.

#### Returns

* `void`

<a id="function-set:KEY_VALUE">

### `set( name, value ) → void` _(inherited)_

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

This method throws the `observable-set-cannot-override` error if the observable instance already has 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`.

In TypeScript, those properties should be declared in class using `declare` keyword. In example:

```typescript
public declare myProp: number;

constructor() {
	this.set( 'myProp', 2 );
}
```

[See source](https://github.com/ckeditor/ckeditor5/blob/master/packages/ckeditor5-utils/src/observablemixin.ts#L815)

#### Type parameters

* `K`

#### Parameters

* `name: K`

  The property's name.

* `value: FocusTracker[ K ]`

  The property's value.

#### Returns

* `void`

<a id="function-stopDelegating">

### `stopDelegating( event?, emitter? ) → void` _(inherited)_

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

* To stop delegating all events.
* To stop delegating a specific event to all emitters.
* To stop delegating a specific event to a specific emitter.

[See source](https://github.com/ckeditor/ckeditor5/blob/master/packages/ckeditor5-utils/src/emittermixin.ts#L552)

#### 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.

#### Returns

* `void`

<a id="function-stopListening:BASE_STOP">

### `stopListening( emitter?, event?, callback? ) → void` _(inherited)_

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

* 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.

[See source](https://github.com/ckeditor/ckeditor5/blob/master/packages/ckeditor5-utils/src/emittermixin.ts#L497)

#### 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`.

#### Returns

* `void`

<a id="function-stopListening:DOM_STOP">

### `stopListening( emitter?, event?, callback? ) → void` _(inherited)_

Stops listening for events. It can be used at different levels: It is backwards compatible with [`listenTo`](module_utils_emittermixin-Emitter.md#function-listenTo:BASE_EMITTER).

* 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.

[See source](https://github.com/ckeditor/ckeditor5/blob/master/packages/ckeditor5-utils/src/dom/emittermixin.ts#L481)

#### Parameters

* `emitter?: Window | EventTarget | Node | 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`.

#### Returns

* `void`

<a id="function-unbind">

### `unbind( unbindProperties ) → void` _(inherited)_

Removes the binding created with [`bind`](#function-bind:SINGLE_BIND).

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

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

[See source](https://github.com/ckeditor/ckeditor5/blob/master/packages/ckeditor5-utils/src/observablemixin.ts#L1090)

#### Parameters

* `unbindProperties: Array<'off' | 'set' | 'bind' | 'unbind' | 'decorate' | 'stopListening' | 'on' | 'once' | 'listenTo' | 'fire' | 'delegate' | 'stopDelegating' | 'destroy' | 'remove' | 'add' | 'isFocused' | 'focusedElement' | '_elements' | '_externalViews' | 'elements' | 'externalViews'>`

  Observable properties to be unbound. All the bindings will be released if no properties are provided.

#### Returns

* `void`

<a id="function-_addElement">

### `_addElement( element ) → void` _(private)_

Adds a DOM element to the focus tracker and starts listening to the `focus` and `blur` events on it.

[See source](https://github.com/ckeditor/ckeditor5/blob/master/packages/ckeditor5-utils/src/focustracker.ts#L159)

#### Parameters

* `element: Element`

#### Returns

* `void`

<a id="function-_addView">

### `_addView( view ) → void` _(private)_

Adds an external [view instance](module_ui_view-View.md) to this focus tracker and makes it contribute to this focus tracker's state either by its `View#element` or by its `View#focusTracker` instance.

[See source](https://github.com/ckeditor/ckeditor5/blob/master/packages/ckeditor5-utils/src/focustracker.ts#L208)

#### Parameters

* `view: ViewWithFocusTracker`

#### Returns

* `void`

<a id="function-_blur">

### `_blur() → void` _(private)_

Clears currently [`focusedElement`](#member-focusedElement) and sets [`isFocused`](#member-isFocused) `false`.

This method uses `setTimeout()` to change order of `blur` and `focus` events calls, ensuring that moving focus between two elements within a single focus tracker's scope, will not cause `[ blurA, focusB ]` sequence but just `[ focusB ]`. The former would cause a momentary change of `#isFocused` to `false` which is not desired because any logic listening to a focus tracker state would experience UI flashes and glitches as the user focus travels across the UI.

[See source](https://github.com/ckeditor/ckeditor5/blob/master/packages/ckeditor5-utils/src/focustracker.ts#L278)

#### Returns

* `void`

<a id="function-_clearBlurTimeout">

### `_clearBlurTimeout() → void` _(private)_

Clears the asynchronous blur event timeout on demand. See [`_blur`](#function-_blur) to learn more.

[See source](https://github.com/ckeditor/ckeditor5/blob/master/packages/ckeditor5-utils/src/focustracker.ts#L309)

#### Returns

* `void`

<a id="function-_focus">

### `_focus( element ) → void` _(private)_

Stores currently focused element as [`focusedElement`](#member-focusedElement) and sets [`isFocused`](#member-isFocused) `true`.

[See source](https://github.com/ckeditor/ckeditor5/blob/master/packages/ckeditor5-utils/src/focustracker.ts#L261)

#### Parameters

* `element: Element`

#### Returns

* `void`

<a id="function-_removeElement">

### `_removeElement( element ) → void` _(private)_

Removes a DOM element from the focus tracker.

[See source](https://github.com/ckeditor/ckeditor5/blob/master/packages/ckeditor5-utils/src/focustracker.ts#L193)

#### Parameters

* `element: Element`

#### Returns

* `void`

<a id="function-_removeView">

### `_removeView( view ) → void` _(private)_

Removes an external [view instance](module_ui_view-View.md) from this focus tracker.

[See source](https://github.com/ckeditor/ckeditor5/blob/master/packages/ckeditor5-utils/src/focustracker.ts#L234)

#### Parameters

* `view: ViewWithFocusTracker`

#### Returns

* `void`

<a id="events">

## Events

<a id="event-change:focusedElement">

### `change:focusedElement( eventInfo, name, value, oldValue )`

Fired when the `focusedElement` property changed value.

[See source](https://github.com/ckeditor/ckeditor5/blob/master/packages/ckeditor5-utils/src/focustracker.ts#L54)

#### Parameters

* `eventInfo: EventInfo`

  An object containing information about the fired event.

* `name: string`

  Name of the changed property (`focusedElement`).

* `value: Element | null`

  New value of the `focusedElement` property with given key or `null`, if operation should remove property.

* `oldValue: Element | null`

  Old value of the `focusedElement` property with given key or `null`, if property was not set before.

<a id="event-change:isFocused">

### `change:isFocused( eventInfo, name, value, oldValue )`

Fired when the `isFocused` property changed value.

[See source](https://github.com/ckeditor/ckeditor5/blob/master/packages/ckeditor5-utils/src/focustracker.ts#L40)

#### Parameters

* `eventInfo: EventInfo`

  An object containing information about the fired event.

* `name: string`

  Name of the changed property (`isFocused`).

* `value: boolean`

  New value of the `isFocused` property with given key or `null`, if operation should remove property.

* `oldValue: boolean`

  Old value of the `isFocused` property with given key or `null`, if property was not set before.

<a id="event-change:%7Bproperty%7D">

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

Fired when a property changed value.

```typescript
observable.set( 'prop', 1 );

observable.on<ObservableChangeEvent<number>>( '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'
```

[See source](https://github.com/ckeditor/ckeditor5/blob/master/packages/ckeditor5-utils/src/observablemixin.ts#L1190)

#### Parameters

* `eventInfo: EventInfo`

  An object containing information about the fired event.

* `name: string`

  The property name.

* `value: TValue`

  The new property value.

* `oldValue: TValue`

  The previous property value.

<a id="event-set:focusedElement">

### `set:focusedElement( eventInfo, name, value, oldValue )`

Fired when the `focusedElement` property is going to be set but is not set yet (before the `change` event is fired).

[See source](https://github.com/ckeditor/ckeditor5/blob/master/packages/ckeditor5-utils/src/focustracker.ts#L54)

#### Parameters

* `eventInfo: EventInfo`

  An object containing information about the fired event.

* `name: string`

  Name of the changed property (`focusedElement`).

* `value: Element | null`

  New value of the `focusedElement` property with given key or `null`, if operation should remove property.

* `oldValue: Element | null`

  Old value of the `focusedElement` property with given key or `null`, if property was not set before.

<a id="event-set:isFocused">

### `set:isFocused( eventInfo, name, value, oldValue )`

Fired when the `isFocused` property is going to be set but is not set yet (before the `change` event is fired).

[See source](https://github.com/ckeditor/ckeditor5/blob/master/packages/ckeditor5-utils/src/focustracker.ts#L40)

#### Parameters

* `eventInfo: EventInfo`

  An object containing information about the fired event.

* `name: string`

  Name of the changed property (`isFocused`).

* `value: boolean`

  New value of the `isFocused` property with given key or `null`, if operation should remove property.

* `oldValue: boolean`

  Old value of the `isFocused` property with given key or `null`, if property was not set before.

<a id="event-set:%7Bproperty%7D">

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

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

You can control the final value of the property by using the [event's `return` property](module_utils_eventinfo-EventInfo.md#member-return).

```typescript
observable.set( 'prop', 1 );

observable.on<ObservableSetEvent<number>>( '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<ObservableChangeEvent<number>>( '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:** The event is fired even when the new value is the same as the old value.

[See source](https://github.com/ckeditor/ckeditor5/blob/master/packages/ckeditor5-utils/src/observablemixin.ts#L1228)

#### Parameters

* `eventInfo: EventInfo`

  An object containing information about the fired event.

* `name: string`

  The property name.

* `value: TValue`

  The new property value.

* `oldValue: TValue`

  The previous property value.

---

Full index of the CKEditor 5 API reference: [llms.txt](llms.txt)