iOS 7新出了一个非常好玩的特性，就是View Controllers之间的切换动画，开发者也可以在程序中自行定制切换动画，例如翻转视图、缩放视图、旋转视图等等。另一方面，对于iPhone的使用来说，通过pan手势pop回到之前的视图，使用起来无疑是非常方便的。最开始使用这个特性时感觉非常惊喜。

本文就来说说如何定制我们自己的动画切换方案。

首先定义一个实现UINavigationControllerDelegate协议的类NavigationPerfomer。

故事板非常简单，就是一个UINavigationController和两个View Controller。但是要在UINavigationController中加入一个Object（这个貌似是Xcode 5的新特性吧，以前比较少用故事板，所以不是很确定），该对象的类为NavigaitonPerfomer类：

随后要为NavigationPerformer添加一个outlet：当前的UINavigatinController。并将UINavigationController的委托设为NavigationPerformer类。

故事板中其他的View Controller一切照旧，这里做得非常好，在提供了新的动画接口之后，在UINavigationController类中实现这些接口并不会影响到其他的View Controller的内容。

接着定义两个动画类：PushAnimation和PopAnimation，分别对应Push和Pop动画。

代码如下：

PushAnimtaion类：

#import <Foundation/Foundation.h>

@interface PushAnimation : NSObject <UIViewControllerAnimatedTransitioning>

@end

#pragma mark - UIViewControllerAnimatedTransitioning Delegate

/* 动画持续的时间 */
- (NSTimeInterval)transitionDuration:(id<UIViewControllerContextTransitioning>)transitionContext {
    return 1;
}

/* 动画的内容 */
- (void)animateTransition:(id<UIViewControllerContextTransitioning>)transitionContext {
    UIViewController *desController = [transitionContext viewControllerForKey:UITransitionContextToViewControllerKey];
    UIViewController *srcController = [transitionContext viewControllerForKey:UITransitionContextFromViewControllerKey];
    [[transitionContext containerView] addSubview:desController.view];
    desController.view.alpha = 0.0;
    
    [UIView animateWithDuration:[self transitionDuration:transitionContext]
                     animations:^{
                         srcController.view.transform = CGAffineTransformMakeTranslation(-srcController.view.bounds.size.width, 0.0);
                         desController.view.alpha = 1.0;
                     }
                     completion:^(BOOL finished) {
                         srcController.view.transform = CGAffineTransformIdentity;
                         [transitionContext completeTransition:![transitionContext transitionWasCancelled]];
                     }];
}

协议中的两个必选方法：

// This is used for percent driven interactive transitions, as well as for container controllers that have companion animations that might need to
// synchronize with the main animation. 
- (NSTimeInterval)transitionDuration:(id <UIViewControllerContextTransitioning>)transitionContext;
// This method can only  be a nop if the transition is interactive and not a percentDriven interactive transition.
- (void)animateTransition:(id <UIViewControllerContextTransitioning>)transitionContext;

第二个方法定制动画的内容。通过transitionContext这个上下文环境可以获取源视图控制器，目标视图控制器：

    UIViewController *desController = [transitionContext viewControllerForKey:UITransitionContextToViewControllerKey];
    UIViewController *srcController = [transitionContext viewControllerForKey:UITransitionContextFromViewControllerKey];

在进行动画前，先要将desController的视图添加到当前容器视图中。

+ (void)animateWithDuration:(NSTimeInterval)duration animations:(void (^)(void))animations completion:(void (^)(BOOL finished))completion NS_AVAILABLE_IOS(4_0); // delay = 0.0, options = 0

这里Push Animation的动画方案是将srcViewController自左向右划出窗口。

另外一个是PopAnimation类：

@interface PopAnimation : NSObject <UIViewControllerAnimatedTransitioning>

@end

#pragma mark - UIViewControllerAnimatedTransitioning Delegate

- (NSTimeInterval)transitionDuration:(id<UIViewControllerContextTransitioning>)transitionContext {
    return 1;
}

- (void)animateTransition:(id<UIViewControllerContextTransitioning>)transitionContext {
    UIViewController *desController = [transitionContext viewControllerForKey:UITransitionContextToViewControllerKey];
    UIViewController *srcController = [transitionContext viewControllerForKey:UITransitionContextFromViewControllerKey];
    [[transitionContext containerView] addSubview:desController.view];
    desController.view.alpha = 0.0;
    
    [UIView animateWithDuration:[self transitionDuration:transitionContext]
                     animations:^{
                         srcController.view.transform = CGAffineTransformMakeScale(-1.0, 1.0);
                         desController.view.alpha = 1.0;
                     }
                     completion:^(BOOL finished) {
                         srcController.view.transform = CGAffineTransformIdentity;
                         [transitionContext completeTransition:![transitionContext transitionWasCancelled]];
                     }];
}

@end

接着是关键的NavigationPerformer类：

#import <Foundation/Foundation.h>

@class PushAnimation;
@class PopAnimation;

@interface NavigationPerfomer : NSObject <UINavigationControllerDelegate>

@property (weak, nonatomic) IBOutlet UINavigationController *navigationController;

@property (strong, nonatomic) PushAnimation *pushAnimation;

@property (strong, nonatomic) PopAnimation *popAnimation;

@property (strong, nonatomic) UIPercentDrivenInteractiveTransition *interactionController;

@end

首先看看UINavigationControllerDelegate的方法：

#pragma mark - UINavigationController Delegate

- (id<UIViewControllerAnimatedTransitioning>)navigationController:(UINavigationController *)navigationController
                                  animationControllerForOperation:(UINavigationControllerOperation)operation
                                               fromViewController:(UIViewController *)fromVC
                                                 toViewController:(UIViewController *)toVC
{
    if (operation == UINavigationControllerOperationPush) {
        return self.pushAnimation;
    }
    else if (operation == UINavigationControllerOperationPop) {
        return self.popAnimation;
    }
    
    return nil;
}

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

第二个方法指定提供交互性的控制器对象，如果返回nil，那么视图将无法响应用户的手势，如滑动等。

下面为导航控制器的视图添加一个Pan手势：

- (void)awakeFromNib {
    // 在导航控制器的视图上添加pan手势
    UIPanGestureRecognizer *panGesture = [[UIPanGestureRecognizer alloc] initWithTarget:self action:@selector(pan:)];
    [self.navigationController.view addGestureRecognizer:panGesture];
    
    // 初始化动画方案
    self.pushAnimation = [[PushAnimation alloc] init];
    self.popAnimation  = [[PopAnimation alloc] init];
}

- (void)pan:(UIPanGestureRecognizer *)pan {
    UIView *view = self.navigationController.view;
    if (pan.state == UIGestureRecognizerStateBegan) { // 只有从左向右滑动的手势才作出响应：pop
        CGPoint location = [pan locationInView:view];
        if (location.x < CGRectGetMidX(view.bounds) && self.navigationController.viewControllers.count > 1) {
            self.interactionController = [[UIPercentDrivenInteractiveTransition alloc] init];
            [self.navigationController popViewControllerAnimated:YES];
        }
    }
    else if (pan.state == UIGestureRecognizerStateChanged) { // 以当前的位移作为进度执行动画
        CGPoint translation = [pan translationInView:view];
        CGFloat progress = fabs(translation.x / CGRectGetWidth(view.bounds));
        [self.interactionController updateInteractiveTransition:progress];
    }
    else if (pan.state == UIGestureRecognizerStateEnded) {
        if ([pan velocityInView:view].x > 0) {
            [self.interactionController finishInteractiveTransition];
        }
        else { // 如果手势的终点相对于起点在x正向上的位移小于0，即为取消手势
            [self.interactionController cancelInteractiveTransition];
        }
        self.interactionController = nil; // 最后必须将interactionController清空，确保不会影响到后面的动画执行
    }
}

在切换动画完毕后，一定要将interactionController设为nil，因为某些动画是不需要手势交互的，这样会带来奇怪的行为。

最后是一些运行结果：

Push时的效果：

使用Pan手势Pop时的效果：

通过这个新的Transition接口我们可以实现一些比较好玩的动画，Demo已经上传，有兴趣的可以下载看看。

这部分还是挺好玩的，Just enjoy.

参考资料：

iOS 7 新特性：视图控制器切换API

objc.io #5 View Controller Transitions

objcio / issue5-view-controller-transitions

iOS 7学习：定制View Controllers之间的切换动画