Introduction to working with movie clips
Movie clips are a key element for people who create animated content with the Flash authoring tool and want to control that content with ActionScript. Whenever you create a movie clip symbol in Flash, Flash adds the symbol to the library of that Flash document. By default, this symbol becomes an instance of the MovieClip class, and as such has the properties and methods of the MovieClip class.

When an instance of a movie clip symbol is placed on the Stage, the movie clip automatically progresses through its timeline (if it has more than one frame) unless its playback is altered using ActionScript. It is this timeline that distinguishes the MovieClip class, allowing you to create animation through motion or shape tweens through the Flash authoring tool. By contrast, with a display object that is an instance of the Sprite class, you can create animation only by programmatically changing the object's values.

In previous versions of ActionScript, the MovieClip class was the base class of all instances on the Stage. In ActionScript 3.0, a movie clip is only one of many display objects that can appear on the screen. If a timeline is not necessary for the function of a display object, using the Shape class or Sprite class in lieu of the MovieClip class may improve rendering performance. For more information on choosing the appropriate display object for a task, see Choosing a DisplayObject subclass.

Common movie clip tasks
The following common movie clips tasks are described in this chapter:

Making movie clips play and stop
Playing movie clips in reverse
Moving the playhead to specific points in a movie clip's timeline
Working with frame labels in ActionScript
Accessing scene information in ActionScript
Creating instances of library movie clip symbols using ActionScript
Loading and controlling external SWF files, including files created for previous Flash Player versions
Building an ActionScript system for creating graphical assets to be loaded and used at run time
Important concepts and terms
The following reference list contains important terms used in this chapter:

AVM1 SWF: A SWF file created using ActionScript 1.0 or ActionScript 2.0, usually targeting Flash Player 8 or earlier.
AVM2 SWF: A SWF file created using ActionScript 3.0 for Adobe Flash Player 9 or Adobe AIR.
External SWF: A SWF file that is created separately from the project SWF file and is intended to be loaded into the project SWF file and played back within that SWF file.
Frame: The smallest division of time on the timeline. As with a motion picture filmstrip, each frame is like a snapshot of the animation in time, and when frames are played quickly in sequence, the effect of animation is created.
Timeline: The metaphorical representation of the series of frames that make up a movie clip's animation sequence. The timeline of a MovieClip object is equivalent to the timeline in the Flash authoring tool.
Playhead: A marker identifying the location (frame) in the timeline that is being displayed at a given moment.