Why do I get error when using yield?

Questions & Answers
1

I'm trying to create a loading bar for loading the next level/scene. I'm following unity3d's scripting manual but i get and error. here is the code:

   var loadLevel:boolean = false;
   var async:AsyncOperation;

   function OnGUI(){
    if(GUI.Button(Rect(0,0,100,50),"LoadLevel 1")){
    loadLevel = true;
    }
   }
   function Update(){
    if(loadLevel){
      async = Application.LoadLevelAsync ("Level_01");
      yield async;
      if(async.isDone){
        print ("Loading complete");
       }
     }
    }

The error is: "Script error: Update() can not be a coroutine." when i remove the yield the error is gone but the result is not what i expect. It prints nothing:( ?

2

Update runs every frame without exception and cannot be a coroutine. (Neither can OnGUI.) You won't find any example code in the scripting manual that has yield in Update. Instead of running checks every frame in Update, just call a coroutine when the button is pressed.