再谈iOS 7的手势滑动返回功能

之前随手写过一篇《使用UIScreenEdgePanGestureRecognizer实现swipe to pop效果》，挺粗糙的。

现在使用默认模板创建的iOS App都支持手势返回功能，如果导航栏的返回按钮是自定义的那么则会失效，也可以参考这里手动设置无效。

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if ([self.navigationController respondsToSelector:@selector(interactivePopGestureRecognizer)]) {  
    self.navigationController.interactivePopGestureRecognizer.enabled = NO;  
}  

如果是因为自定义导航按钮而导致手势返回失效，那么可以在NavigationController的viewDidLoad函数中添加如下代码：

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- (void)viewDidLoad  
{  
    [super viewDidLoad];  
    // Do any additional setup after loading the view.  
      
    __weak typeof (self) weakSelf = self;  
    if ([self respondsToSelector:@selector(interactivePopGestureRecognizer)]) {  
        self.interactivePopGestureRecognizer.delegate = weakSelf;  
    }  
}  

这样写了以后就可以通过手势滑动返回上一层了，但是如果在push过程中触发手势滑动返回，会导致导航栏崩溃（从日志中可以看出）。针对这个问题，我们需要在pop过程禁用手势滑动返回功能：

[cpp]
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- (void)pushViewController:(UIViewController *)viewController animated:(BOOL)animated  
{    
    // fix 'nested pop animation can result in corrupted navigation bar'  
    if ([self respondsToSelector:@selector(interactivePopGestureRecognizer)]) {  
        self.interactivePopGestureRecognizer.enabled = NO;  
    }  
      
    [super pushViewController:viewController animated:animated];  
}  

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- (void)navigationController:(UINavigationController *)navigationController  
       didShowViewController:(UIViewController *)viewController  
                    animated:(BOOL)animated  
{  
    if ([navigationController respondsToSelector:@selector(interactivePopGestureRecognizer)]) {  
        navigationController.interactivePopGestureRecognizer.enabled = YES;  
    }  
}  

除了使用系统默认的动画，还可以使用自定义过渡动画（丰满的文档）：

[cpp]
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- (id<UIViewControllerAnimatedTransitioning>)navigationController:(UINavigationController *)navigationController  
                                  animationControllerForOperation:(UINavigationControllerOperation)operation  
                                               fromViewController:(UIViewController *)fromVC  
                                                 toViewController:(UIViewController *)toVC  
{  
    if (operation == UINavigationControllerOperationPop) {  
        if (self.popAnimator == nil) {  
            self.popAnimator = [WQPopAnimator new];  
        }  
        return self.popAnimator;  
    }  
      
    return nil;  
}  
  
- (id<UIViewControllerInteractiveTransitioning>)navigationController:(UINavigationController *)navigationController  
                         interactionControllerForAnimationController:(id<UIViewControllerAnimatedTransitioning>)animationController  
{  
    return self.popInteractionController;  
}  
  
#pragma mark -   
  
- (void)enablePanToPopForNavigationController:(UINavigationController *)navigationController  
{  
    UIScreenEdgePanGestureRecognizer *left2rightSwipe = [[UIScreenEdgePanGestureRecognizer alloc]  
                                                         initWithTarget:self  
                                                         action:@selector(didPanToPop:)];  
    //[left2rightSwipe setDelegate:self];  
    [left2rightSwipe setEdges:UIRectEdgeLeft];  
    [navigationController.view addGestureRecognizer:left2rightSwipe];  
      
    self.popAnimator = [WQPopAnimator new];  
    self.supportPan2Pop = YES;  
}  
  
- (void)didPanToPop:(UIPanGestureRecognizer *)panGesture  
{  
    if (!self.supportPan2Pop) return ;  
      
    UIView *view = self.navigationController.view;  
      
    if (panGesture.state == UIGestureRecognizerStateBegan) {  
        self.popInteractionController = [UIPercentDrivenInteractiveTransition new];  
        [self.navigationController popViewControllerAnimated:YES];  
    } else if (panGesture.state == UIGestureRecognizerStateChanged) {  
        CGPoint translation = [panGesture translationInView:view];  
        CGFloat d = fabs(translation.x / CGRectGetWidth(view.bounds));  
        [self.popInteractionController updateInteractiveTransition:d];  
    } else if (panGesture.state == UIGestureRecognizerStateEnded) {  
        if ([panGesture velocityInView:view].x > 0) {  
            [self.popInteractionController finishInteractiveTransition];  
        } else {  
            [self.popInteractionController cancelInteractiveTransition];  
        }  
        self.popInteractionController = nil;  
    }  
}  

如下这个代理方法是用来提供一个非交互式的过渡动画的：

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- (id<UIViewControllerAnimatedTransitioning>)navigationController:(UINavigationController *)navigationController  
                                  animationControllerForOperation:(UINavigationControllerOperation)operation  
                                               fromViewController:(UIViewController *)fromVC  
                                                 toViewController:(UIViewController *)toVC  
{  
    if (operation == UINavigationControllerOperationPop) {  
        if (self.popAnimator == nil) {  
            self.popAnimator = [WQPopAnimator new];  
        }  
        return self.popAnimator;  
    }  
      
    return nil;  
}  

而下面这个代理方法则是提供交互式动画：

[cpp]
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- (id<UIViewControllerInteractiveTransitioning>)navigationController:(UINavigationController *)navigationController  
                         interactionControllerForAnimationController:(id<UIViewControllerAnimatedTransitioning>)animationController  
{  
    return self.popInteractionController;  
}  

这两个组合起来使用。首先，我们需要有个动画：

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@interface WQPopAnimator : NSObject <UIViewControllerAnimatedTransitioning>  
  
@end  

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#import "WQPopAnimator.h"  
  
@implementation WQPopAnimator  
  
- (NSTimeInterval)transitionDuration:(id <UIViewControllerContextTransitioning>)transitionContext  
{  
    return 0.4;  
}  
  
- (void)animateTransition:(id<UIViewControllerContextTransitioning>)transitionContext  
{  
    UIViewController *toViewController = [transitionContext viewControllerForKey:UITransitionContextToViewControllerKey];  
    UIViewController *fromViewController = [transitionContext viewControllerForKey:UITransitionContextFromViewControllerKey];  
    [[transitionContext containerView] insertSubview:toViewController.view belowSubview:fromViewController.view];  
      
    __block CGRect toRect = toViewController.view.frame;  
    CGFloat originX = toRect.origin.x;  
    toRect.origin.x -= toRect.size.width / 3;  
    toViewController.view.frame = toRect;  
      
    [UIView animateWithDuration:[self transitionDuration:transitionContext] animations:^{  
        CGRect fromRect = fromViewController.view.frame;  
        fromRect.origin.x += fromRect.size.width;  
        fromViewController.view.frame = fromRect;  
          
        toRect.origin.x = originX;  
        toViewController.view.frame = toRect;  
    } completion:^(BOOL finished) {  
        [transitionContext completeTransition:![transitionContext transitionWasCancelled]];  
    }];  
}  
  
@end  

其次，交互式动画是通过

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UIPercentDrivenInteractiveTransition  

来维护的，在滑动过程中根据滑动距离来进行更新：

[cpp]
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} else if (panGesture.state == UIGestureRecognizerStateChanged) {  
        CGPoint translation = [panGesture translationInView:view];  
        CGFloat d = fabs(translation.x / CGRectGetWidth(view.bounds));  
        [self.popInteractionController updateInteractiveTransition:d];  

当手势结束时要做出收尾动作：

[cpp]
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} else if (panGesture.state == UIGestureRecognizerStateEnded) {  
        if ([panGesture velocityInView:view].x > 0) {  
            [self.popInteractionController finishInteractiveTransition];  
        } else {  
            [self.popInteractionController cancelInteractiveTransition];  
        }  
        self.popInteractionController = nil;  
    }  

同样地，自定义的动画也会有上面提到的导航栏崩溃问题，也可以通过类似的方法来解决：

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- (void)navigationController:(UINavigationController *)navigationController  
       didShowViewController:(UIViewController *)viewController  
                    animated:(BOOL)animated  
{  
    if (viewController == self.navigationController.pushingViewController) {  
        self.supportPan2Pop = YES;  
        self.navigationController.pushingViewController = nil;  
    }  

补充：位于当前navgationController的第一个（[0]）viewController时需要设置手势代理，不响应。