AS1 OOP: Object Basics
         by senocular  

MovieClips as Objects
Youíve probably heard of movieclips being called movieclip objects. The reason for that is that movieclips are themselves just as much an object as any other object. They have their own properties, can have added to those more properties and can contain yet other objects or movieclips within themselves which behave similarly. Basically the only difference is that with movieclips, you can physically see the object as itís a visual object with a timeline. Other visual objects include buttons and textfields, though movieclips have a greater importance because they are the only type of those visual objects that can have other nested timelines (movieclips) within them. That is not the case so much with buttons and textfields.

Movieclips are also a special case as they exist as soft references. Because of a movieclipís timeline and the fact that its visually seen on the screen, itís handled differently from other objects. With other objects, if you delete all or have no remaining reference variables to an object in memory, Flash sees that object as non-existent and forgets it ever existed (Actionscript handles garbage collection on its own in that manner). However, the only method of removing a movieclip is through using the removeMovieClip() method. This method physically extracts and removes the movieclip from the timeline it which exists. Any references to that movieclip can still very well exist in your Actionscript even though the movieclip itself does not. These remaining soft references technically still point to the movieclip object but do so blindly. All is not lost, however, since if the movieclip returns to the stage, the reference will be able to again correctly reference the clip.

Itís like sending smoke signals to your best friend in a far off land who doesnít have a fire. You know you can give him messages but he has no way of returning them. What happens when your friend moves (or gets eaten by a horde of rabid buffalo)? You would have no way of knowing that so you would continue sending your smoke signals as though nothing has changed even though no one is able to read them. Should your friend ever come back, he will again be able to receive those smoke signals.

Take a look at the following example.

// create a temporary movieclip
this.createEmptyMovieClip("temp",1);

 
// assign a reference variable
tempRef = temp;
trace(temp); // traces _level0.temp
trace(tempRef); // traces _level0.temp

 
// remove the movieclip
 
temp.removeMovieClip();
 
trace(tempRef); // traces nothing
trace(typeof tempRef); // traces "movieclip"

 
// re-create the movieclip
this.createEmptyMovieClip("temp",1);
trace(tempRef); // traces _level0.temp

You can see there, that the tempRef variable, though the movieclip was removed, remained defined in the script. It had no base value when traced, though it still knew it was a movieclip reference. When the movieclip was reattached (using the same name as it was when originally created), the tempRef variable again was able to correctly reference that clip once again.

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