从iOS7开始，苹果更新了自定义ViewController转场的API，这些新增的类和接口让很多人困惑，望而却步。本文就从这些API入口，让读者理清这些API错综复杂的关系。

几个protocol

讲自定义转场就离不开这几个protocol：

UIViewControllerContextTransitioning
UIViewControllerAnimatedTransitioning
UIViewControllerInteractiveTransitioning
UIViewControllerTransitioningDelegate
UINavigationControllerDelegate
UITabBarControllerDelegate

乍一看很多，其实很简单，我们可以将其分为三类：

1. 描述ViewController转场的：
   UIViewControllerTransitioningDelegate,UINavigationControllerDelegate,UITabBarControllerDelegate
2. 定义动画内容的
   UIViewControllerAnimatedTransitioning,UIViewControllerInteractiveTransitioning
3. 表示动画上下文的
   UIViewControllerContextTransitioning

描述ViewController转场的

细说之前先扯个蛋：

为什么苹果要引入这一套API？因为在iOS7之前，做转场动画很麻烦，要写一大堆代码在ViewController中。引入这一套API之后，在丰富功能的同时极大程度地降低了代码耦合，实现方式就是将之前在ViewController里面的代码通过protocol分离了出来。

顺着这个思路往下想，实现自定义转场动画首先需要找到ViewController的delegate。苹果告诉我们切换ViewController有三种形式：UITabBarController内部切换，UINavigationController切换，present modal ViewController。这三种方式是不是需要不同的protocol呢？

我们分别来看下：

• UIViewControllerTransitioningDelegate自定义模态转场动画时使用。
  设置UIViewController的属性transitioningDelegate。
  @property (nullable, nonatomic, weak) id <UIViewControllerTransitioningDelegate> transitioningDelegate
• UINavigationControllerDelegate自定义navigation转场动画时使用。
  设置UINavigationController的属性delegate
  @property(nullable, nonatomic, weak) id<UINavigationControllerDelegate> delegate
• UITabBarControllerDelegate自定义tab转场动画时使用。
  设置UITabBarController的属性delegate
  @property(nullable, nonatomic,weak) id<UITabBarControllerDelegate> delegate

实际上这三个protocol干的事情是一样的，就是我们“扯淡”的内容，只不过他们的应用场景不同罢了。我们下面以UINavigationControllerDelegate为例，其他的类似。

定义动画内容的

UINavigationControllerDelegate主要包含这两个方法：

- (nullable id <UIViewControllerInteractiveTransitioning>)navigationController:(UINavigationController *)navigationControllerinteractionControllerForAnimationController:(id <UIViewControllerAnimatedTransitioning>) animationController NS_AVAILABLE_IOS(7_0);- (nullable id <UIViewControllerAnimatedTransitioning>)navigationController:(UINavigationController *)navigationControlleranimationControllerForOperation:(UINavigationControllerOperation)operationfromViewController:(UIViewController *)fromVCtoViewController:(UIViewController *)toVC  NS_AVAILABLE_IOS(7_0);

两个方法分别返回UIViewControllerInteractiveTransitioning和UIViewControllerAnimatedTransitioning，它们的任务是描述动画行为（转场动画如何执行，就看它俩的）。
从名字可以看出，这两个protocol的区别在于是否是interactive的。如何理解？****interactive动画可以根据输入信息的变化改变动画的进程。****例如iOS系统为UINavigationController提供的默认右滑退出手势就是一个interactive 动画，整个动画的进程由用户手指的移动距离控制。

我们来看下相对简单的UIViewControllerAnimatedTransitioning:

- (NSTimeInterval)transitionDuration:(nullable id <UIViewControllerContextTransitioning>)transitionContext;
- (void)animateTransition:(id <UIViewControllerContextTransitioning>)transitionContext; @optional - (void)animationEnded:(BOOL) transitionCompleted; 

transitionDuration返回动画的执行时间，animateTransition处理具体的动画，animationEnded是optional，大部分情况下不需要处理。
这里出现了我们要讲的最后一个protocol：UIViewControllerContextTransitioning。

表示动画上下文的

UIViewControllerContextTransitioning也是唯一一个不需要我们实现的protocol。

Do not adopt this protocol in your own classes, nor should you directly create objects that adopt this protocol.

UIViewControllerContextTransitioning提供了一系列方法，为interactive和非interactive动画提供上下文：

//转场动画发生在该View中
- (nullable UIView *)containerView;
//上报动画执行完毕
- (void)completeTransition:(BOOL)didComplete; //根据key返回一个ViewController。我们通过UITransitionContextFromViewControllerKey找到将被替换掉的ViewController，通过UITransitionContextToViewControllerKey找到将要显示的ViewController - (nullable __kindof UIViewController *)viewControllerForKey:(NSString *)key; 

还有一些其他的方法，我们以后用到再说。

下面我们通过一个简单的Demo串联理解下。

DEMO

这是一个缩放同时修改透明度的动画，我们来看下如何实现。
在上面的讲解中，我们通过倒推的方式来理解转场动画中用到的protocol，在Demo 中，我们会从创建动画开始。

第一步：创建动画

由上面的解析得知，动画是在UIViewControllerAnimatedTransitioning中定义的，所以我们首先创建实现UIViewControllerAnimatedTransitioning的对象：JLScaleTransition。

JLScaleTransition.h

@interface JLScaleTransition : NSObject<UIViewControllerAnimatedTransitioning>
@end
JLScaleTransition.m@implementation JLScaleTransition- (NSTimeInterval)transitionDuration:(nullable id <UIViewControllerContextTransitioning>)transitionContext
{return 0.5f;
}- (void)animateTransition:(id <UIViewControllerContextTransitioning>)transitionContext
{UIViewController *toVC = (UIViewController*)[transitionContext viewControllerForKey:UITransitionContextToViewControllerKey];UIViewController *fromVC = (UIViewController*)[transitionContext viewControllerForKey:UITransitionContextFromViewControllerKey];UIView * containerView = [transitionContext containerView];UIView * fromView = fromVC.view;UIView * toView = toVC.view;[containerView addSubview:toView];[[transitionContext containerView] bringSubviewToFront:fromView];NSTimeInterval duration = [self transitionDuration:transitionContext];[UIView animateWithDuration:duration animations:^{fromView.alpha = 0.0;fromView.transform = CGAffineTransformMakeScale(0.2, 0.2);toView.alpha = 1.0;} completion:^(BOOL finished) {fromView.transform = CGAffineTransformMakeScale(1, 1);[transitionContext completeTransition:YES];}];
}
在animateTransition中，我们分别获取两个ViewController的view，将toView添加到containerView中，然后执行动画。为了理解containerView和fromView,toView的关系，我们添加几个log来分析一下：- (void)animateTransition:(id <UIViewControllerContextTransitioning>)transitionContext
{UIViewController *toVC = (UIViewController*)[transitionContext viewControllerForKey:UITransitionContextToViewControllerKey];UIViewController *fromVC = (UIViewController*)[transitionContext viewControllerForKey:UITransitionContextFromViewControllerKey];UIView * containerView = [transitionContext containerView];UIView * fromView = fromVC.view;UIView * toView = toVC.view;NSLog(@"startAnimation! fromView = %@", fromView);NSLog(@"startAnimation! toView = %@", toView);for(UIView * view in containerView.subviews){NSLog(@"startAnimation! list container subviews: %@", view);}[containerView addSubview:toView];[[transitionContext containerView] bringSubviewToFront:fromView];NSTimeInterval duration = [self transitionDuration:transitionContext];[UIView animateWithDuration:duration animations:^{fromView.alpha = 0.0;fromView.transform = CGAffineTransformMakeScale(0.2, 0.2);toView.alpha = 1.0;} completion:^(BOOL finished) {fromView.transform = CGAffineTransformMakeScale(1, 1);[transitionContext completeTransition:YES];for(UIView * view in containerView.subviews){NSLog(@"endAnimation! list container subviews: %@", view);}}];
}

运行log如下：

2016-06-29 13:50:48.512 JLTransition[1970:177922] startAnimation! fromView = <UIView: 0x7aaef4e0; frame = (0 0; 320 568); autoresize = W+H; layer = <CALayer: 0x7aaef5a0>> 2016-06-29 13:50:48.513 JLTransition[1970:177922] startAnimation! toView = <UIView: 0x796ac5a0; frame = (0 0; 320 568); autoresize = W+H; layer = <CALayer: 0x796ac050>> 2016-06-29 13:50:48.513 JLTransition[1970:177922] startAnimation! list container subviews: <UIView: 0x7aaef4e0; frame = (0 0; 320 568); autoresize = W+H; layer = <CALayer: 0x7aaef5a0>> 2016-06-29 13:50:49.017 JLTransition[1970:177922] endAnimation! list container subviews: <UIView: 0x796ac5a0; frame = (0 0; 320 568); autoresize = W+H; layer = <CALayer: 0x796ac050>> 

可见，转场执行的时候，containerView中只包含fromView，转场动画执行完毕之后，containerView会将fromView移除。因为containerView不负责toView的添加，所以我们需要主动将toView添加到containerView中。

注意！非interactive转场中，动画结束之后需要执行[transitionContext completeTransition:YES];（如果动画被取消，传NO）；但是在interactive转场中，动画是否结束是由外界控制的（用户行为或者特定函数），需要在外部调用。

第二步：定义转场

在第二部，我们需要实现UIViewControllerAnimatedTransitioning，并将第一步创建的JLScaleTransition对象返回。

JLScaleNavControlDelegate.h

@interface JLScaleNavControlDelegate : NSObject<UINavigationControllerDelegate>
@end

JLScaleNavControlDelegate.m

@implementation JLScaleNavControlDelegate
- (nullable id <UIViewControllerAnimatedTransitioning>) navigationController:(UINavigationController *)navigationController animationControllerForOperation:(UINavigationControllerOperation)operation fromViewController:(UIViewController *)fromVC toViewController:(UIViewController *)toVC
{return [JLScaleTransition new];
}
@end

这一步很简单，实现UIViewControllerAnimatedTransitioning对应方法即可。

第三步：设置转场

设置转场其实就是设置delegate（还记得我们“扯淡”的内容吧）。

   self.navigationController.delegate = id<UINavigationControllerDelegate>self.transitioningDelegate = id<UIViewControllerTransitioningDelegate>self.tabBarController.delegate = id<UITabBarControllerDelegate>

设置delegate有两种方式：通过代码；通过StoryBoard。

通过代码设置

@interface ViewController ()
@property (nonatomic, strong) JLScaleNavControlDelegate * scaleNavDelegate;
@end@implementation ViewController- (void)viewDidLoad {[super viewDidLoad];self.scaleNavDelegate = [JLScaleNavControlDelegate new];
}- (IBAction)triggerTransitionDelegate:(id)sender
{self.navigationController.delegate = self.scaleNavDelegate;[self.navigationController pushViewController:[TargetViewController new] animated:YES];
}