Flash MX Prototyping Basics

An important factor when writing prototypes is getting your paths correct. In the centerAlign function, the _x and _y are set using this as the target. That's because it's written in the scope of the movie clip and not _root where the code is actually written. Let's run through some examples of what can go wrong.

You have a movie clip that has a child movie clip and they both have the same instance name, let's call them both subMenu. You write a new prototype in the first key frame of _root that should change the position of the child clip, like expanding a sub menu.  Inside the prototype you have:

subMenu._x += 100;

That's not actually referring to the child as you might imagine, instead it is referring to the main movie clip in _root. To get the scope right, you must refer to it like:

this.subMenu._x += 100;

A similar problem occurs when using variables. If the variable is not used in the right scope, all kinds of chaos can arise. A typical example of variable problems might be when a user reports "I'm sure the data is getting added correctly but when I try and use it it's not there". That's because the data has been successfully added to a variable in a different scope.

MovieClip.prototype.mover = function()
   {
      this.createEmptyTextField("debugText", 1, 0, 0, 100, 10);
      this.debugText.variable = debugVar;
      debugVar = "This is in the wrong scope";     
      // The correct way to address this
      // this.debugVar = "This is the correct scope";
   }
myMovie.mover();
// This will return undefined
trace(myMovie.debugVar);
// This shows that the sentence we meant to assign to the text box in myMovie was actually created in the parent.
Trace(debugVar);