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。

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：

- (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：

- (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];
}

- (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）：

- (id)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)navigationController:(UINavigationController *)navigationController
                         interactionControllerForAnimationController:(id)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：

- (id)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：

- (id)navigationController:(UINavigationController *)navigationController
                         interactionControllerForAnimationController:(id)animationController
{
    return self.popInteractionController;
}

这两个组合起来使用。首先，我们需要有个动画：

@interface WQPopAnimator : NSObject 

@end

#import "WQPopAnimator.h"

@implementation WQPopAnimator

- (NSTimeInterval)transitionDuration:(id )transitionContext
{
    return 0.4;
}

- (void)animateTransition:(id)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

UIPercentDrivenInteractiveTransition

来维护的，在滑动过程中根据滑动距离来进行更新：

} 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：

} 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：

- (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。