前言
如果有两个手势，我们需要对手势进行优先级的设置，会用到下面的方法。
// create a relationship with another gesture recognizer that will prevent this gesture's actions from being called until otherGestureRecognizer transitions to UIGestureRecognizerStateFailed
// if otherGestureRecognizer transitions to UIGestureRecognizerStateRecognized or UIGestureRecognizerStateBegan then this recognizer will instead transition to UIGestureRecognizerStateFailed
// example usage: a single tap may require a double tap to fail
- (void)requireGestureRecognizerToFail:(UIGestureRecognizer *)otherGestureRecognizer;
大概意思是，创建另一个有联系的手势识别器，只要另一个手势状态为UIGestureRecognizerStateFailed这个手势才会被相应。如果另一个手势正在被响应，那么这个手势不会被响应。举个例子，单击与双击的冲突，scrollView的平移手势与系统的边缘手势冲突的时候可以使用。
比如分段控制器
如果用scrollview作为控制器的容器，那么此时可以这样设置：
[self.scrollView.panGestureRecognizer requireGestureRecognizerToFail:self.navigationController.interactivePopGestureRecognizer];
其他一些手势代理方法
//手指触摸屏幕后回调的方法，返回NO则不再进行手势识别，方法触发等
- (BOOL)gestureRecognizer:(UIGestureRecognizer *)gestureRecognizer shouldReceiveTouch:(UITouch *)touch;
//开始进行手势识别时调用的方法，返回NO则结束，不再触发手势
- (BOOL)gestureRecognizerShouldBegin:(UIGestureRecognizer *)gestureRecognizer;
//是否支持多时候触发，返回YES，则可以多个手势一起触发方法，返回NO则为互斥
- (BOOL)gestureRecognizer:(UIGestureRecognizer *)gestureRecognizer shouldRecognizeSimultaneouslyWithGestureRecognizer:(UIGestureRecognizer *)otherGestureRecognizer;
//下面这个两个方法也是用来控制手势的互斥执行的
//这个方法返回YES，第一个手势和第二个互斥时，第一个会失效
- (BOOL)gestureRecognizer:(UIGestureRecognizer *)gestureRecognizer shouldRequireFailureOfGestureRecognizer:(UIGestureRecognizer *)otherGestureRecognizer NS_AVAILABLE_IOS(7_0);
//这个方法返回YES，第一个和第二个互斥时，第二个会失效
- (BOOL)gestureRecognizer:(UIGestureRecognizer *)gestureRecognizer shouldBeRequiredToFailByGestureRecognizer:(UIGestureRecognizer *)otherGestureRecognizer NS_AVAILABLE_IOS(7_0);
