在iOS的手势识别中，有时我们要用手势实现页面pop（比如微信的左滑返回），实现时UIGestureRecognizerDelegate，会有这么一个（可选的）方法：

// called when a gesture recognizer attempts to transition out of UIGestureRecognizerStatePossible. returning NO causes it to transition to UIGestureRecognizerStateFailed

- (BOOL)gestureRecognizerShouldBegin:(UIGestureRecognizer *)gestureRecognizer;

实现它有一些常见的处理，比如：

- (BOOL)gestureRecognizerShouldBegin:(UIGestureRecognizer *)gestureRecognizer
{
    // Don't handle gestures if navigation controller is still animating a transition
    if ([self.navigationController.transitionCoordinator isAnimated])
        return NO;

    if (self.navigationController.viewControllers.count < 2)
        return NO;

    return YES;
}

解释一下，第一个是说如果仍然在transaction的动画中，不识别，第二个是说如果小于2个不识别（因为没的可pop了）。

顺便说一下，这个左滑pop的手势是iOS的navigationController自带的：

self.navigationController.interactivePopGestureRecognizer.delegate = self;
self.navigationController.interactivePopGestureRecognizer.enabled = YES;