The random utterances of David Arno

Consider, if you will, the following piece of non-language-specific code:

  void someMethod()
  {
    for (byte i=0; i<5; i++)
    {
      ...
    }
    for (int i=0; i<10; i++)
    {
      ...
    }
  }

If you are a C#, Java, C++ etc developer, such a piece of code might look decidedly unspectacular. Not so with ActionScript 3 (AS3) though as I recently discovered.

The equivalent in AS3 would be:

  function someMethod():void
  {
    for (var i:uint=0; i<5; i++)
    {
      ...
    }
    for (var i:int=0; i<10; i++)
    {
      ...
    }
  }

and this code won’t compile. Instead it will report an error due to variable i being redefined as a different type. Further, if you change i in the first loop to be an int too, the compiler will then issue a warning that you are redefining i.

Being used to languages such as Java, C# and C++, I put this down to a compiler bug, disabled what I thought was a pointless warning message in my projects and carried on happy I’d hidden an annoying bug. How wrong I was. I recently discovered a bug in my code was due to a variable defined inside a loop changing the value of a variable of the same name at the method-scope level. It turns out that AS3 doesn’t support block-level scope for variables. Variables can only have one of three scopes:

• Global
• Class
• Function

Whilst a function-level variable can override a class-level one, variables defined inside code blocks (such as loops, try blocks etc) have method-level scope.

The following code illustrates how AS3 variable scoping works:

<?xml version="1.0" encoding="utf-8"?>
<mx:Application xmlns:mx="http://www.adobe.com/2006/mxml"
                layout="absolute" width="450" height="200"
                borderColor="0" borderStyle="solid"
                borderThickness="1" backgroundColor="#ffffff">
  <mx:Button x="20" y="10" label="Go" click="go()" fontSize="14"/>
  <mx:Label x="20" y="50" text="aVariable v1:" fontSize="14"/>
  <mx:Label x="20" y="90" text="aVariable v2:" fontSize="14"/>
  <mx:Label x="20" y="130" text="aVariable v3:" fontSize="14"/>
  <mx:Label x="20" y="170" text="aVariable v4:" fontSize="14"/>
  <mx:Label x="130" y="50" text="" fontSize="14" id="label1"/>
  <mx:Label x="130" y="90" text="" fontSize="14" id="label2"/>
  <mx:Label x="130" y="130" text="" fontSize="14" id="label3"/>
  <mx:Label x="130" y="170" text="" fontSize="14" id="label4"/>

  <mx:Script>
    <![CDATA[
      private var aVariable:String = "1";

      private function go():void
      {
        label1.text = ""+aVariable;

        var aVariable:int = 2;
        label2.text = ""+aVariable;

        for (var aVariable:int=3; aVariable<4; aVariable++)
        {
          label3.text = ""+aVariable;
        }

        label4.text = ""+aVariable;
      }
    ]]>
  </mx:Script>
</mx:Application>

Tracing through the code, I would have expected the following output:

  aVariable v1: 1
  aVariable v1: 2
  aVariable v1: 3
  aVariable v1: 2

The resultant SWF is included below. The results are really rather different:

The version of aVariable defined in go() is in scope from the start of the method, so label1 picks up its default value, rather than the class-scoped variable’s value. And the loop uses the same variable, so label4 is 4, rather than 2.

For more information on AS3 variable scoping, I refer you to the “Understanding variable scope” section of Adobe’s official AS3 language reference.

You can download a zip of the FlexBuilder project used to create the above SWF here.