Scope in ActionScript 3 is so fubared I can barely express it in words
Published 20 October 2008
Hi! You've stumbled upon a blog post by a guy named Ryan. I'm not that guy anymore, but I've left his posts around because cool URIs don't change and to remind me how much I've learned and grown over time.
Ryan was a well-meaning but naïve and priviledged person. His views don't necessarily represent the views of anyone.
This looks wrong:
package {
public class ScopeFu {
private static var var0:String = 'var 0';
public static var var1:String = 'var 1';
private var var2:string = 'var 2';
public var var3:String = 'var 3';
public function ScopeFu(var4:String = 'var 4') {
var var5 = 'var 5';
trace(var0);
trace(var1);
trace(var2);
trace(var3);
trace(var4);
trace(var5);
}
}
}And yet it works.
Compared to JavaScript:
var ScopeFu;
(function () {
var var0 = "var 0";
ScopeFu = function (var4) {
this.var2 = "var 2";
this.var3 = "var3";
var4 = var4 || "var 4";
var var5 = "var 5";
console.log(var0);
console.log(ScopeFu.var1);
console.log(this.var2);
console.log(this.var3);
console.log(var4);
console.log(var5);
};
ScopeFu.var1 = "var 1";
})();Still a mess. The JavaScript syntax is a little more cryptic and var2 isn't private, yet there are half as many scope collisions. The ones that do exist are only because JavaScript doesn't really support private members; I'm using closures to fake it.
In either case, scope can become a nightmare in large applications when you're trying to figure out where something is defined, especially when you're jumping back and forth between the otherwise-similar languages. Underscores and access methods can help, so long as you are disciplined enough to be consistent:
| Scope | Prefix | Access | Example |
|---|---|---|---|
| Private Static | Three Underscores | local | ___var0 |
| Public Static | none | via class | ScopeFu.var1 |
| Private Instance | Two Underscores | this (JavaScript only) | this.__var2 or __var2 |
| Public Instance | none | this | this.var3 |
| Function Argument | One Underscore | local | _var4 |
| Function Member | none | local | var5 |
Adopting these kinds of conventions may help keep scope sorted amongst a group of developers.