title: gesture recognizer doesn't work on subview
body: <p>At first,I attach UISwipeGestureRecognizer to a image view,the action method can't be triggered.Then I attach UISwipeGestureRecognizer to view controller's view,it works well.I don't know why.
  here is the code that doesn't work</p>
  
  <pre><code> - (void)viewDidLoad
  {
      [super viewDidLoad];
      // Do any additional setup after loading the view.
  
      UISwipeGestureRecognizer *swipeRight=[[UISwipeGestureRecognizer alloc] initWithTarget:self action:@selector(swipeRightAction)];
      swipeRight.direction=UISwipeGestureRecognizerDirectionRight;
      //imageView is an outlet of image view
      [imageView addGestureRecognizer:swipeRight];
  }
  </code></pre>
  
  <p>here is the code that works well</p>
  
  <pre><code>- (void)viewDidLoad
      {
          [super viewDidLoad];
          // Do any additional setup after loading the view.
  
      UISwipeGestureRecognizer *swipeRight=[[UISwipeGestureRecognizer alloc] initWithTarget:self action:@selector(swipeRightAction)];
      swipeRight.direction=UISwipeGestureRecognizerDirectionRight;
      //imageView is an outlet of image view
      [self.view addGestureRecognizer:swipeRight];
  }
  </code></pre>