how do I make non-document-class classes 'aware' of stage components in Flash AS3?

Question

I am working on a text adventure game which will have at least a few components (a text area for narrative and text input for user input) on the stage at all times. Therefore, I have created those components statically through Flash's WYSIWYG design environment. I gave them instance names "myTA" and "myTI" respectively. I was able to get my main class (the document class for the stage) to interact with them (dynamically adding text one character at a time like a typewriter at runtime), but other classes in the same package don't seem able to recognize the stage components. Below is the relevant code:

Case A, in which everything happens within the Main class:

package { 
    public class Main extends MovieClip { 
      public var myTA:TextArea; 
      var displayedChar:String = new String(); 
      var textToWrite:String = new String(); 
      var i:int = 0; var intervalId:uint; 
      var done:int = 0; 

      public function Main { 
       setUpTA(); 
      } 

      public function setUpTA(){ 
       myTA.text = "" + playAtInterval("Hello Player!"); 
      }         

      public function writeCharsSlowly(){     
        textToWrite = arguments[0];     
        displayedChar=textToWrite.substring(i,i+1); 
        myTA.appendText(displayedChar);                             
        i++;     
        if (i == textToWrite.length) {         
          done = 1;         
          clearInterval(intervalId);     
        }             
      }                 

      public function playAtInterval(theText:String) {                
        i = 0;              
        intervalId = setInterval(writeCharsSlowly, 100, theText);           
      }  
    } 
}

Case B, where Main calls on a second class 'TypeWriter' to handle the typewriter-printing:

Main:

package { 
    public class Main extends MovieClip { 
      public var myTA:TextArea; 
      public var myTI:TextInput; 
      var str:String = new String(); 

      public function Main{ 
        testTypeWriter(); 
      } 

      public function testTypeWriter(){ 
        typeW.playAtInterval("Hello Player");
        typeW.addEventListener(MouseEvent.CLICK,testTypeWriter2);
        typeW.addEventListener(KeyboardEvent.KEY_DOWN,inputEngine2) 
        addChild(typeW); 
      } 

      public function testTypeWriter2(event:MouseEvent){ 
        if (myTI.text == "a") {   
          typeW.playAtInterval("yo");  
        } else {   
          typeW.playAtInterval("Greetings, I am a test...");  
        }             
        addChild(typeW); 
      } 

      public function inputEngine2(event:KeyboardEvent){ 
        str = String.fromCharCode(event.charCode); 
        myTI.appendText(str); 
      }

TypeWriter:

package { 
    public class TypeWriter extends MovieClip { 
      public var myTI:TextInput; 
      public var myTA:TextArea; 
      var i:int = 0; 
      var done:int = 0; 
      var intervalId:uint; 
      var displayedChar:String = new String(); 
      var textToWrite:String = new String(); 

      public function TypeWriter(){ 
        ///nothing here 
      } 

      public function writeCharsSlowly(){     
        textToWrite = arguments[0];     
        displayedChar = textToWrite.substring(i,i+1);
        myTA.appendText(displayedChar);                             
        i++;     
        if (i == textToWrite.length) {         
          done = 1;         
          clearInterval(intervalId);     
        }             
      }                 

      public function playAtInterval(theText:String) {                
        i = 0;              
        intervalId = setInterval(writeCharsSlowly, 100, theText);           
      }  
    } 
}

Case A works, but in case B Flash is giving me the error "Error #1009: Cannot access a property or method of a null object reference" and notes the first line in TypeWriter where I try to operate on myTA as the problem.

how can I make other classes besides the document class 'aware' of existing stage components?

Thanks,

CCJ

Allan Allan · Accepted Answer · 2010-12-03T04:01:41

I would recommend the Service Locator Pattern for this. The most naive approach would be to create a resource class which contains public static variables. Then in your document class you assign the stage instances to the corresponding static variable in the resource class. Then you can simply access these stage components anywhere.

var someTextArea = Resource.TA; //probably should rename to something more meaningful

For something a little more ingenious you should read the article I linked to.

I think this is better than the dependency injection as constructor injection could lead to huge parameter list as you might add more items to the stage, and I am not so fond on setter injection as it is easy to forget to set them.

EDIT:

Just to make it a bit more clear I thought I would add some code :)

Resource class

package
{
    //TODO imports
    public class Resource
    {
        public static var TA:TextArea;
        public static var TI:TextInput;
    }
}

Document class

//....setup function
Resource.TA = myTA; //myTA is the name of the instance on stage
Resource.TI = myTI;

Foo class

Resource.TA.x = 100;
//or
_myClassMemberVariable = Resource.TA;
_myClassMemberVariable.x = 100;

how do I make non-document-class classes 'aware' of stage components in Flash AS3?

5 Answers