iOS限制拖拽手势（UIPanGestureRecognizer）的范围

UIPanGestureRecognizer * panGestureRecognizer = [[UIPanGestureRecognizer alloc] initWithTarget:self action:@selector(doMoveAction:)];
[<UIView> addGestureRecognizer:panGestureRecognizer];

-(void)doMoveAction:(UIPanGestureRecognizer *)recognizer{
    // Figure out where the user is trying to drag the view.
    // 找出现在在父视图中的坐标
    CGPoint translation = [recognizer translationInView:self.view];
    CGPoint newCenter = CGPointMake(recognizer.view.center.x+ translation.x,
                                    recognizer.view.center.y + translation.y);
    newCenter.y = MAX(recognizer.view.frame.size.height/2, newCenter.y);
    newCenter.y = MIN(self.view.frame.size.height - recognizer.view.frame.size.height/2,  newCenter.y);
    newCenter.x = MAX(recognizer.view.frame.size.width/2, newCenter.x);
    newCenter.x = MIN(self.view.frame.size.width - recognizer.view.frame.size.width/2,newCenter.x);
    recognizer.view.center = newCenter;
    [recognizer setTranslation:CGPointZero inView:self.view];
}