iOS 下如果存在UIScrollerView 使用UIScreenEdgePanGestureRecognizer实现侧滑效果失效的问题

当你在使用UIScreenEdgePanGestureRecognizer手势实现侧滑的时候，如果后期你导航控制器push出的界面中包含UIScrollerView，这个时候你会发现，侧滑效果无法实现了，这个首先你会想到肯定是UIScrollerView,把这个手势给拦截了，执行了UIScrollerView中包含的手势。

问题所在

滑动返回事实上也是由于存在已久的UIScreenEdgePanGestureRecognizer来识别并且相应地，它直接与UINavigationController的view进行了绑定，绑定的方法是写在UINavgationController 的基类中的，正如一下：

UIPanGestureRecongnizer -- bind-- UIScrollerView
UIScreenEdgePanGestureRecognizer --bind-- UINavigationController.view

滑动返回无法触发，说明UIScreenEdgePanGestureRecongnizer并没有接受到手势事件。
根据苹果的官方文档说明 UIGestureRecongnizer 和UIview 是多对一的关系，UIGestureRecognizer 一定要和UIView进行绑定才能发挥作用，因此UIGestureRecongnizer对于屏幕上的手势事件，其接受顺序和UIView的层次结构是一致的，如下关系

UINavgataionController.view -->UIviewController.view -- > UIScrollerView.view -->screen and user'finger 既UIScrollView的panGestureRecognizer
先接受到了手势事件，直接就处理而没有往下传递实际上就是两个手势共存的问题

解决方案

UIGestureRecognizerDelegate 代理方法中包含，支持多个UIGestureRecongnizer共存，其中一个方法是

1 // called when the recognition of one of gestureRecognizer or otherGestureRecognizer would be blocked by the other
2 // return YES to allow both to recognize simultaneously. the default implementation returns NO (by default no two gestures can be recognized simultaneously)
3 // 
4 // note: returning YES is guaranteed to allow simultaneous recognition. returning NO is not guaranteed to prevent simultaneous recognition, as the other gesture's delegate may return YES
5 - (BOOL)gestureRecognizer:(UIGestureRecognizer *)gestureRecognizer shouldRecognizeSimultaneouslyWithGestureRecognizer:(UIGestureRecognizer *)otherGestureRecognizer;

总结就是此方法返回YES,手势事件会一直往下传递，不论当前层次是否对该事件进行响应

看看UIScrollerView的头文件的描述：

1 // Use these accessors to configure the scroll view's built-in gesture recognizers.
2 // Do not change the gestures' delegates or override the getters for these properties.
3 @property(nonatomic, readonly) UIPanGestureRecognizer *panGestureRecognizer NS_AVAILABLE_IOS(5_0);

UIScrollView本身是其panGestureRecognizer的delegate，且apple君明确表明不能修改它的delegate（修改的时候也会有警告）

UIScrollView作为delegate，说明UIScrollView中实现了上文提到的shouldRecognizeSimultaneouslyWithGestureRecognizer方法，返回了NO。创建一个UIScrollView的category，由于category中的同名方法会覆盖原有.m文件中的实现，使得可以自定义手势事件的传递，如下：

- (BOOL)gestureRecognizer:(UIGestureRecognizer *)gestureRecognizer shouldRecognizeSimultaneouslyWithGestureRecognizer:(UIGestureRecognizer *)otherGestureRecognizer{ 
      if ([gestureRecognizer isKindOfClass:[UIPanGestureRecognizer class]]  && [otherGestureRecognizer isKindOfClass:[UIScreenEdgePanGestureRecognizer class]]) { 
            return YES; 
      } else  {
            return NO; 
     }
 }