.AS File Format – Functions

ActionScript 2.0

Functions in ActionScript 2.0 are the functions of ActionScript 1.0 with optional types on their parameters and results. In class files they are methods; in FLA scripts and include files they are also free-standing functions and function literals, as before.

Declaring functions

FormExample
Method, in a classpublic function area():Number { return w * h; }
Declaration, in an FLA scriptfunction clamp(n:Number, lo:Number, hi:Number):Number { … }
Function literalvar double:Function = function (n:Number):Number { return n * 2; };
Handler propertystart_btn.onRelease = function ():Void { … };
  • A parameter or result without a type is unchecked.
  • Void as the return type says the function returns nothing; a return with a value is then an error.
  • There are no default parameter values and no rest parameters. A caller may still pass fewer arguments, which are undefined, or more, which are only in arguments.
  • There is no overloading: a class has one method of each name, and a subclass method of the same name overrides it.
  • The type of a function value is Function; its parameter types are not part of it.

The arguments object

MemberIs
arguments[i], arguments.lengthThe arguments passed, from 0, and their number
arguments.calleeThe function being run
arguments.callerThe function that called it

Methods as callbacks

A method stored as a handler or passed to another object loses its object: when it runs, this is whatever object calls it, such as the button for an onRelease handler. Inside a class, members compiled as this.name then point at the wrong object. Three ways round this were used:

import mx.utils.Delegate;

class com.example.Player {
    private var play_btn:Button;
    private var playing:Boolean = false;

    public function Player(button:Button) {
        play_btn = button;

        // 1. Delegate: a function that calls toggle with this set to the Player
        play_btn.onRelease = Delegate.create(this, toggle);

        // 2. A local variable kept by a nested function
        var owner:Player = this;
        play_btn.onRollOver = function () { owner.hint(); };

        // 3. setInterval's object form
        setInterval(this, "tick", 1000);
    }
    private function toggle():Void { playing = !playing; }
    private function hint():Void { trace("Play or pause"); }
    private function tick():Void { if (playing) trace("tick"); }
}
Keeping this when a method is called back.

Delegate.create(scope, method) comes with Flash from the Flash MX 2004 7.2 update. It is an ordinary class, compiled into each movie that uses it. It passes on the callback’s arguments but not its original this.

Functions as objects

MemberDoes
f.call(thisObject, arg1, …)Calls the function with this set.
f.apply(thisObject, argsArray)The same, with the arguments in an array.
f.prototypeThe prototype of objects the function makes with new; for a class, where its methods are.

A class is itself a function, its constructor, so typeof Player is "function".

Limits

  • The player stops a script that recurses more than 256 calls deep.
  • A script that runs longer than the time limit, 15 seconds by default, makes the player offer to stop it.
  • From Flash Player 7, functions compiled for that player keep their local variables, arguments and this in fast registers, and run faster than those of earlier versions; see Compiler.