UIGestureRecognizer是一个定义基本手势的抽象类，具体包含：
1、拍击UITapGestureRecognizer (任意次数的拍击)
2、向里或向外捏UIPinchGestureRecognizer (用于缩放)
3、摇动或者拖拽UIPanGestureRecognizer (拖动)
4、擦碰UISwipeGestureRecognizer (以任意方向)
5、旋转UIRotationGestureRecognizer (手指朝相反方向移动)
6、长按UILongPressGestureRecognizer (长按)
本文主要使用到的是拖拽手势：UIPanGestureRecognizer

限制方法：

-(void)doMoveAction:(UIPanGestureRecognizer *)recognizer{
    // Figure out where the user is trying to drag the view. 
    CGPoint translation = [recognizer translationInView:self.view];
    CGPoint newCenter = CGPointMake(recognizer.view.center.x+ translation.x,
                                    recognizer.view.center.y + translation.y);//    限制屏幕范围：
    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];
}

也可以使用UITouch来实现：
当手指接触到屏幕，不管是单点触摸还是多点触摸，事件都会开始，直到用户所有的手指都离开屏幕。期间所有的UITouch对象都被包含在UIEvent事件对象中，由程序分发给处理者。事件记录了这个周期中所有触摸对象状态的变化。
限制方法如下：

BOOL isMove;
CGPoint legend_point;
-(void)touchesBegan:(NSSet<uitouch *> *)touches withEvent:(UIEvent *)event{
    [super touchesBegan:touches withEvent:event];
    isMove = NO;
    UITouch *touch = [touches anyObject];
    CGPoint point = [touch locationInView:self.view];
    if (CGRectContainsPoint(V2.frame, point)) {
        legend_point = [touch locationInView:V2];
        isMove = YES;
    }
}
 
-(void)touchesMoved:(NSSet<uitouch *> *)touches withEvent:(UIEvent *)event{
    [super touchesMoved:touches withEvent:event];
    if (!isMove) {
        return;
    }
    @autoreleasepool {
        UITouch *touch = [touches anyObject];
        CGPoint point = [touch locationInView:self.view];
        //转化成相对的中心
        point.x += V2.frame.size.width/2.0f - legend_point.x;
        point.y += V2.frame.size.height/2.0f - legend_point.y;
//        限制范围
        if (point.x < V2.frame.size.width / 2.0f) {
            point.x = V2.frame.size.width / 2.0f;
        }
        if (point.y < V2.frame.size.height / 2.0f) {
            point.y = V2.frame.size.height / 2.0f;
        }
 
        if (point.x > self.view.frame.size.width - V2.frame.size.width / 2.0f) {
            point.x = self.view.frame.size.width - V2.frame.size.width / 2.0f;
        }
        if (point.y > self.view.frame.size.height - V2.frame.size.height / 2.0f) {
            point.y = self.view.frame.size.height - V2.frame.size.height / 2.0f;
        }
        V2.center = point;
 
    }
}