ActionScript 2.0 Overview

ActionScript 1.0 is based on the ECMAScript 3 standard (as is JavaScript 1.5), whereas ActionScript 2.0 is based on the emerging ECMAScript 4 standard (as is the theoretical JavaScript 2.0). As we learned in the Preface, under “ActionScript 2.0 Versus ActionScript 1.0,” this common heritage gives the two versions a strong family resemblance; they share the same syntax for most non-OOP features, such as loops, conditionals, and operators.

Although ActionScript 2.0 is now the preferred version of ActionScript, ActionScript 1.0 syntax continues to be fully supported by Flash Player 7 and is not deprecated. As we’ll see shortly, you can author either ActionScript 1.0 or ActionScript 2.0 in Flash MX 2004 and Flash MX Professional 2004 (but you cannot author ActionScript 2.0 in Flash MX). With a few minor exceptions, noted throughout the text, ActionScript 2.0 code is also backward compatible with Flash Player 6. However, ActionScript 2.0 is not compatible with older versions such as Flash Player 5 or Flash Player 4.

If you’re an ActionScript 1.0 programmer, you can think of ActionScript 2.0 as a syntactic façade over ActionScript 1.0. That is, both ActionScript 2.0 and ActionScript 1.0

compile to the same .swf bytecode (with a few minor additions for ActionScript 2.0). To the Flash Player, at runtime, there’s effectively no difference between ActionScript 1.0 and ActionScript 2.0 (barring the aforementioned minor additions). For example, once an ActionScript 2.0 class, such as Rectangle, is compiled to a .swf file, it exists as a Function object at runtime, just as an older ActionScript 1.0 function declaration used as a class constructor would. Similarly, at runtime, an ActionScript 2.0 Rectangle instance (r) is given a __proto__ property that refers to Rectangle.prototype, again making it look to the Flash Player just like its ActionScript 1.0 counterpart.

But for the most part, you don’t need to worry about these behind-the-scenes compiler and runtime issues. If you’re moving to ActionScript 2.0 (and I think you should!), you can permanently forget ActionScript 1.0’s prototype-based programming. In fact, most ActionScript 1.0 techniques for dynamically manipulating objects and classes at runtime are considered bad practice in ActionScript 2.0, and will actually lead to compiler errors when mixed with ActionScript 2.0 code. But never fear, this book highlights problematic ActionScript 1.0 practices and show you how to replace them with their modern ActionScript 2.0 counterparts.

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