Connecting Movie Clips in _root to AS Classes

AS classes, derived from the movieClip class, can be used to govern the behavior and appearance of movie clips on the stage. The AS class can:

define method handlers for movie clip events,
manipulate movie clip properties, such as _alpha.

Instances of a AS movieClip class are per default not connected to any clip on the stage.
Also instantiating a movieClip subclass does not create a clip on stage (in the _root hierarchy).

There are two ways to create a movieClip on stage and connect it to an AS class.

1) Connect a Symbol from the Library to an AS Class

A movie clip template existing as symbol in the library can be connected to an AS class with its linked identifier. This is called "registering":

Afterwards the AS class and its event handlers are automatically associated with every instance of the movie clip.

When a symbol is placed on the stage an instance of the movie clip is created as well as an instance of the connected AS class.

2) Load the Movie Clip from an SWF File and Connect "by hand"

A movie clip subclass (derived from the MovieClip class) can be associated with a movie clip on the stage via the _prototype attribute:

class Ball extends MovieClip 
{
  function Ball()
  {
      _root.createEmptyMovieClip( "ball", 100 );
      this.prototype = _root["ball"];
  }
}

The "ball" can be loaded directly in the AS class from an SWF file:

  var mcl:MovieClipLoader = new MovieClipLoader();
  mcl.loadClip( "ball.swf", _root["ball"] );