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Using Symbol Instances

A symbol instance is a pointer to the full description of the symbol. This is a space-efficient way of reusing vector objects. If you converted a graphic on the stage to a symbol, you already have one symbol instance on the Stage. If you want to use the symbol again, or if you created your symbol from scratch in symbol-editing mode, you'll need to get a copy out of the library and onto the Stage.

Why Use Symbols?

Symbols help you keep file sizes small. You've already learned how Flash uses vectors to hold down file size: Each vector shape is really just a set of instructions, a recipe for creating the shape. So you could duplicate a vector graphic that you want to reuse, and it would be smaller than a bitmapped version of the same graphic. But symbols are even more efficient than duplicate vector shapes.

A symbol is a master recipe. Imagine a busy restaurant that serves three kinds of soup—chicken noodle, cream of chicken rice, and chicken with garden vegetables—and each pot of soup has its own cook. The head chef could go over with each cook all the steps required to make a chicken broth. But that would involve a lot of repetition and take a lot of time. If the restaurant has a master recipe for chicken broth, the chef can simply tell all the cooks to make a pot of chicken broth and then tell each cook just those additional steps that distinguish each dish—add noodles for chicken noodle; add rice and cream for cream of chicken rice; add potatoes, carrots, and peas for garden vegetable.

Symbols act the same way in your movie file. The full recipe is in the library. Each instance on the Stage contains just the instructions that say which recipe to start with and how to modify it—for example, use the recipe for the red rectangle but make it twice as large, change the color to blue, and rotate it 45 degrees clockwise. Because symbols can themselves contain other symbols, it really pays to break your graphic elements into their lowest-common-denominator parts, make each individual part a symbol, and then combine the parts into larger symbols or graphics.



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