UIAlertView点击灰色部分dismiss

模态弹出来的窗口，需要点击窗口以外的区域，也就是模态区域让这个窗口消失。一般这种场景会出现在iPad里，当你modalPresentationStyle为UIModalPresentationFormSheet时，会有这种需求。UIAlertView可以实现:

- (IBAction)showAlert:(id)sender {
  alert = [[UIAlertView alloc] initWithTitle:@"模态测试"
                                     message:@"请点击四周的模态区域我就消失"
                                    delegate:nil
                           cancelButtonTitle:@"确定"
                           otherButtonTitles:nil];
  [alert show];
  recognizerTap = [[UITapGestureRecognizer alloc] initWithTarget:self
                                                 action:@selector(handleTapBehind:)];

  [recognizerTap setNumberOfTapsRequired:1];
  recognizerTap.cancelsTouchesInView = NO;
  [alert.window 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];
    }
  }
}