Movie Clips (ActionScript: The Definitive Guide)

13.1. The "Objectness" of Movie Clips

As of Flash 5, movie clips can be manipulated like the objects we learned about in Chapter 12, "Objects and Classes". We may retrieve and set the properties of a clip, and we may invoke built-in or custom methods on a clip. Unlike other objects, an operation performed on a clip may have a visible or audible result in the Player.

Movie clips are not truly a type of object; there is no MovieClip class or constructor, nor can we use an object literal to instantiate a movie clip in our code. So what, then, are movie clips if not objects? They are members of their very own object-like datatype, called movieclip (we can prove it by executing typeof on a movie clip, which returns the string "movieclip"). The main difference between movie clips and true objects is how they are allocated (created) and deallocated (disposed of, or freed). For details, see Chapter 15, "Advanced Topics". Despite this technicality, however, we nearly always treat movie clips exactly like objects.

So how does the "objectness" of movie clips affect our use of them in ActionScript? Most notably, it dictates the way we control clips and examine their properties. Movie clips can be controlled directly through built-in methods. For example:

eyes.play( );

We can retrieve and set a movie clip's properties using the dot operator, just as we would access the properties of any object:

ball._xscale = 90;
var radius = ball._width / 2;

A variable in a movie clip is simply a property of that clip, and we can use the dot operator to set and retrieve variable values:

myClip.myVariable = 14;
x = myClip.myVariable;

Submovie clips can be treated as object properties of their parent movie clips. We therefore use the dot operator to access "nested" clips:

clipA.clipB.clipC.play( );

and we use the reserved _ parent property to refer to the clip containing the current clip:

_ parent.clipC.play( );

Treating clips as objects affords us all the luxuries of convenient syntax and flexible playback control. But our use of clips as objects also lets us manage clips as data; we can store a movie clip in an array element or a variable and even pass a clip reference to a function as an argument! Here, for example, is a function that moves a clip to a particular location on the screen:

function moveClip (clip, x, y) {
  clip._x = x;
  clip._y = y;
}
moveClip(ball, 14, 399);

Throughout the rest of this chapter, we'll learn the specifics of referencing, controlling, and manipulating movie clips as data objects.

Chapter 13. Movie Clips

Contents:

13.1. The "Objectness" of Movie Clips