拖动手势设置边界

- (void)handlePanGesture:(UIPanGestureRecognizer *)panGesture {
    
       CGPoint translation = [panGesture translationInView:panGesture.view.superview];
       CGFloat space = 20;
       // Figure out where the user is trying to drag the view.
       CGPoint newCenter = CGPointMake(panGesture.view.center.x+ translation.x,
                                       panGesture.view.center.y + translation.y);//    限制屏幕范围：
    
       newCenter.y = MAX(panGesture.view.frame.size.height/2, newCenter.y);
       newCenter.y = MIN(panGesture.view.superview.frame.size.height - panGesture.view.frame.size.height/2,  newCenter.y);
       newCenter.x = MAX(panGesture.view.frame.size.width/2+space, newCenter.x);
       newCenter.x = MIN(panGesture.view.superview.frame.size.width - panGesture.view.frame.size.width/2 - space,newCenter.x);
        panGesture.view.center = newCenter;
       [panGesture setTranslation:CGPointZero inView:panGesture.view.superview];
    
    // Don't need any logic for ended/failed/canceled states
}