ChangeWatcher

Package	mx.binding.utils
Class	public class ChangeWatcher
Inheritance	ChangeWatcher Object

Language Version:

ActionScript 3.0

Product Version:

Flex 3

Runtime Versions:

Flash Player 9, AIR 1.1

The ChangeWatcher class defines utility methods that you can use with bindable Flex properties. These methods let you define an event handler that is executed whenever a bindable property is updated.

More examples

Defining binding watchers

Public Properties

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	Property		Defined By
		constructor : Object A reference to the class object or constructor function for a given object instance.	Object
		useWeakReference : Boolean Determines whether the reference to handler is strong or weak.	ChangeWatcher

Public Methods

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	Method		Defined By
		ChangeWatcher(access:Object, handler:Function, commitOnly:Boolean = false, next:ChangeWatcher = null) Constructor.	ChangeWatcher
		canWatch(host:Object, name:String, commitOnly:Boolean = false):Boolean [static] Lets you determine if the host exposes a data-binding event on the property.	ChangeWatcher
		getEvents(host:Object, name:String, commitOnly:Boolean = false):Object [static] Returns all binding events for a bindable property in the host object.	ChangeWatcher
		getValue():Object Retrieves the current value of the watched property or property chain, or null if the host object is null.	ChangeWatcher
		hasOwnProperty(name:String):Boolean Indicates whether an object has a specified property defined.	Object
		isPrototypeOf(theClass:Object):Boolean Indicates whether an instance of the Object class is in the prototype chain of the object specified as the parameter.	Object
		isWatching():Boolean Returns true if each watcher in the chain is attached to at least one change event.	ChangeWatcher
		propertyIsEnumerable(name:String):Boolean Indicates whether the specified property exists and is enumerable.	Object
		reset(newHost:Object):void Resets this ChangeWatcher instance to use a new host object.	ChangeWatcher
		setHandler(handler:Function):void Sets the handler function.	ChangeWatcher
		setPropertyIsEnumerable(name:String, isEnum:Boolean = true):void Sets the availability of a dynamic property for loop operations.	Object
		toLocaleString():String Returns the string representation of this object, formatted according to locale-specific conventions.	Object
		toString():String Returns the string representation of the specified object.	Object
		unwatch():void Detaches this ChangeWatcher instance, and its handler function, from the current host.	ChangeWatcher
		valueOf():Object Returns the primitive value of the specified object.	Object
		watch(host:Object, chain:Object, handler:Function, commitOnly:Boolean = false, useWeakReference:Boolean = false):ChangeWatcher [static] Creates and starts a ChangeWatcher instance.	ChangeWatcher

Property Detail

useWeakReference

property

public var useWeakReference:Boolean

Language Version:

ActionScript 3.0

Product Version:

Flex 3

Runtime Versions:

Flash Player 9, AIR 1.1

Determines whether the reference to handler is strong or weak. A strong reference (the default) prevents handler from being garbage-collected. A weak reference does not.

The default value is false.

Constructor Detail

()

Constructor

public function ChangeWatcher(access:Object, handler:Function, commitOnly:Boolean = false, next:ChangeWatcher = null)

Language Version:

ActionScript 3.0

Product Version:

Flex 3

Runtime Versions:

Flash Player 9, AIR 1.1

Constructor. Not for public use. This method is called only from the watch() method. See the watch() method for parameter usage.

Parameters

	`access:Object`

	`handler:Function`

	`commitOnly:Boolean` (default = `false`)

	`next:ChangeWatcher` (default = `null`)

Method Detail

canWatch

()

method

public static function canWatch(host:Object, name:String, commitOnly:Boolean = false):Boolean

Language Version:

ActionScript 3.0

Product Version:

Flex 3

Runtime Versions:

Flash Player 9, AIR 1.1

Lets you determine if the host exposes a data-binding event on the property.

NOTE: Property chains are not supported by the canWatch() method. They are supported by the watch() method.

Parameters

	`host:Object` — The host of the property. See the `watch()` method for more information.

	`name:String` — The name of the property. See the `watch()` method for more information.

	`commitOnly:Boolean` (default = `false`) — Set to `true` if the handler should be called only on committing change events. See the `watch()` method for more information.

Returns

Boolean — true if host exposes any change events on name.

getEvents

()

method

public static function getEvents(host:Object, name:String, commitOnly:Boolean = false):Object

Language Version:

ActionScript 3.0

Product Version:

Flex 3

Runtime Versions:

Flash Player 9, AIR 1.1

Returns all binding events for a bindable property in the host object.

Parameters

	`host:Object` — The host of the property. See the `watch()` method for more information.

	`name:String` — The name of the property, or property chain. See the `watch()` method for more information.

	`commitOnly:Boolean` (default = `false`) — Controls inclusion of non-committing change events in the returned value.

Returns

Object — Object of the form { eventName: isCommitting, ... } containing all change events for the property.

getValue

()

method

public function getValue():Object

Language Version:

ActionScript 3.0

Product Version:

Flex 3

Runtime Versions:

Flash Player 9, AIR 1.1

Retrieves the current value of the watched property or property chain, or null if the host object is null. For example:

      watch(obj, ["a","b","c"], ...).getValue() === obj.a.b.c

Returns

Object — The current value of the watched property or property chain.

isWatching

()

method

public function isWatching():Boolean

Language Version:

ActionScript 3.0

Product Version:

Flex 3

Runtime Versions:

Flash Player 9, AIR 1.1

Returns true if each watcher in the chain is attached to at least one change event. Note that the isWatching() method varies with host, since different hosts may expose different change events for the watcher's chosen property.

Returns

Boolean — true if each watcher in the chain is attached to at least one change event.

reset

()

method

public function reset(newHost:Object):void

Language Version:

ActionScript 3.0

Product Version:

Flex 3

Runtime Versions:

Flash Player 9, AIR 1.1

Resets this ChangeWatcher instance to use a new host object. You can call this method to reuse a watcher instance on a different host.

Parameters

newHost:Object — The new host of the property. See the watch() method for more information.

setHandler

()

method

public function setHandler(handler:Function):void

Language Version:

ActionScript 3.0

Product Version:

Flex 3

Runtime Versions:

Flash Player 9, AIR 1.1

Sets the handler function.

Parameters

handler:Function — The handler function. This argument must not be null.

unwatch

()

method

public function unwatch():void

Language Version:

ActionScript 3.0

Product Version:

Flex 3

Runtime Versions:

Flash Player 9, AIR 1.1

Detaches this ChangeWatcher instance, and its handler function, from the current host. You can use the reset() method to reattach the ChangeWatcher instance, or watch the same property or chain on a different host object.

watch

()

method

 public static function watch(host:Object, chain:Object, handler:Function, commitOnly:Boolean = false, useWeakReference:Boolean = false):ChangeWatcher

Language Version:

ActionScript 3.0

Product Version:

Flex 3

Runtime Versions:

Flash Player 9, AIR 1.1

Creates and starts a ChangeWatcher instance. A single ChangeWatcher instance can watch one property, or a property chain. A property chain is a sequence of properties accessible from a host object. For example, the expression obj.a.b.c contains the property chain (a, b, c).

Parameters

	`host:Object` — The object that hosts the property or property chain to be watched. You can use the use the `reset()` method to change the value of the `host` argument after creating the ChangeWatcher instance. The `host` maintains a list of `handlers` to invoke when `prop` changes.

	`chain:Object` — A value specifying the property or chain to be watched. Legal values are: A String containing the name of a public bindable property of the host object. An Object in the form: `{ name: property name, getter: function(host) { return host[name] } }`. The Object contains the name of a public bindable property, and a function which serves as a getter for that property. A non-empty Array containing any combination of the first two options. This represents a chain of bindable properties accessible from the host. For example, to watch the property `host.a.b.c`, call the method as: `watch(host, ["a","b","c"], ...)`. Note: The property or properties named in the `chain` argument must be public, because the `describeType()` method suppresses all information about non-public properties, including the bindability metadata that ChangeWatcher scans to find the change events that are exposed for a given property. However, the getter function supplied when using the `{ name, getter }` argument form described above can be used to associate an arbitrary computed value with the named (public) property.

	`handler:Function` — An event handler function called when the value of the watched property (or any property in a watched chain) is modified. The modification is signaled when any host object in the watcher chain dispatches the event that has been specified in that host object's `[Bindable]` metadata tag for the corresponding watched property. The default event is named `propertyChange`. The event object dispatched by the bindable property is passed to this handler function without modification. By default, Flex dispatches an event object of type PropertyChangeEvent. However, you can define your own event type when you use the `[Bindable]` metadata tag to define a bindable property.

	`commitOnly:Boolean` (default = `false`) — Set to `true` if the handler should be called only on committing change events; set to `false` if the handler should be called on both committing and non-committing change events. Note: the presence of non-committing change events for a property is indicated by the `[NonCommittingChangeEvent(<event-name>)]` metadata tag. Typically these tags are used to indicate fine-grained value changes, such as modifications in a text field prior to confirmation.

	`useWeakReference:Boolean` (default = `false`) — (default = false) Determines whether the reference to `handler` is strong or weak. A strong reference (the default) prevents `handler` from being garbage-collected. A weak reference does not.

Returns

ChangeWatcher — The ChangeWatcher instance, if at least one property name has been specified to the chain argument; null otherwise. Note that the returned watcher is not guaranteed to have successfully discovered and attached itself to change events, since none may have been exposed on the given property or chain by the host. You can use the isWatching() method to determine the watcher's state.

Related API Elements

mx.events.PropertyChangeEvent

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ChangeWatcher - AS3 Flex

Classes x

useWeakReference

ChangeWatcher

canWatch

getEvents

getValue

isWatching

reset

setHandler

unwatch

watch