Quick Tip: Understanding getDefinitionByName()

In this Quick Tip, you’ll learn how to construct a reference to a class from a String, and then create an instance of that class, using a built-in AS3 function called getDefinitionByName(). You’ll also learn the best methods for using this in different situations.

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Why Is getDefinitionByName() Useful?

getDefinitionByName() is very useful if you need to make new instances of classes by using a String. For example, if you had seven different tiles – each represented by a class called Tile1, Tile2, etc – and you needed to create an instance of each, you would have to write the following code:

private function createTiles():void
{
 var tile1:Tile1 = new Tile1();
 var tile2:Tile2 = new Tile2();
 var tile3:Tile3 = new Tile3();
 var tile4:Tile4 = new Tile4();
 var tile5:Tile5 = new Tile5();
 var tile6:Tile6 = new Tile6();
 var tile7:Tile7 = new Tile7();
 stage.addChild( tile1 );
 stage.addChild( tile2 );
 stage.addChild( tile3 );
 // You get the idea, it is very lengthy!
}

getDefinitionByName() allows you to solve this problem!

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How to Use It

Now, the above code was a little messy and we had to type many lines just to make a few different tiles. This is how we could achieve the same goal using getDefinitionByName():

private function createTiles():void
{
 for( var i:int = 1; i < 8; i++ )
 {

  var tileRef:Class = getDefinitionByName( "Tile" + i ) as Class;
  var tile:tileRef = new tileRef();
  stage.addChild( tile );

 }
}

In line 6, getDefinitionByName() returns a reference of the class called “Tile + the current value of i in the for-loop“. So, when i is equal to 1, getDefinitionByName("Tile" + i); returns a reference to the class Tile1. We then create the tile and add it to the stage.

However, when you run this code, it will not work! You’ll get a variable is undefined error message, in most cases, because “Tile1″ might not be enough information for Flash to find the class. Let’s look at some workarounds.

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Make It Work

There are a few commonly-used methods to solve the problem of the variable is undefined error you’ll get when you run the above code, and I am going to teach you what they are. I would also like to give credit to Gert-Jan van der Well of Floorplanner Tech Blog for this blog post.

Here are some of the methods you can use [...]

Quick Tip: Understanding getDefinitionByName()