.AS File Format – Events

Handlers in the FLA

on(event) on buttons and movie clips, and onClipEvent(event) on movie clips, are written in the Actions panel on the instance and saved in the FLA, never in a class file. Their events are those of ActionScript 1.0: press, release, releaseOutside, rollOver, rollOut, dragOver, dragOut and keyPress for on; load, unload, enterFrame, mouseDown, mouseUp, mouseMove, keyDown, keyUp and data for onClipEvent. With classes, the same work is normally done by handler methods instead.

Handler properties and methods

An object whose class has a method named after an event receives it. A movie clip subclass simply defines onPress, onEnterFrame and the rest as methods:

class com.example.Draggable extends MovieClip {
    public function onPress():Void {
        startDrag();
    }
    public function onRelease():Void {
        stopDrag();
        dispatchDrop();
    }
    public function onReleaseOutside():Void {
        onRelease();
    }
    private function dispatchDrop():Void {
        _parent.itemDropped(this, _droptarget);
    }
}

A movie clip class’s onLoad method runs when an instance is created, after the constructor; setting onLoad on a clip that already exists has no effect.

Listener objects

AsBroadcaster.initialize(obj) gives any object the same addListener, removeListener and broadcastMessage(eventName, args…). It was undocumented in Flash MX, and appears in Adobe’s later ActionScript 2.0 reference.

EventDispatcher

The version 2 components of Flash MX 2004 and their framework use mx.events.EventDispatcher, a class shipped as source. It adds three methods to an object or to a class’s prototype, and events are objects with at least a type and a target:

import mx.events.EventDispatcher;

class com.example.Timer {
    // filled in by EventDispatcher.initialize
    public var addEventListener:Function;
    public var removeEventListener:Function;
    private var dispatchEvent:Function;

    public function Timer() {
        EventDispatcher.initialize(this);
    }
    public function finish():Void {
        dispatchEvent({type: "complete", target: this});
    }
}

// elsewhere
var t:Timer = new Timer();
t.addEventListener("complete", Delegate.create(this, onTimerDone));

A listener can be a function, an object with a method named after the event type, or an object with a handleEvent method. The component events, such as a Button’s click or a List’s change, are received this way. The declarations of the three function members are only there to satisfy the compiler.

Delegate

Handlers and listeners run with this set to the object that calls them. mx.utils.Delegate.create(scope, method), from the Flash MX 2004 7.2 update, returns a function that calls method with this set to scope, so a class can pass its own methods as handlers; see Functions.

Timers

For players before Flash Player 8, a single delayed call is made with setInterval, cleared on its first call.