/** 1.相对于父坐标系而言,表示当前触摸点所在的位置 */
CGPoint locationPoint = [panGestureRecognizer locationInView:panGestureRecognizer.view];
/** 2.相对于当前位置而言,以当前坐标点为标准(0, 0) */
CGPoint translationPoint = [panGestureRecognizer translationInView:panGestureRecognizer.view];
/** 此句相当于重新初始化当前坐标点 */
[panGestureRecognizer setTranslation:CGPointZero inView:panGestureRecognizer.view];
/** 3.每秒拖动像素点的速率(相对于父视图而言) */
CGPoint velocityPoint = [panGestureRecognizer velocityInView:panGestureRecognizer.view];