flash.system

# MessageChannel - AS3

Package | flash.system
Class | public final class MessageChannel
Inheritance | MessageChannel EventDispatcher Object

Language Version: | ActionScript 3.0

Runtime Versions: | Flash Player 11.4, AIR 3.4

The MessageChannel class provides a mechanism for a worker to communicate with another worker. A message channel is a one-way communication channel. The message channel's sending worker uses the message channel to send objects to the receiving worker. A MessageChannel object is the only way to send a message between workers that dispatches an event indicating to the receiver that the message is available. Other mechanisms for sharing data allow a value to be set but do not provide an event to notify you of the changed data.

Each MessageChannel object contains a queue of message objects sent from the sending worker to the receiving worker. Each call to `send()` adds an object to the queue. Each call to `receive()` retrieves the oldest message object from the queue.

You do not create MessageChannel instances directly by calling the `MessageChannel()` constructor. To create a MessageChannel instance, call the `createMessageChannel()` method of the Worker object that will send messages on the channel, passing the receiving Worker object as an argument.

The typical workflow for sending messages with a MessageChannel object is as follows:

1. Call the sending worker's `createMessageChannel()` method to create the message channel

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           // In the sending worker swf
           var sendChannel:MessageChannel;
           sendChannel = Worker.current.createMessageChannel(receivingWorker);

2. Pass the message channel to the other worker, either by calling `Worker.setSharedProperty()` or by sending it through an existing message channel

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           receivingWorker.setSharedProperty("incomingChannel", sendChannel);

3. Code in the receiving worker registers a listener with the MessageChannel object for the `channelMessage` event

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           // In the receiving worker swf
           var incomingChannel:MessageChannel;
           incomingChannel = Worker.current.getSharedProperty("incomingChannel");
           incomingChannel.addEventListener(Event.CHANNEL_MESSAGE, handleIncomingMessage);

4. Code in the sending worker sends a message by calling the `send()` method

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           // In the sending worker swf
           sendChannel.send("This is a message");

5. The runtime calls the event handler in the receiving worker code, indicating that a message has been sent

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           // In the receiving worker swf
           // This method is called when the message channel gets a message
           private function handleIncomingMessage(event:Event):void
           {
               // Do something with the message, as shown in the next code listing
           }

6. Code in the receiving worker calls the `receive()` method to get the message. The object returned by the `receive()` method has the same data type as the object passed to the `send()` method.

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           var message:String = incomingChannel.receive() as String;

In addition to the asynchronous workflow outlined above, you can use an alternative workflow with the `receive()` method to pause the code in the receiving worker and wait until a message is sent. See the `receive()` method description for more information.

The MessageChannel class is one of the special object types that are shared between workers rather than copied between them. When you pass a message channel from one worker to another worker either by calling the Worker object's `setSharedProperty()` method or by using a MessageChannel object, both workers have a reference to the same MessageChannel object in the runtime's memory.

Related API Elements

Worker class
Worker.createMessageChannel() method

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
|  | messageAvailable : Boolean [read-only] Indicates whether the MessageChannel has one or more messages from the sending worker in its internal message queue. | MessageChannel
|  | state : String [read-only] Indicates the current state of the MessageChannel object (open, closing, or closed). | MessageChannel

Public Methods
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| Method | Defined By
--- | --- | ---
|  | addEventListener(type:String, listener:Function, useCapture:Boolean = false, priority:int = 0, useWeakReference:Boolean = false):void [override] Registers an event listener object with an EventDispatcher object so that the listener receives notification of an event. | MessageChannel
|  | close():void Instructs the current MessageChannel to close once all messages have been received. | MessageChannel
|  | dispatchEvent(event:Event):Boolean Dispatches an event into the event flow. | EventDispatcher
|  | hasEventListener(type:String):Boolean Checks whether the EventDispatcher object has any listeners registered for a specific type of event. | EventDispatcher
|  | 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
|  | propertyIsEnumerable(name:String):Boolean Indicates whether the specified property exists and is enumerable. | Object
|  | receive(blockUntilReceived:Boolean = false):* Retrieves a single message object from the queue of messages sent through this message channel. | MessageChannel
|  | removeEventListener(type:String, listener:Function, useCapture:Boolean = false):void [override] Removes a listener from the EventDispatcher object. | MessageChannel
|  | send(arg:*, queueLimit:int = -1):void Sends an object from the sending worker, adding it to the message queue for the receiving worker. | MessageChannel
|  | 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 [override] Returns the string representation of the specified object. | MessageChannel
|  | valueOf():Object Returns the primitive value of the specified object. | Object
|  | willTrigger(type:String):Boolean Checks whether an event listener is registered with this EventDispatcher object or any of its ancestors for the specified event type. | EventDispatcher

Events
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| Event | Summary | Defined By
--- | --- | --- | ---
|  | activate | [broadcast event] Dispatched when the Flash Player or AIR application gains operating system focus and becomes active. | EventDispatcher
|  | channelMessage | Dispatched each time the sending worker calls this MessageChannel object's send() method, indicating that a new message object is available in the MessageChannel instance's queue. | MessageChannel
|  | channelState | Dispatched when the value of the message channel's state property changes. | MessageChannel
|  | deactivate | [broadcast event] Dispatched when the Flash Player or AIR application operating loses system focus and is becoming inactive. | EventDispatcher

Property Detail

### messageAvailable

property

`messageAvailable:Boolean` [read-only]

Language Version: | ActionScript 3.0

Runtime Versions: | Flash Player 11.4, AIR 3.4

Indicates whether the MessageChannel has one or more messages from the sending worker in its internal message queue.

Implementation
`public function get messageAvailable():Boolean`

### state

property

`state:String` [read-only]

Language Version: | ActionScript 3.0

Runtime Versions: | Flash Player 11.4, AIR 3.4

Indicates the current state of the MessageChannel object (open, closing, or closed). The possible values for this property are defined as constants in the MessageChannelState class.

Implementation
`public function get state():String`

Related API Elements

MessageChannelState class
Method Detail

### addEventListener

()
method

`override public function addEventListener(type:String, listener:Function, useCapture:Boolean = false, priority:int = 0, useWeakReference:Boolean = false):void`

Language Version: | ActionScript 3.0

Runtime Versions: | Flash Player 11.4, AIR 3.4

Registers an event listener object with an EventDispatcher object so that the listener receives notification of an event. You can register event listeners on all nodes in the display list for a specific type of event, phase, and priority.

After you successfully register an event listener, you cannot change its priority through additional calls to `addEventListener()`. To change a listener's priority, you must first call `removeListener()`. Then you can register the listener again with the new priority level.

Keep in mind that after the listener is registered, subsequent calls to `addEventListener()` with a different `type` or `useCapture` value result in the creation of a separate listener registration. For example, if you first register a listener with `useCapture` set to `true`, it listens only during the capture phase. If you call `addEventListener()` again using the same listener object, but with `useCapture` set to `false`, you have two separate listeners: one that listens during the capture phase and another that listens during the target and bubbling phases.

You cannot register an event listener for only the target phase or the bubbling phase. Those phases are coupled during registration because bubbling applies only to the ancestors of the target node.

If you no longer need an event listener, remove it by calling `removeEventListener()`, or memory problems could result. Event listeners are not automatically removed from memory because the garbage collector does not remove the listener as long as the dispatching object exists (unless the `useWeakReference` parameter is set to `true`).

Copying an EventDispatcher instance does not copy the event listeners attached to it. (If your newly created node needs an event listener, you must attach the listener after creating the node.) However, if you move an EventDispatcher instance, the event listeners attached to it move along with it.

If the event listener is being registered on a node while an event is being processed on this node, the event listener is not triggered during the current phase but can be triggered during a later phase in the event flow, such as the bubbling phase.

If an event listener is removed from a node while an event is being processed on the node, it is still triggered by the current actions. After it is removed, the event listener is never invoked again (unless registered again for future processing).

Parameters

| `type:String` — The type of event.
| `listener:Function` — The listener function that processes the event. This function must accept an Event object as its only parameter and must return nothing, as this example shows: Copy
    function(evt:Event):void
The function can have any name.
| `useCapture:Boolean` (default = `false`)`` — Determines whether the listener works in the capture phase or the target and bubbling phases. If `useCapture` is set to `true`, the listener processes the event only during the capture phase and not in the target or bubbling phase. If `useCapture` is `false`, the listener processes the event only during the target or bubbling phase. To listen for the event in all three phases, call `addEventListener` twice, once with `useCapture` set to `true`, then again with `useCapture` set to `false`.
| `priority:int` (default = `0`)`` — The priority level of the event listener. The priority is designated by a signed 32-bit integer. The higher the number, the higher the priority. All listeners with priority n are processed before listeners of priority n-1. If two or more listeners share the same priority, they are processed in the order in which they were added. The default priority is 0.
| `useWeakReference:Boolean` (default = `false`)`` — Determines whether the reference to the listener is strong or weak. A strong reference (the default) prevents your listener from being garbage-collected. A weak reference does not. Class-level member functions are not subject to garbage collection, so you can set `useWeakReference` to `true` for class-level member functions without subjecting them to garbage collection. If you set `useWeakReference` to `true` for a listener that is a nested inner function, the function will be garbage-collected and no longer persistent. If you create references to the inner function (save it in another variable) then it is not garbage-collected and stays persistent.

### close

()
method

`public function close():void`

Language Version: | ActionScript 3.0

Runtime Versions: | Flash Player 11.4, AIR 3.4

Instructs the current MessageChannel to close once all messages have been received.

Once you call this method, you can no longer call the `send()` method to add messages to the queue. The `send()` call will fail and return `false`.

You can also only call the `receive()` method to receive messages that are already waiting in the queue. If the queue is empty, the `receive()` call will return `null`.

Events

| `channelState:`Event`` — dispatched when the `close()` method is called (which sets the `state` property is to `MessageChannelState.CLOSING`). Dispatched again when all the messages have been received and the `state` property is set to `MessageChannelState.CLOSED`.

### receive

()
method

`public function receive(blockUntilReceived:Boolean = false):*`

Language Version: | ActionScript 3.0

Runtime Versions: | Flash Player 11.4, AIR 3.4

Retrieves a single message object from the queue of messages sent through this message channel.

Each time the sending worker's code calls the MessageChannel object's `send()` method, a single object is added to the message channel's internal message queue. These objects stack up in the queue until they are removed one at a time by the receiving worker calling the `receive()` method. The message objects are received in the order they are sent.

To check if the queue contains a message object to receive, use the `messageAvailable` property.

In the standard case, the object passed into `send()` is serialized in AMF3 format. When it is removed from the queue by the `receive()` call, it is deserialized into an ActionScript object (a copy of the original object) in the receiving worker and the worker receives a reference to that copy. Certain types of objects are shared between workers rather than copied. In that case the object that the receiving worker gets is a reference to the shared object itself rather than a new copy of the object. For more information about this case see the `send()` method description.

The method's behavior changes if the message queue is empty and you pass `true` for the `blockUntilReceived` parameter. In that case the worker pauses its execution thread at the `receive()` call and does not execute more code. Once the sending worker calls `send()`, the `receive()` call completes by receiving the message. The worker then resumes code execution at the next line of code following the receive call.

Parameters

| `blockUntilReceived:Boolean` (default = `false`)`` — indicates whether the worker's execution thread should receive a message object and then continue execution (`false`), or if it should pause at the `receive()` call and wait for a message to be sent if the queue is empty (`true`)

Returns

| `*` — a copy of the object passed into the `send()` method by the sending worker. If the object is one of the special types that are shared between workers, the return value is a reference to the shared object rather than to a copy of it. If no message is available on the queue, the method returns `null`.

Throws

| `IOError` — if the channel is closed at the time the method is called or if the `blockUntilReceived` argument results in the execution being paused and the channel is then closed by another worker.
| `ArgumentError` — if the calling code is not in the receiving worker
| `ScriptTimeoutError` — if the method is called from code in the primordial worker in Flash Player and the `blockUntilReceived` argument causes the worker to pause longer than the script timeout limit (15 seconds by default)

Related API Elements

send() method
messageAvailable property

### removeEventListener

()
method

`override public function removeEventListener(type:String, listener:Function, useCapture:Boolean = false):void`

Language Version: | ActionScript 3.0

Runtime Versions: | Flash Player 11.4, AIR 3.4

Removes a listener from the EventDispatcher object. If there is no matching listener registered with the EventDispatcher object, a call to this method has no effect.

Parameters

| `type:String` — The type of event.
| `listener:Function` — The listener object to remove.
| `useCapture:Boolean` (default = `false`)`` — Specifies whether the listener was registered for the capture phase or the target and bubbling phases. If the listener was registered for both the capture phase and the target and bubbling phases, two calls to `removeEventListener()` are required to remove both, one call with `useCapture()` set to `true`, and another call with `useCapture()` set to `false`.

### send

()
method

`public function send(arg:*, queueLimit:int = -1):void`

Language Version: | ActionScript 3.0

Runtime Versions: | Flash Player 11.4, AIR 3.4

Sends an object from the sending worker, adding it to the message queue for the receiving worker.

The object passed to the `arg` parameter can be almost any object. Other than the exceptions noted below, any object that is passed to the `arg` parameter is not passed by reference. Any changes made to the object in one worker after `send()` is called are not carried over to the other worker. The object is copied by serializing it to AMF3 format and deserializing it into a new object in the receiving worker when `receive()` is called. For this reason, any object that can't be serialized in AMF3 format, including display objects, can't be passed to the `arg` parameter. In order for a custom class to be passed properly, the class definition must be registered using the `flash.net.registerClassAlias()` function or `[RemoteClass]` metadata. With either technique the same alias must be used for both worker's versions of the class.

There are five types of objects that are an exception to the rule that objects aren't shared between workers:

- Worker
- MessageChannel
- shareable ByteArray (a ByteArray object with its `shareable` property set to `true`
- Mutex
- Condition

If you pass an instance of these objects to the `arg` parameter, each worker has a reference to the same underlying object. Changes made to an instance in one worker are immediately available in other workers. In addition, if you pass the same instance of these objects more than once using `send()`, the runtime doesn't create a new copy of the object in the receiving worker. Instead, the same reference is re-used, reducing system memory use.

By default, this method adds the object to the queue and immediately returns, continuing execution with the next line of code. If you want to prevent the queue from growing beyond a certain size, you can use the `queueLimit` parameter to specify the maximum number of items to allow in the queue. If at the time you call `send()` the number of items in the queue is greater than the limit you specify, the worker pauses the execution thread at the `send()` call. Once the receiving worker calls `receive()` enough times that the queue size is less than the specified queue limit, the send() call completes. The worker then continues execution at the next line of code.

Parameters

| `arg:*` — the object to add to the message queue
| `queueLimit:int` (default = `-1`)`` — the maximum number of message objects that the message queue can contain. If the queue contains more objects than the limit, the sending worker pauses execution until messages are received and the queue size drops below the limit.

Events

| `channelMessage:`Event`` — dispatched to notify the receiving worker that a message object is available in the queue

Throws

| `IOError` — if the channel is closed at the time the method is called or if the `queueLimit` argument results in the execution being paused and the channel is then closed by another worker.
| `ArgumentError` — if the calling code is not in the sending worker
| `ScriptTimeoutError` — if the method is called from code in the primordial worker in Flash Player and the `queueLimit` argument causes the worker to pause longer than the script timeout limit (15 seconds by default)

Related API Elements

receive() method
Worker class
ByteArray.shareable property
Mutex class
Condition class

### toString

()
method

`override public function toString():String`

Language Version: | ActionScript 3.0

Runtime Versions: | Flash Player 11.4, AIR 3.4

Returns the string representation of the specified object.

Note: Methods of the Object class are dynamically created on Object's prototype. To redefine this method in a subclass of Object, do not use the `override` keyword. For example, a subclass of Object implements `function toString():String` instead of using an override of the base class.

Returns

| `String` — A string representation of the object.

Event Detail

### channelMessage

Event

Event Object Type: `flash.events.Event`
property Event.type = `flash.events.Event.CHANNEL_MESSAGE`

Language Version: | ActionScript 3.0

Runtime Versions: | Flash Player 11.4, AIR 3.4

Dispatched each time the sending worker calls this MessageChannel object's `send()` method, indicating that a new message object is available in the MessageChannel instance's queue.

The `Event.CHANNEL_MESSAGE` constant defines the value of the `type` property of a `channelMessage` event object.

This event has the following properties:

Property | Value
--- | ---
`bubbles` | `false`
`cancelable` | `false`; there is no default behavior to cancel.
`currentTarget` | The object that is actively processing the Event object with an event listener.
`target` | The object that dispatched this event.

### channelState

Event

Event Object Type: `flash.events.Event`
property Event.type = `flash.events.Event.CHANNEL_STATE`

Language Version: | ActionScript 3.0

Runtime Versions: | Flash Player 11.4, AIR 3.4

Dispatched when the value of the message channel's `state` property changes.

The `Event.CHANNEL_STATE` constant defines the value of the `type` property of a `channelState` event object.

This event has the following properties:

Property | Value
--- | ---
`bubbles` | `false`
`cancelable` | `false`; there is no default behavior to cancel.
`currentTarget` | The object that is actively processing the Event object with an event listener.
`target` | The object that dispatched this event.

© 2015 Adobe Systems Incorporated. All rights reserved.
Mon Apr 10 2017, 12:28 PM -07:00