Question

I have couple of questions about AS3 variables handling by AVM/compiler/scope

.1. This code in Flash will throw an error:

function myFunction() {
    var mc:MovieClip=new MovieClip();
    var mc:MovieClip=new MovieClip();
}

but it won`t throw an error in Flex (only warning in Editor). Why?

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.2. How Flash sees variables in loops? Apparently this:

for (var i:int=0; i<2; i++) {
    var mc:MovieClip=new MovieClip();
}

isn`t equal to just:

var mc:MovieClip=new MovieClip();
var mc:MovieClip=new MovieClip();

because it will throw an error again as earlier in Flash, but in Flex in function not? Is Flash changing somehow my loop before compilation?

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.3. Where in a class in equivalent to timeline in Flash - where in class I would put code which I put normally on timeline (I assume it is not constructor because of what I have written earlier, or maybe it`s a matter of Flash/Flex compiler)?

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@fenomas thanks for explaining, but I checked 1. answer and it is not enitirely true 🙂 this code:

function myFunction() {
    var mc:MovieClip=new MovieClip();
    mc.graphics.beginFill(0x0000FF);
    mc.graphics.drawRect(0,0,100,100);
    mc.graphics.endFill();
    addChild(mc);
    var mc:MovieClip=new MovieClip();
    mc.graphics.beginFill(0x000000);
    mc.graphics.drawRect(0,0,30,30);
    mc.graphics.endFill();
    addChild(mc);
}
myFunction();

will compile in Flash in strict mode but with warning mode turned off and won`t throw an error during compile or runtime.

And it will also compile and execute nicely in Flex (event with -strict -warnings compiler commands) (checked with Flash CS3 and FlashBuilder 4).

The same code, but not wrapped in function will generate compile time error regardless off any error modes turned on (strict/warning)in Flash.

Is that what @back2dos said about Flash Compiler that behaves weirdly?

What is the differences between these two compilers Flash/Flex (why I have to change errors mode in Flash while Flex does not care about anything:) )?

Answer 1

Well, I will explain to you, how package level ActionScript (classes and global functions) is scoping.

The var statement declares a variable within the scope of the function body it is in. It’s visibility is within the whole body. thus the following is completely valid.

a = 3;
if (Math.random()>0.5) {
    var a:int = 0;
}
else {
    a = 6;
}

this is horrible, but it’s based on the abandonend ECMA-Script draft AS3 is based on … yay! 🙁

for simplicity, imagine that all variable declarations are actually at the start of the containing function body (while their initialisation is actually performed in the place where you put it)

thus

for (var i:int=0; i<2; i++) {
    var mc:MovieClip=new MovieClip();
}

is equal to

var i:int, mc:MovieClip;
for (i=0; i<2; i++) {
    mc=new MovieClip();
}

the first piece of code from your first question to a duplicate variable defininition, which causes a compiler warning, but works as if you had made only one declaration.

as for your third question: there is no equivalent at all.

AS3 in the flash IDE and many designer friendly concepts (such as frames) are highly ambiguous. from a developer’s point of view the flash IDE is about the worst piece of cr*p you can get for money (which stop it from being a great tool for design, drawing and animation). if you want clear and consistent behaviour, I advise you not to use the flash IDE for compiling ActionScript or to waste time on trying to find out why it behaves so weirdly. Apart from its quirks, it takes a long time to compile and the strange things it does to your ActionScript (such as converting local variable declaration to instance field declaration (which is probably the source of your problem)).