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For example:\n\t\t\t *\n\t\t\t *\t\tlet observable = new Model();\n\t\t\t *\t\tobservable.property = 1;\n\t\t\t *\t\tobservable.set( 'property', 2 );\t\t\t// throws\n\t\t\t *\n\t\t\t *\t\tobservable.set( 'property', 1 );\n\t\t\t *\t\tobservable.set( 'property', 2 );\t\t\t// ok, because this is an existing property.\n\t\t\t *\n\t\t\t * @error observable-set-cannot-override\n\t\t\t */\n\t\t\tthrow new CKEditorError( 'observable-set-cannot-override', this );\n\t\t}\n\n\t\tObject.defineProperty( this, name, {\n\t\t\tenumerable: true,\n\t\t\tconfigurable: true,\n\n\t\t\tget() {\n\t\t\t\treturn properties.get( name );\n\t\t\t},\n\n\t\t\tset( value ) {\n\t\t\t\tconst oldValue = properties.get( name );\n\n\t\t\t\t// Fire `set` event before the new value will be set to make it possible\n\t\t\t\t// to override observable property without affecting `change` event.\n\t\t\t\t// See https://github.com/ckeditor/ckeditor5-utils/issues/171.\n\t\t\t\tlet newValue = this.fire( 'set:' + name, name, value, oldValue );\n\n\t\t\t\tif ( newValue === undefined ) {\n\t\t\t\t\tnewValue = value;\n\t\t\t\t}\n\n\t\t\t\t// Allow undefined as an initial value like A.define( 'x', undefined ) (#132).\n\t\t\t\t// Note: When properties map has no such own property, then its value is undefined.\n\t\t\t\tif ( oldValue !== newValue || !properties.has( name ) ) {\n\t\t\t\t\tproperties.set( name, newValue );\n\t\t\t\t\tthis.fire( 'change:' + name, name, newValue, oldValue );\n\t\t\t\t}\n\t\t\t}\n\t\t} );\n\n\t\tthis[ name ] = value;\n\t},\n\n\t/**\n\t * @inheritDoc\n\t */\n\tbind( ...bindProperties ) {\n\t\tif ( !bindProperties.length || !isStringArray( bindProperties ) ) {\n\t\t\t/**\n\t\t\t * All properties must be strings.\n\t\t\t *\n\t\t\t * @error observable-bind-wrong-properties\n\t\t\t */\n\t\t\tthrow new CKEditorError( 'observable-bind-wrong-properties', this );\n\t\t}\n\n\t\tif ( ( new Set( bindProperties ) ).size !== bindProperties.length ) {\n\t\t\t/**\n\t\t\t * Properties must be unique.\n\t\t\t *\n\t\t\t * @error observable-bind-duplicate-properties\n\t\t\t */\n\t\t\tthrow new CKEditorError( 'observable-bind-duplicate-properties', this );\n\t\t}\n\n\t\tinitObservable( this );\n\n\t\tconst boundProperties = this[ boundPropertiesSymbol ];\n\n\t\tbindProperties.forEach( propertyName => {\n\t\t\tif ( boundProperties.has( propertyName ) ) {\n\t\t\t\t/**\n\t\t\t\t * Cannot bind the same property more than once.\n\t\t\t\t *\n\t\t\t\t * @error observable-bind-rebind\n\t\t\t\t */\n\t\t\t\tthrow new CKEditorError( 'observable-bind-rebind', this );\n\t\t\t}\n\t\t} );\n\n\t\tconst bindings = new Map();\n\n\t\t// @typedef {Object} Binding\n\t\t// @property {Array} property Property which is bound.\n\t\t// @property {Array} to Array of observable–property components of the binding (`{ observable: ..., property: .. }`).\n\t\t// @property {Array} callback A function which processes `to` components.\n\t\tbindProperties.forEach( a => {\n\t\t\tconst binding = { property: a, to: [] };\n\n\t\t\tboundProperties.set( a, binding );\n\t\t\tbindings.set( a, binding );\n\t\t} );\n\n\t\t// @typedef {Object} BindChain\n\t\t// @property {Function} to See {@link ~ObservableMixin#_bindTo}.\n\t\t// @property {Function} toMany See {@link ~ObservableMixin#_bindToMany}.\n\t\t// @property {module:utils/observablemixin~Observable} _observable The observable which initializes the binding.\n\t\t// @property {Array} _bindProperties Array of `_observable` properties to be bound.\n\t\t// @property {Array} _to Array of `to()` observable–properties (`{ observable: toObservable, properties: ...toProperties }`).\n\t\t// @property {Map} _bindings Stores bindings to be kept in\n\t\t// {@link ~ObservableMixin#_boundProperties}/{@link ~ObservableMixin#_boundObservables}\n\t\t// initiated in this binding chain.\n\t\treturn {\n\t\t\tto: bindTo,\n\t\t\ttoMany: bindToMany,\n\n\t\t\t_observable: this,\n\t\t\t_bindProperties: bindProperties,\n\t\t\t_to: [],\n\t\t\t_bindings: bindings\n\t\t};\n\t},\n\n\t/**\n\t * @inheritDoc\n\t */\n\tunbind( ...unbindProperties ) {\n\t\t// Nothing to do here if not inited yet.\n\t\tif ( !( this[ observablePropertiesSymbol ] ) ) {\n\t\t\treturn;\n\t\t}\n\n\t\tconst boundProperties = this[ boundPropertiesSymbol ];\n\t\tconst boundObservables = this[ boundObservablesSymbol ];\n\n\t\tif ( unbindProperties.length ) {\n\t\t\tif ( !isStringArray( unbindProperties ) ) {\n\t\t\t\t/**\n\t\t\t\t * Properties must be strings.\n\t\t\t\t *\n\t\t\t\t * @error observable-unbind-wrong-properties\n\t\t\t\t */\n\t\t\t\tthrow new CKEditorError( 'observable-unbind-wrong-properties', this );\n\t\t\t}\n\n\t\t\tunbindProperties.forEach( propertyName => {\n\t\t\t\tconst binding = boundProperties.get( propertyName );\n\n\t\t\t\t// Nothing to do if the binding is not defined\n\t\t\t\tif ( !binding ) {\n\t\t\t\t\treturn;\n\t\t\t\t}\n\n\t\t\t\tlet toObservable, toProperty, toProperties, toPropertyBindings;\n\n\t\t\t\tbinding.to.forEach( to => {\n\t\t\t\t\t// TODO: ES6 destructuring.\n\t\t\t\t\ttoObservable = to[ 0 ];\n\t\t\t\t\ttoProperty = to[ 1 ];\n\t\t\t\t\ttoProperties = boundObservables.get( toObservable );\n\t\t\t\t\ttoPropertyBindings = toProperties[ toProperty ];\n\n\t\t\t\t\ttoPropertyBindings.delete( binding );\n\n\t\t\t\t\tif ( !toPropertyBindings.size ) {\n\t\t\t\t\t\tdelete toProperties[ toProperty ];\n\t\t\t\t\t}\n\n\t\t\t\t\tif ( !Object.keys( toProperties ).length ) {\n\t\t\t\t\t\tboundObservables.delete( toObservable );\n\t\t\t\t\t\tthis.stopListening( toObservable, 'change' );\n\t\t\t\t\t}\n\t\t\t\t} );\n\n\t\t\t\tboundProperties.delete( propertyName );\n\t\t\t} );\n\t\t} else {\n\t\t\tboundObservables.forEach( ( bindings, boundObservable ) => {\n\t\t\t\tthis.stopListening( boundObservable, 'change' );\n\t\t\t} );\n\n\t\t\tboundObservables.clear();\n\t\t\tboundProperties.clear();\n\t\t}\n\t},\n\n\t/**\n\t * @inheritDoc\n\t */\n\tdecorate( methodName ) {\n\t\tconst originalMethod = this[ methodName ];\n\n\t\tif ( !originalMethod ) {\n\t\t\t/**\n\t\t\t * Cannot decorate an undefined method.\n\t\t\t *\n\t\t\t * @error observablemixin-cannot-decorate-undefined\n\t\t\t * @param {Object} object The object which method should be decorated.\n\t\t\t * @param {String} methodName Name of the method which does not exist.\n\t\t\t */\n\t\t\tthrow new CKEditorError(\n\t\t\t\t'observablemixin-cannot-decorate-undefined',\n\t\t\t\tthis,\n\t\t\t\t{ object: this, methodName }\n\t\t\t);\n\t\t}\n\n\t\tthis.on( methodName, ( evt, args ) => {\n\t\t\tevt.return = originalMethod.apply( this, args );\n\t\t} );\n\n\t\tthis[ methodName ] = function( ...args ) {\n\t\t\treturn this.fire( methodName, args );\n\t\t};\n\n\t\tthis[ methodName ][ _decoratedOriginal ] = originalMethod;\n\n\t\tif ( !this[ _decoratedMethods ] ) {\n\t\t\tthis[ _decoratedMethods ] = [];\n\t\t}\n\n\t\tthis[ _decoratedMethods ].push( methodName );\n\t}\n};\n\nextend( ObservableMixin, EmitterMixin );\n\n// Override the EmitterMixin stopListening method to be able to clean (and restore) decorated methods.\n// This is needed in case of:\n//  1. Have x.foo() decorated.\n//  2. Call x.stopListening()\n//  3. Call x.foo(). Problem: nothing happens (the original foo() method is not executed)\nObservableMixin.stopListening = function( emitter, event, callback ) {\n\t// Removing all listeners so let's clean the decorated methods to the original state.\n\tif ( !emitter && this[ _decoratedMethods ] ) {\n\t\tfor ( const methodName of this[ _decoratedMethods ] ) {\n\t\t\tthis[ methodName ] = this[ methodName ][ _decoratedOriginal ];\n\t\t}\n\n\t\tdelete this[ _decoratedMethods ];\n\t}\n\n\tEmitterMixin.stopListening.call( this, emitter, event, callback );\n};\n\nexport default ObservableMixin;\n\n// Init symbol properties needed for the observable mechanism to work.\n//\n// @private\n// @param {module:utils/observablemixin~ObservableMixin} observable\nfunction initObservable( observable ) {\n\t// Do nothing if already inited.\n\tif ( observable[ observablePropertiesSymbol ] ) {\n\t\treturn;\n\t}\n\n\t// The internal hash containing the observable's state.\n\t//\n\t// @private\n\t// @type {Map}\n\tObject.defineProperty( observable, observablePropertiesSymbol, {\n\t\tvalue: new Map()\n\t} );\n\n\t// Map containing bindings to external observables. It shares the binding objects\n\t// (`{ observable: A, property: 'a', to: ... }`) with {@link module:utils/observablemixin~ObservableMixin#_boundProperties} and\n\t// it is used to observe external observables to update own properties accordingly.\n\t// See {@link module:utils/observablemixin~ObservableMixin#bind}.\n\t//\n\t//\t\tA.bind( 'a', 'b', 'c' ).to( B, 'x', 'y', 'x' );\n\t//\t\tconsole.log( A._boundObservables );\n\t//\n\t//\t\t\tMap( {\n\t//\t\t\t\tB: {\n\t//\t\t\t\t\tx: Set( [\n\t//\t\t\t\t\t\t{ observable: A, property: 'a', to: [ [ B, 'x' ] ] },\n\t//\t\t\t\t\t\t{ observable: A, property: 'c', to: [ [ B, 'x' ] ] }\n\t//\t\t\t\t\t] ),\n\t//\t\t\t\t\ty: Set( [\n\t//\t\t\t\t\t\t{ observable: A, property: 'b', to: [ [ B, 'y' ] ] },\n\t//\t\t\t\t\t] )\n\t//\t\t\t\t}\n\t//\t\t\t} )\n\t//\n\t//\t\tA.bind( 'd' ).to( B, 'z' ).to( C, 'w' ).as( callback );\n\t//\t\tconsole.log( A._boundObservables );\n\t//\n\t//\t\t\tMap( {\n\t//\t\t\t\tB: {\n\t//\t\t\t\t\tx: Set( [\n\t//\t\t\t\t\t\t{ observable: A, property: 'a', to: [ [ B, 'x' ] ] },\n\t//\t\t\t\t\t\t{ observable: A, property: 'c', to: [ [ B, 'x' ] ] }\n\t//\t\t\t\t\t] ),\n\t//\t\t\t\t\ty: Set( [\n\t//\t\t\t\t\t\t{ observable: A, property: 'b', to: [ [ B, 'y' ] ] },\n\t//\t\t\t\t\t] ),\n\t//\t\t\t\t\tz: Set( [\n\t//\t\t\t\t\t\t{ observable: A, property: 'd', to: [ [ B, 'z' ], [ C, 'w' ] ], callback: callback }\n\t//\t\t\t\t\t] )\n\t//\t\t\t\t},\n\t//\t\t\t\tC: {\n\t//\t\t\t\t\tw: Set( [\n\t//\t\t\t\t\t\t{ observable: A, property: 'd', to: [ [ B, 'z' ], [ C, 'w' ] ], callback: callback }\n\t//\t\t\t\t\t] )\n\t//\t\t\t\t}\n\t//\t\t\t} )\n\t//\n\t// @private\n\t// @type {Map}\n\tObject.defineProperty( observable, boundObservablesSymbol, {\n\t\tvalue: new Map()\n\t} );\n\n\t// Object that stores which properties of this observable are bound and how. It shares\n\t// the binding objects (`{ observable: A, property: 'a', to: ... }`) with\n\t// {@link module:utils/observablemixin~ObservableMixin#_boundObservables}. This data structure is\n\t// a reverse of {@link module:utils/observablemixin~ObservableMixin#_boundObservables} and it is helpful for\n\t// {@link module:utils/observablemixin~ObservableMixin#unbind}.\n\t//\n\t// See {@link module:utils/observablemixin~ObservableMixin#bind}.\n\t//\n\t//\t\tA.bind( 'a', 'b', 'c' ).to( B, 'x', 'y', 'x' );\n\t//\t\tconsole.log( A._boundProperties );\n\t//\n\t//\t\t\tMap( {\n\t//\t\t\t\ta: { observable: A, property: 'a', to: [ [ B, 'x' ] ] },\n\t//\t\t\t\tb: { observable: A, property: 'b', to: [ [ B, 'y' ] ] },\n\t//\t\t\t\tc: { observable: A, property: 'c', to: [ [ B, 'x' ] ] }\n\t//\t\t\t} )\n\t//\n\t//\t\tA.bind( 'd' ).to( B, 'z' ).to( C, 'w' ).as( callback );\n\t//\t\tconsole.log( A._boundProperties );\n\t//\n\t//\t\t\tMap( {\n\t//\t\t\t\ta: { observable: A, property: 'a', to: [ [ B, 'x' ] ] },\n\t//\t\t\t\tb: { observable: A, property: 'b', to: [ [ B, 'y' ] ] },\n\t//\t\t\t\tc: { observable: A, property: 'c', to: [ [ B, 'x' ] ] },\n\t//\t\t\t\td: { observable: A, property: 'd', to: [ [ B, 'z' ], [ C, 'w' ] ], callback: callback }\n\t//\t\t\t} )\n\t//\n\t// @private\n\t// @type {Map}\n\tObject.defineProperty( observable, boundPropertiesSymbol, {\n\t\tvalue: new Map()\n\t} );\n}\n\n// A chaining for {@link module:utils/observablemixin~ObservableMixin#bind} providing `.to()` interface.\n//\n// @private\n// @param {...[Observable|String|Function]} args Arguments of the `.to( args )` binding.\nfunction bindTo( ...args ) {\n\tconst parsedArgs = parseBindToArgs( ...args );\n\tconst bindingsKeys = Array.from( this._bindings.keys() );\n\tconst numberOfBindings = bindingsKeys.length;\n\n\t// Eliminate A.bind( 'x' ).to( B, C )\n\tif ( !parsedArgs.callback && parsedArgs.to.length > 1 ) {\n\t\t/**\n\t\t * Binding multiple observables only possible with callback.\n\t\t *\n\t\t * @error observable-bind-to-no-callback\n\t\t */\n\t\tthrow new CKEditorError( 'observable-bind-to-no-callback', this );\n\t}\n\n\t// Eliminate A.bind( 'x', 'y' ).to( B, callback )\n\tif ( numberOfBindings > 1 && parsedArgs.callback ) {\n\t\t/**\n\t\t * Cannot bind multiple properties and use a callback in one binding.\n\t\t *\n\t\t * @error observable-bind-to-extra-callback\n\t\t */\n\t\tthrow new CKEditorError(\n\t\t\t'observable-bind-to-extra-callback',\n\t\t\tthis\n\t\t);\n\t}\n\n\tparsedArgs.to.forEach( to => {\n\t\t// Eliminate A.bind( 'x', 'y' ).to( B, 'a' )\n\t\tif ( to.properties.length && to.properties.length !== numberOfBindings ) {\n\t\t\t/**\n\t\t\t * The number of properties must match.\n\t\t\t *\n\t\t\t * @error observable-bind-to-properties-length\n\t\t\t */\n\t\t\tthrow new CKEditorError( 'observable-bind-to-properties-length', this );\n\t\t}\n\n\t\t// When no to.properties specified, observing source properties instead i.e.\n\t\t// A.bind( 'x', 'y' ).to( B ) -> Observe B.x and B.y\n\t\tif ( !to.properties.length ) {\n\t\t\tto.properties = this._bindProperties;\n\t\t}\n\t} );\n\n\tthis._to = parsedArgs.to;\n\n\t// Fill {@link BindChain#_bindings} with callback. When the callback is set there's only one binding.\n\tif ( parsedArgs.callback ) {\n\t\tthis._bindings.get( bindingsKeys[ 0 ] ).callback = parsedArgs.callback;\n\t}\n\n\tattachBindToListeners( this._observable, this._to );\n\n\t// Update observable._boundProperties and observable._boundObservables.\n\tupdateBindToBound( this );\n\n\t// Set initial values of bound properties.\n\tthis._bindProperties.forEach( propertyName => {\n\t\tupdateBoundObservableProperty( this._observable, propertyName );\n\t} );\n}\n\n// Binds to an attribute in a set of iterable observables.\n//\n// @private\n// @param {Array.<Observable>} observables\n// @param {String} attribute\n// @param {Function} callback\nfunction bindToMany( observables, attribute, callback ) {\n\tif ( this._bindings.size > 1 ) {\n\t\t/**\n\t\t * Binding one attribute to many observables only possible with one attribute.\n\t\t *\n\t\t * @error observable-bind-to-many-not-one-binding\n\t\t */\n\t\tthrow new CKEditorError( 'observable-bind-to-many-not-one-binding', this );\n\t}\n\n\tthis.to(\n\t\t// Bind to #attribute of each observable...\n\t\t...getBindingTargets( observables, attribute ),\n\t\t// ...using given callback to parse attribute values.\n\t\tcallback\n\t);\n}\n\n// Returns an array of binding components for\n// {@link Observable#bind} from a set of iterable observables.\n//\n// @param {Array.<Observable>} observables\n// @param {String} attribute\n// @returns {Array.<String|Observable>}\nfunction getBindingTargets( observables, attribute ) {\n\tconst observableAndAttributePairs = observables.map( observable => [ observable, attribute ] );\n\n\t// Merge pairs to one-dimension array of observables and attributes.\n\treturn Array.prototype.concat.apply( [], observableAndAttributePairs );\n}\n\n// Check if all entries of the array are of `String` type.\n//\n// @private\n// @param {Array} arr An array to be checked.\n// @returns {Boolean}\nfunction isStringArray( arr ) {\n\treturn arr.every( a => typeof a == 'string' );\n}\n\n// Parses and validates {@link Observable#bind}`.to( args )` arguments and returns\n// an object with a parsed structure. For example\n//\n//\t\tA.bind( 'x' ).to( B, 'a', C, 'b', call );\n//\n// becomes\n//\n//\t\t{\n//\t\t\tto: [\n//\t\t\t\t{ observable: B, properties: [ 'a' ] },\n//\t\t\t\t{ observable: C, properties: [ 'b' ] },\n//\t\t\t],\n//\t\t\tcallback: call\n// \t\t}\n//\n// @private\n// @param {...*} args Arguments of {@link Observable#bind}`.to( args )`.\n// @returns {Object}\nfunction parseBindToArgs( ...args ) {\n\t// Eliminate A.bind( 'x' ).to()\n\tif ( !args.length ) {\n\t\t/**\n\t\t * Invalid argument syntax in `to()`.\n\t\t *\n\t\t * @error observable-bind-to-parse-error\n\t\t */\n\t\tthrow new CKEditorError( 'observable-bind-to-parse-error', null );\n\t}\n\n\tconst parsed = { to: [] };\n\tlet lastObservable;\n\n\tif ( typeof args[ args.length - 1 ] == 'function' ) {\n\t\tparsed.callback = args.pop();\n\t}\n\n\targs.forEach( a => {\n\t\tif ( typeof a == 'string' ) {\n\t\t\tlastObservable.properties.push( a );\n\t\t} else if ( typeof a == 'object' ) {\n\t\t\tlastObservable = { observable: a, properties: [] };\n\t\t\tparsed.to.push( lastObservable );\n\t\t} else {\n\t\t\tthrow new CKEditorError( 'observable-bind-to-parse-error', null );\n\t\t}\n\t} );\n\n\treturn parsed;\n}\n\n// Synchronizes {@link module:utils/observablemixin#_boundObservables} with {@link Binding}.\n//\n// @private\n// @param {Binding} binding A binding to store in {@link Observable#_boundObservables}.\n// @param {Observable} toObservable A observable, which is a new component of `binding`.\n// @param {String} toPropertyName A name of `toObservable`'s property, a new component of the `binding`.\nfunction updateBoundObservables( observable, binding, toObservable, toPropertyName ) {\n\tconst boundObservables = observable[ boundObservablesSymbol ];\n\tconst bindingsToObservable = boundObservables.get( toObservable );\n\tconst bindings = bindingsToObservable || {};\n\n\tif ( !bindings[ toPropertyName ] ) {\n\t\tbindings[ toPropertyName ] = new Set();\n\t}\n\n\t// Pass the binding to a corresponding Set in `observable._boundObservables`.\n\tbindings[ toPropertyName ].add( binding );\n\n\tif ( !bindingsToObservable ) {\n\t\tboundObservables.set( toObservable, bindings );\n\t}\n}\n\n// Synchronizes {@link Observable#_boundProperties} and {@link Observable#_boundObservables}\n// with {@link BindChain}.\n//\n// Assuming the following binding being created\n//\n// \t\tA.bind( 'a', 'b' ).to( B, 'x', 'y' );\n//\n// the following bindings were initialized by {@link Observable#bind} in {@link BindChain#_bindings}:\n//\n// \t\t{\n// \t\t\ta: { observable: A, property: 'a', to: [] },\n// \t\t\tb: { observable: A, property: 'b', to: [] },\n// \t\t}\n//\n// Iterate over all bindings in this chain and fill their `to` properties with\n// corresponding to( ... ) arguments (components of the binding), so\n//\n// \t\t{\n// \t\t\ta: { observable: A, property: 'a', to: [ B, 'x' ] },\n// \t\t\tb: { observable: A, property: 'b', to: [ B, 'y' ] },\n// \t\t}\n//\n// Then update the structure of {@link Observable#_boundObservables} with updated\n// binding, so it becomes:\n//\n// \t\tMap( {\n// \t\t\tB: {\n// \t\t\t\tx: Set( [\n// \t\t\t\t\t{ observable: A, property: 'a', to: [ [ B, 'x' ] ] }\n// \t\t\t\t] ),\n// \t\t\t\ty: Set( [\n// \t\t\t\t\t{ observable: A, property: 'b', to: [ [ B, 'y' ] ] },\n// \t\t\t\t] )\n//\t\t\t}\n// \t\t} )\n//\n// @private\n// @param {BindChain} chain The binding initialized by {@link Observable#bind}.\nfunction updateBindToBound( chain ) {\n\tlet toProperty;\n\n\tchain._bindings.forEach( ( binding, propertyName ) => {\n\t\t// Note: For a binding without a callback, this will run only once\n\t\t// like in A.bind( 'x', 'y' ).to( B, 'a', 'b' )\n\t\t// TODO: ES6 destructuring.\n\t\tchain._to.forEach( to => {\n\t\t\ttoProperty = to.properties[ binding.callback ? 0 : chain._bindProperties.indexOf( propertyName ) ];\n\n\t\t\tbinding.to.push( [ to.observable, toProperty ] );\n\t\t\tupdateBoundObservables( chain._observable, binding, to.observable, toProperty );\n\t\t} );\n\t} );\n}\n\n// Updates an property of a {@link Observable} with a value\n// determined by an entry in {@link Observable#_boundProperties}.\n//\n// @private\n// @param {Observable} observable A observable which property is to be updated.\n// @param {String} propertyName An property to be updated.\nfunction updateBoundObservableProperty( observable, propertyName ) {\n\tconst boundProperties = observable[ boundPropertiesSymbol ];\n\tconst binding = boundProperties.get( propertyName );\n\tlet propertyValue;\n\n\t// When a binding with callback is created like\n\t//\n\t// \t\tA.bind( 'a' ).to( B, 'b', C, 'c', callback );\n\t//\n\t// collect B.b and C.c, then pass them to callback to set A.a.\n\tif ( binding.callback ) {\n\t\tpropertyValue = binding.callback.apply( observable, binding.to.map( to => to[ 0 ][ to[ 1 ] ] ) );\n\t} else {\n\t\tpropertyValue = binding.to[ 0 ];\n\t\tpropertyValue = propertyValue[ 0 ][ propertyValue[ 1 ] ];\n\t}\n\n\tif ( Object.prototype.hasOwnProperty.call( observable, propertyName ) ) {\n\t\tobservable[ propertyName ] = propertyValue;\n\t} else {\n\t\tobservable.set( propertyName, propertyValue );\n\t}\n}\n\n// Starts listening to changes in {@link BindChain._to} observables to update\n// {@link BindChain._observable} {@link BindChain._bindProperties}. Also sets the\n// initial state of {@link BindChain._observable}.\n//\n// @private\n// @param {BindChain} chain The chain initialized by {@link Observable#bind}.\nfunction attachBindToListeners( observable, toBindings ) {\n\ttoBindings.forEach( to => {\n\t\tconst boundObservables = observable[ boundObservablesSymbol ];\n\t\tlet bindings;\n\n\t\t// If there's already a chain between the observables (`observable` listens to\n\t\t// `to.observable`), there's no need to create another `change` event listener.\n\t\tif ( !boundObservables.get( to.observable ) ) {\n\t\t\tobservable.listenTo( to.observable, 'change', ( evt, propertyName ) => {\n\t\t\t\tbindings = boundObservables.get( to.observable )[ propertyName ];\n\n\t\t\t\t// Note: to.observable will fire for any property change, react\n\t\t\t\t// to changes of properties which are bound only.\n\t\t\t\tif ( bindings ) {\n\t\t\t\t\tbindings.forEach( binding => {\n\t\t\t\t\t\tupdateBoundObservableProperty( observable, binding.property );\n\t\t\t\t\t} );\n\t\t\t\t}\n\t\t\t} );\n\t\t}\n\t} );\n}\n\n/**\n * An interface which adds \"observable properties\" and data binding functionality.\n *\n * Can be easily implemented by a class by mixing the {@link module:utils/observablemixin~ObservableMixin} mixin.\n *\n * Read more about the usage of this interface in the:\n * * {@glink framework/guides/architecture/core-editor-architecture#event-system-and-observables Event system and observables}\n * section of the {@glink framework/guides/architecture/core-editor-architecture Core editor architecture} guide,\n * * {@glink framework/guides/deep-dive/observables Observables deep dive} guide.\n *\n * @interface Observable\n * @extends module:utils/emittermixin~Emitter\n */\n\n/**\n * Fired when a property changed value.\n *\n *\t\tobservable.set( 'prop', 1 );\n *\n *\t\tobservable.on( 'change:prop', ( evt, propertyName, newValue, oldValue ) => {\n *\t\t\tconsole.log( `${ propertyName } has changed from ${ oldValue } to ${ newValue }` );\n *\t\t} );\n *\n *\t\tobservable.prop = 2; // -> 'prop has changed from 1 to 2'\n *\n * @event change:{property}\n * @param {String} name The property name.\n * @param {*} value The new property value.\n * @param {*} oldValue The previous property value.\n */\n\n/**\n * Fired when a property value is going to be set but is not set yet (before the `change` event is fired).\n *\n * You can control the final value of the property by using\n * the {@link module:utils/eventinfo~EventInfo#return event's `return` property}.\n *\n *\t\tobservable.set( 'prop', 1 );\n *\n *\t\tobservable.on( 'set:prop', ( evt, propertyName, newValue, oldValue ) => {\n *\t\t\tconsole.log( `Value is going to be changed from ${ oldValue } to ${ newValue }` );\n *\t\t\tconsole.log( `Current property value is ${ observable[ propertyName ] }` );\n *\n *\t\t\t// Let's override the value.\n *\t\t\tevt.return = 3;\n *\t\t} );\n *\n *\t\tobservable.on( 'change:prop', ( evt, propertyName, newValue, oldValue ) => {\n *\t\t\tconsole.log( `Value has changed from ${ oldValue } to ${ newValue }` );\n *\t\t} );\n *\n *\t\tobservable.prop = 2; // -> 'Value is going to be changed from 1 to 2'\n *\t\t                     // -> 'Current property value is 1'\n *\t\t                     // -> 'Value has changed from 1 to 3'\n *\n * **Note:** The event is fired even when the new value is the same as the old value.\n *\n * @event set:{property}\n * @param {String} name The property name.\n * @param {*} value The new property value.\n * @param {*} oldValue The previous property value.\n */\n\n/**\n * Creates and sets the value of an observable property of this object. Such a property becomes a part\n * of the state and is observable.\n *\n * It accepts also a single object literal containing key/value pairs with properties to be set.\n *\n * This method throws the `observable-set-cannot-override` error if the observable instance already\n * has a property with the given property name. This prevents from mistakenly overriding existing\n * properties and methods, but means that `foo.set( 'bar', 1 )` may be slightly slower than `foo.bar = 1`.\n *\n * @method #set\n * @param {String|Object} name The property's name or object with `name=>value` pairs.\n * @param {*} [value] The property's value (if `name` was passed in the first parameter).\n */\n\n/**\n * Binds {@link #set observable properties} to other objects implementing the\n * {@link module:utils/observablemixin~Observable} interface.\n *\n * Read more in the {@glink framework/guides/deep-dive/observables#property-bindings dedicated guide}\n * covering the topic of property bindings with some additional examples.\n *\n * Consider two objects: a `button` and an associated `command` (both `Observable`).\n *\n * A simple property binding could be as follows:\n *\n *\t\tbutton.bind( 'isEnabled' ).to( command, 'isEnabled' );\n *\n * or even shorter:\n *\n *\t\tbutton.bind( 'isEnabled' ).to( command );\n *\n * which works in the following way:\n *\n * * `button.isEnabled` **instantly equals** `command.isEnabled`,\n * * whenever `command.isEnabled` changes, `button.isEnabled` will immediately reflect its value.\n *\n * **Note**: To release the binding, use {@link module:utils/observablemixin~Observable#unbind}.\n *\n * You can also \"rename\" the property in the binding by specifying the new name in the `to()` chain:\n *\n *\t\tbutton.bind( 'isEnabled' ).to( command, 'isWorking' );\n *\n * It is possible to bind more than one property at a time to shorten the code:\n *\n *\t\tbutton.bind( 'isEnabled', 'value' ).to( command );\n *\n * which corresponds to:\n *\n *\t\tbutton.bind( 'isEnabled' ).to( command );\n *\t\tbutton.bind( 'value' ).to( command );\n *\n * The binding can include more than one observable, combining multiple data sources in a custom callback:\n *\n *\t\tbutton.bind( 'isEnabled' ).to( command, 'isEnabled', ui, 'isVisible',\n *\t\t\t( isCommandEnabled, isUIVisible ) => isCommandEnabled && isUIVisible );\n *\n * Using a custom callback allows processing the value before passing it to the target property:\n *\n *\t\tbutton.bind( 'isEnabled' ).to( command, 'value', value => value === 'heading1' );\n *\n * It is also possible to bind to the same property in an array of observables.\n * To bind a `button` to multiple commands (also `Observables`) so that each and every one of them\n * must be enabled for the button to become enabled, use the following code:\n *\n *\t\tbutton.bind( 'isEnabled' ).toMany( [ commandA, commandB, commandC ], 'isEnabled',\n *\t\t\t( isAEnabled, isBEnabled, isCEnabled ) => isAEnabled && isBEnabled && isCEnabled );\n *\n * @method #bind\n * @param {...String} bindProperties Observable properties that will be bound to other observable(s).\n * @returns {Object} The bind chain with the `to()` and `toMany()` methods.\n */\n\n/**\n * Removes the binding created with {@link #bind}.\n *\n *\t\t// Removes the binding for the 'a' property.\n *\t\tA.unbind( 'a' );\n *\n *\t\t// Removes bindings for all properties.\n *\t\tA.unbind();\n *\n * @method #unbind\n * @param {...String} [unbindProperties] Observable properties to be unbound. All the bindings will\n * be released if no properties are provided.\n */\n\n/**\n * Turns the given methods of this object into event-based ones. This means that the new method will fire an event\n * (named after the method) and the original action will be plugged as a listener to that event.\n *\n * Read more in the {@glink framework/guides/deep-dive/observables#decorating-object-methods dedicated guide}\n * covering the topic of decorating methods with some additional examples.\n *\n * Decorating the method does not change its behavior (it only adds an event),\n * but it allows to modify it later on by listening to the method's event.\n *\n * For example, to cancel the method execution the event can be {@link module:utils/eventinfo~EventInfo#stop stopped}:\n *\n *\t\tclass Foo {\n *\t\t\tconstructor() {\n *\t\t\t\tthis.decorate( 'method' );\n *\t\t\t}\n *\n *\t\t\tmethod() {\n *\t\t\t\tconsole.log( 'called!' );\n *\t\t\t}\n *\t\t}\n *\n *\t\tconst foo = new Foo();\n *\t\tfoo.on( 'method', ( evt ) => {\n *\t\t\tevt.stop();\n *\t\t}, { priority: 'high' } );\n *\n *\t\tfoo.method(); // Nothing is logged.\n *\n *\n * **Note**: The high {@link module:utils/priorities~PriorityString priority} listener\n * has been used to execute this particular callback before the one which calls the original method\n * (which uses the \"normal\" priority).\n *\n * It is also possible to change the returned value:\n *\n *\t\tfoo.on( 'method', ( evt ) => {\n *\t\t\tevt.return = 'Foo!';\n *\t\t} );\n *\n *\t\tfoo.method(); // -> 'Foo'\n *\n * Finally, it is possible to access and modify the arguments the method is called with:\n *\n *\t\tmethod( a, b ) {\n *\t\t\tconsole.log( `${ a }, ${ b }`  );\n *\t\t}\n *\n *\t\t// ...\n *\n *\t\tfoo.on( 'method', ( evt, args ) => {\n *\t\t\targs[ 0 ] = 3;\n *\n *\t\t\tconsole.log( args[ 1 ] ); // -> 2\n *\t\t}, { priority: 'high' } );\n *\n *\t\tfoo.method( 1, 2 ); // -> '3, 2'\n *\n * @method #decorate\n * @param {String} methodName Name of the method to decorate.\n */\n"],"sourceRoot":""}