Flash CS5: The Missing Manual

The Art of Debugging

Imagine, for an instant, that your animation isn't behaving the way you think it should. Testing it on the stage or in Flash Player, and then eyeballing the results, as described in the previous section, is a good place to start tracking down the problem. But if you've added ActionScript to your animation, chances are you need more firepower. You need to be able to examine the inner workings of your ActionScript code—the variables, instance names, methods, and so on—to help you figure out what's wrong. Debugging is one of those activities that's part art, part craft, and part science. Flash and ActionScript provide several tools that help you track down and eliminate those pesky bugs of all types. These are some of the tools at your disposal:

• The Check syntax button catches the most obvious typos. If you've got too many parentheses in a line or you misplaced a comma or semicolon, the syntax checker is likely to notice. Still, it lets lots of the bad guys through.

• The Compiler Errors panel is the next layer of defense against bugs. If there's a flaw in your code's logic (for example, a reference to some object that doesn't exist or is misnamed), a message is likely to appear in the Compiler Errors panel. Sometimes your animation will run anyway; other times it won't.

• The Output panel displays messages. Using the trace() statement, you can display the values of variable and object properties in the Output panel. So, you get to tell ActionScript what to report on. ...