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Essential ActionScript 3.0 by Colin Moock

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Control-Flow Changes in try/catch/finally

As we've seen throughout this chapter, the throw statement changes the flow of a program. When ActionScript encounters a throw statement, it immediately stops what it's doing and transfers program control to eligible catch and finally blocks. However, it is also quite legal for those catch and finally blocks to change program flow again via return (in the case of a method or function) or break or continue (in the case of a loop). Furthermore, a return, break, or continue statement can also appear in a try block.

To learn the rules of flow changes in the try/catch/finally statement, let's look at how the return statement affects program flow in a try, catch, and finally block. The following code examples contain a function, changeFlow( ), which demonstrates a control flow in various hypothetical situations.

Example 13-1 shows a return statement in a try block, placed before an error is thrown. In this case, the method returns normally, and no error is ever thrown or handled. However, before the method returns, the finally block is executed. Note that you're unlikely to see code exactly like Example 13-1 in the real world. In most applied cases, the return statement would occur in a conditional statement and execute in response to some specific condition in the program.

Example 13-1. Using return in try, before throw

public function changeFlow ():void {
  try {
    return;
    throw new Error("Test error."); } catch (e:Error) { trace("Caught: " + e.message); ...

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