效果图

popAnimation.gif

交互式动画的实现过程

动画
1、给UINavigationController添加代理，需实现UINavigationControllerDelegate协议。
2、创建动画类，该类需实现UIViewControllerAnimatedTransitioning协议。
手势交互
3、给UINavigationController添加手势和完成手势百分比的判定。
4、实现UIViewControllerAnimatedTransitioning协议的手势方法，返回UIPercentDrivenInteractiveTransition对象。

---

给UINavigationController添加代理

创建PJNavigationInteractiveTransition类用于对UINavigationController代理的实现

// PJNavigationInteractiveTransition.h
self.navTransition = 
[[PJNavigationInteractiveTransition alloc] initWithViewController:self];

// PJNavigationInteractiveTransition.h
- (instancetype)initWithViewController:(UINavigationController *)vc {
    
    self = [super init];
    if (self) {
        _navigationController = vc;
        _navigationController.delegate = self;
    }
    return self;
}

实现UINavigationControllerDelegate协议

- (id<UIViewControllerAnimatedTransitioning>)navigationController:(UINavigationController *)navigationController
                                  animationControllerForOperation:(UINavigationControllerOperation)operation
                                               fromViewController:(UIViewController *)fromVC
                                                 toViewController:(UIViewController *)toVC {
    
    if (operation == UINavigationControllerOperationPop) {
        return [[PJPopAnimation alloc] init];
    }
    _fromViewController = fromVC;
    
    return nil;
}

• 当调用- (void)pushViewController:(UIViewController *)viewController animated:(BOOL)animated方法后系统将自动调用该方法。
• 当返回为nil时，将使用默认的转场。否者则使用特定的动画类。

实现动画类

动画类主要就是实现UINavigationControllerDelegate协议中的两个required方法

- (NSTimeInterval)transitionDuration:(id <UIViewControllerContextTransitioning>)transitionContext {
    
    return 0.3;
}

这个方法就是返回动画执行的时间
</br>

- (void)animateTransition:(id <UIViewControllerContextTransitioning>)transitionContext {
    
    // 1
    UIViewController *fromViewController = [transitionContext viewControllerForKey:UITransitionContextFromViewControllerKey];
    UIViewController *toViewController = [transitionContext viewControllerForKey:UITransitionContextToViewControllerKey];
    
    // 2
    UIView *fromView;
    UIView *toView;
    
    if ([transitionContext respondsToSelector:@selector(viewForKey:)]) {
        fromView = [transitionContext viewForKey:UITransitionContextFromViewKey];
        toView = [transitionContext viewForKey:UITransitionContextToViewKey];
    } else {
        fromView = fromViewController.view;
        toView = toViewController.view;
    }
    
    // 3
    fromView.frame = [transitionContext initialFrameForViewController:fromViewController];
    toView.frame = [transitionContext finalFrameForViewController:toViewController];
    
    // 4
    UIView *containerView = [transitionContext containerView];

    // 5
    [containerView insertSubview:toView belowSubview:fromView];
    
    // 6
    [UIView animateWithDuration:[self transitionDuration:transitionContext] animations:^{
        fromView.transform = CGAffineTransformMakeTranslation([UIScreen mainScreen].bounds.size.width, 0);
    } completion:^(BOOL finished) {
        fromView.transform = CGAffineTransformIdentity;
        [transitionContext completeTransition:!transitionContext.transitionWasCancelled];
    }];
}

1、获取fromViewController与toViewController
fromViewController与toViewController是相对的，如下图所示

屏幕快照 2017-03-30 15.41.21.png

2、获取fromView与toView
当系统版本为iOS8.0以上时，需使用viewForKey。

3、设置frame。

4、获取containerView
containerView就是转场动画执行中屏幕所展示的View，相当于是动画展示的平台。

5、将fromView与toView加入至containerView中

6、设置动画
当动画结束后必须调用completeTransition方法以告知系统动画完成。

给UINavigationController添加手势和完成手势百分比的判定

给UINavigationController添加手势

UIGestureRecognizer *gesture = self.interactivePopGestureRecognizer;
gesture.enabled = NO;

UIPanGestureRecognizer *popRecognizer = [[UIPanGestureRecognizer alloc] init];
popRecognizer.delegate = self;
popRecognizer.maximumNumberOfTouches = 1;
[gesture.view addGestureRecognizer:popRecognizer];

self.navTransition = [[PJNavigationInteractiveTransition alloc] initWithViewController:self];
[popRecognizer addTarget:self.navTransition action:@selector(handleControllerPop:)];

• 禁止系统的侧滑返回
• 添加我们自己的Pan手势

手势的限制

#pragma mark - UIGestureRecognizerDelegate
- (BOOL)gestureRecognizerShouldBegin:(UIPanGestureRecognizer *)gestureRecognizer {
    
    // Ignore when no view controller is pushed into the navigation stack.
    if (self.viewControllers.count <= 1) {
        return NO;
    }
    
    // Ignore when the beginning location is beyond max allowed initial distance to left edge.
    CGPoint beginningLocation = [gestureRecognizer locationInView:gestureRecognizer.view];
    if (maxAllowedInitialDistance > 0 && beginningLocation.x > maxAllowedInitialDistance) {
        return NO;
    }
    
    // Ignore pan gesture when the navigation controller is currently in transition.
    if ([[self.navigationController valueForKey:@"_isTransitioning"] boolValue]) {
        return NO;
    }
    
    // Prevent calling the handler when the gesture begins in an opposite direction.
    CGPoint translation = [gestureRecognizer translationInView:gestureRecognizer.view];
    if (translation.x <= 0) {
        return NO;
    }
    
    // When the active view controller allow interactive pop.
    UIViewController *topViewController = self.viewControllers.lastObject;
    if (topViewController.pj_interactivePopDisabled) {
        return NO;
    }
    
    return YES;
}

手势百分比的判定

- (void)handleControllerPop:(UIGestureRecognizer *)gestureRecognizer {
    /**
     * 稳定进度区间，让它在0.0（未完成）～1.0（已完成）之间
     */
    CGFloat progress = 0.0;
    if ([gestureRecognizer isKindOfClass:[UIPanGestureRecognizer class]]) {
        UIPanGestureRecognizer *recognizer = (UIPanGestureRecognizer *)gestureRecognizer;
        progress = [recognizer translationInView:recognizer.view].x / recognizer.view.bounds.size.width;
        progress = MIN(1.0, MAX(0.0, progress));
    }
    
    [self p_handleGestureRecognizer:gestureRecognizer Progress:progress];
}

- (void)p_handleGestureRecognizer:(UIGestureRecognizer *)gestureRecognizer Progress:(CGFloat)progress {
    
    if (gestureRecognizer.state == UIGestureRecognizerStateBegan) {
        self.interactivePopTransition = [[UIPercentDrivenInteractiveTransition alloc] init];
        [self.navigationController popViewControllerAnimated:YES];
    } else if (gestureRecognizer.state == UIGestureRecognizerStateChanged) {
        [self.interactivePopTransition updateInteractiveTransition:progress];
    } else if (gestureRecognizer.state == UIGestureRecognizerStateEnded ||
               gestureRecognizer.state == UIGestureRecognizerStateCancelled) {
        if (progress > 0.5) {
            [self.interactivePopTransition finishInteractiveTransition];
            if (_fromViewController && _fromViewController.pj_transitionFinishedBlock) {
                _fromViewController.pj_transitionFinishedBlock();
            }
        } else {
            [self.interactivePopTransition cancelInteractiveTransition];
        }
        self.interactivePopTransition = nil;
    }
}

UIPercentDrivenInteractiveTransition是什么
这是一个实现了UIViewControllerInteractiveTransitioning接口的类，为我们预先实现和提供了一系列便利的方法，可以用一个百分比来控制交互式切换的过程。具体有以下几个重要方法：
-(void)updateInteractiveTransition:(CGFloat)percentComplete 更新百分比，一般通过手势识别的长度之类的来计算一个值，然后进行更新。
-(void)cancelInteractiveTransition 报告交互取消，返回切换前的状态
–(void)finishInteractiveTransition 报告交互完成，更新到切换后的状态

实现UIViewControllerAnimatedTransitioning协议的手势方法

- (id<UIViewControllerInteractiveTransitioning>)navigationController:(UINavigationController *)navigationController
                         interactionControllerForAnimationController:(id<UIViewControllerAnimatedTransitioning>)animationController {
    
    if ([animationController isKindOfClass:[PJPopAnimation class]]) {
        return self.interactivePopTransition;
    }
    return nil;
}

参考

https://developer.apple.com/library/content/featuredarticles/ViewControllerPGforiPhoneOS/CustomizingtheTransitionAnimations.html#//apple_ref/doc/uid/TP40007457-CH16-SW1
https://developer.apple.com/library/content/samplecode/CustomTransitions/Introduction/Intro.html#//apple_ref/doc/uid/TP40015158-Intro-DontLinkElementID_2
https://onevcat.com/2013/10/vc-transition-in-ios7/
https://github.com/forkingdog/FDFullscreenPopGesture