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Using Symbols and the Library in Adobe Flash CS4

#28. Distinguishing Between Graphic Symbols and Movie Clips

While button symbols are used for a single, specialized purpose, graphic symbols and movie clips have broader applications. Although graphic symbol sounds like something that contains a still image, and movie clip sounds like something that contains motion, the truth is that either type of symbol can have either type of content. The differences lie elsewhere.

If you’re creating a symbol, and you don’t know whether to make it a graphic symbol or a movie clip, consider these issues:

  • Dependent or independent? Any instance of a graphic symbol is dependent on the pace of the movie it’s placed in. If the symbol contains animation that takes 52 frames to complete, then you’ll probably want the instance to remain on the stage for exactly 52 frames. If the movie ends before those frames have elapsed, the action in the symbol will appear to be interrupted in the middle.

    In contrast, a movie clip runs independently of the movie it’s in. As long as a movie clip is on the stage, its animation will continue, even if the main movie ends (Figure 28a). For that matter, when the action in the movie clip reaches its end, it will start playing all over again from the beginning and loop repeatedly—unless you deliberately stop it with ActionScript.

    Figure 28a

    Figure 28a The rotation of this pinwheel is contained in a movie clip. Even after the animation of the child is over, the pinwheel will continue to turn.

  • Passive or interactive? ActionScript can’t control instances of graphic symbols—in fact, it doesn’t even recognize that they’re there. For that reason, graphic symbols are useful mostly for traditional animation—the kind that the user watches, but doesn’t interact with.

    In contrast, a movie can use ActionScript to control the movie clip instances within it, and any of those instances can use ActionScript to control any of the others. (In fact, many interactive movies are only one frame long. The action of the movie takes place not in the timeline, but in sequential commands issued to movie clips by ActionScript.)

    The fact that a movie clip instance can be controlled by ActionScript doesn’t mean it has to be. It’s fine to have a movie clip that simply plays animation. However, in a situation in which a graphic symbol or a movie clip would serve equally well, it makes sense to go with the graphic symbol. Because of their more limited capabilities, graphic symbols take up less space in the SWF file and make fewer demands on the computer’s processor.

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