彈出一個UIAlertView，然後點選視窗之外的區域來關閉UIALertView，程式碼如下：

- (IBAction)showAlert:(id)sender {

  alert = [[UIAlertView alloc] initWithTitle:@"測試"
                                     message:@"點選四周的區域我就消失"
                                    delegate:nil
                           cancelButtonTitle:@"確定"
                           otherButtonTitles:nil];
  [alert show];
  recognizerTap = [[UITapGestureRecognizer alloc] initWithTarget:self action:@selector(handleTapBehind:)];

  [recognizerTap setNumberOfTapsRequired:1];
  recognizerTap.cancelsTouchesInView = NO; 
  [[UIApplication sharedApplication].keyWindow addGestureRecognizer:recognizerTap];
}

- (void)handleTapBehind:(UITapGestureRecognizer *)sender {
  if (sender.state == UIGestureRecognizerStateEnded){
    CGPoint location = [sender locationInView:nil];
    if (![alert pointInside:[alert convertPoint:location fromView:alert.window] withEvent:nil]){
      [alert.window removeGestureRecognizer:sender];
      [alert dismissWithClickedButtonIndex:0 animated:YES];
    }
  }
}