title: SpriteKit -use of AppDelegate - undeclared identifier
body: <p>I'm writing the game in cocos2d (from Pablo Ruiz's book).
  Right now I have to create pause screen, and, according to the book, I have to create new function in AppDelegate.m (and in .h file) :</p>
  
  <pre><code>+(AppDelegate *) get {
  
  return (AppDelegate *) [[UIApplication sharedApplication] delegate];
  }
  </code></pre>
  
  <p>I'm getting errors : Expected a type; expected expression; Missing '[' at start of message send expression; Use of undeclared identifier 'AppDelegate'.</p>
  
  <p>In another file, called GameScene.m, I created those functions:</p>
  
  <pre><code>-(void)resume
  {
  if(![AppDelegate get].paused)
  {
      return;
  }
  [AppDelegate get].paused = NO;
  [self onEnter];
  }
  
  -(void)onExit
  {
  if(![AppDelegate get].paused)
  {
      [AppDelegate get].paused = YES;
      [super onExit];
  }
  }
  
  -(void)onEnter
  {
  if(![AppDelegate get].paused)
  {
      [super onEnter];
  }
  }
  </code></pre>
  
  <p>And I'm getting another set of errors : Use of undeclared identifier 'AppDelegate', four times.</p>
  
  <p>Can someone explain me how to get rid of those errors?</p>