自定义转场动画

写在前面

本文中提到的 presented 和 presenting ，分别指被展示者和展示者，譬如 [viewControllerA presentViewController:viewControllerB animated:NO] 中，viewControllerA 是 presenting，viewControllerB 是 presented。

从熟悉的地方开始

苹果提供的几种转场动画，可以用 UIModalPresentationStyle 和 UIModalTransitionStyle 来设置，从 presentation 和 transition 两个关键词的语意来理解转场动画，presentation 重静态的展示，可通过 UIModalPresentationStyle 来定义 presented view 的展示形式（全屏、formsheet、popover 等）；transition 重动态的转场，可通过 UIModalTransitionStyle 来定义从 presenting view 上呈现 presented view 的动画。

UIModalPresentationStyle	Discussion
UIModalPresentationFullScreen	全屏展示presented view，presenting view 在转场动画完成后被移除
UIModalPresentationPageSheet	horizontally compact 场景下同 UIModalPresentationFullScreen；horizontally regular 场景下 presented view 的 width 和 height 等于 presenting view 在 portrait 模式下的 width 与 height；未遮挡部分虚化禁止用户交互
UIModalPresentationFormSheet	horizontally compact 场景下同 UIModalPresentationFullScreen; horizontally regular 场景下，presented view 的长宽小于 screen 、居中显示，landscape mode 下presented view 会随着键盘的弹出而上移；未遮挡部分虚华禁止用户交互
UIModalPresentationCurrentContext	转场动画开始前，UIKit 开始从 presenting view controller 向上寻找，presented view controller 的内容将覆盖第一个找到的 definesPresentationContext = YES 的 view controller 的内容；转场动画结束时，被覆盖的内容将被移除
UIModalPresentationCustom	将转场动画交由 view controller 的 transitioningDelegate 对象来管理
UIModalPresentationOverFullScreen	presented view 覆盖的 content 不会从 view hierarchy 中移除，未被遮挡的部分对用户可见
UIModalPresentationOverCurrentContext	CurrentContext 和 OverFullScreen 的结合
UIModalPresentationPopover	horizontally regular 场景下， popover 展示；horizontally compact 场景下，同 FullScreen

UIModalTransitionStyle	Discussion
UIModalTransitionStyleCoverVertical	默认值，从屏幕底部上推
UIModalTransitionStyleFlipHorizontal	旋转门 自己体会
UIModalTransitionStyleCrossDissolve	渐入渐出
UIModalTransitionStylePartialCurl	浮夸的翻页

(transition style 和 presentation style 由 presented view controller 来设置，这也符合 presented one 不应该依赖 presenting one 的思想。)

如果上述预定义转场动画不能满足你，那么需要实现自定义动画；自定义动画主要通过实现或集成了 UIViewControllerAnimatedTransitioning 协议和 UIPresentationController 类的动画控制器 animator 来实现，稍作了解后，你会发现 UIViewControllerAnimatedTransitioning 和 UIModalTransitionStyle 相似， UIPresentationController 和 UIModalPresentationStyle 相似。

预定义转场动画通过给 UIViewController 的 modalPresentationStyle 和 modalTransitionStyle 属性赋值来实现，而自定义转场动画则须是令 modalPresentationStyle = UIModalPresentationStyleCustom ，并且给 transitioningDelegate 赋值一个继承了 UIViewControllerTransitioningDelegate 的对象 (初次使用时容易把 UIViewControllerAnimatedTransitioning 和 UIViewControllerTransitioningDelegate 弄混，其实二者是截然不同的概念)，继而在此对象的协议方法中分配动画控制器。通常我们使用当前的 view controller 来实现 UIViewControllerTransitioningDelegate 协议，具体代码如下：

@interface ViewController () <UIViewControllerTransitioningDelegate>
@end

@implementation ViewController 

- (instancetype)initWithNibName:(NSString *)nibNameOrNil bundle:(NSBundle *)nibBundleOrNil
{
  if ((self = [super initWithNibName:nibNameOrNil bundle:nibBundleOrNil]) != nil) {
    [self setModalPresentationStyle:UIModalPresentationCustom];
    [self setTransitioningDelegate:self];
  }

  return self;
}

- (id <UIViewControllerAnimatedTransitioning>)animationControllerForPresentedController:(UIViewController *)presented presentingController:(UIViewController *)presenting sourceController:(UIViewController *)source
{
  return nil; // return a transition animator  
}

- (id <UIViewControllerAnimatedTransitioning>)animationControllerForDismissedController:(UIViewController *)dismissed
{
  return nil; // return a transition animator
}

- (UIPresentationController *)presentationControllerForPresentedViewController:(UIViewController *)presented presentingViewController:(UIViewController *)presenting sourceViewController:(UIViewController *)source
{
  return nil; // return a presentation animator
}

上述代码中，animationControllerForPresentedController 要求 programer 返回一个 present 当前 ViewController 的动画控制器，animationControllerForDismissController 要求 coder 返回一个 dismiss 当前 view controller 的动画控制器，如何编写动画控制器就是我们要发挥想象力的地方了。

好在苹果给了一个很好用的协议，就是上文中提到的 UIViewControllerAnimatedTransitioning ,它有以下方法：

- (void)animateTransition:(id<UIViewControllerContextTransitioning>)transitionContext;

• calls when presenting or dismissing a view controller
• configure your custom transition in it
• use view-based animation or core animation as you like
• all animations must take place in view specified by the containerView property of transitionContext

animateTransition: 前三段解释都好理解，第四条提出了一个 containerView 的属性， containerView 是转场动画中涉及的所有 view 的 superview，且 containerView 默认添加了 presenting view controller 的 view ，你要做的事情是将 presented view controller 的 view 也添加到 containerView 上。
（不得不说 containerView 是 UIKit 作的很大的一个改变，在 iOS5 之前，view controller 被苹果封装得“紧密严合”，虽然知道不同 view controller 之前的转场不过是 view 的叠加，但我们不能真的用[presentingViewController.view addSubview:presentedViewController.view]这样的代码来替代 presentViewController:方法，而在 animateTransition: 方法中，你可以真切地感受到 view 的 present 与 dismiss 是通过 addSubview: 和 removeFromSuperView 来完成的，不同之处是 containerView 必须是所有 view 的 "container"）

- (void)animationEnded:(BOOL)transitionCompleted;

• transitionCompleted = YES if the transition completed successfully and the new view controller is displayed
• transitionCompleted = NO if the transition is canceled and the original view is still visible
• use this method to perform any final cleanup operations required by your transition animator

animationEnded: 类似 completion handler 和 failure handler

- (NSTimeInterval)transitionDuration:(id<UIViewControllerContextTransitioning>)transitionContext;

• return the duration in seconds of your custom transition
• the value you provide should be the same value that you use when configuring the animations in your animateTransition: method.
• UIKit uses the value to synchronize the actions of other objects that might be involved in the transition

了解了必要的知识以后，开始做真正有意思的事情吧~
（如果你想了解实现思路，可以浏览后文中的 GIF 图和伪代码；如果想要看到具体的实现，可以使用【真·代码阅读术】前往 JYCustomTransition 哦）

仿iPhone相册－点击图片放大的转场动画

photo-compressed.gif

深情凝视 iOS 系统自带相册，会发现在照片流里 "点击一张照片放大查看" 的过程，看似是 UIImageView 的 frame 发生了变化，实际上是 UICollectionViewController 切换到了 UIPageViewController 。当然这只是没找到源代码之后基于观察的猜测，两个不同 view controller 切换更符合 Cocoa Frameworks 中一贯的各司其职的原则。
那么，仿制一个苹果系统相册，coder 须要做的是将 presentViewController 的具体过程委托给自定义的第三方（也就是上文提到的动画控制器）来实现，以达到欺骗用户眼球的效果。重点在于我们看到的 “整齐排列的照片中一张照片被放大” 的效果，笔者的做法是创建一个独立于 viewController 的 animateImageView ，设置 animateImageView 的隐式动画来欺骗眼球，动画完成后即 remove，伪代码如下:

- (void)animateTransition:(id <UIViewControllerContextTransitioning>)transitionContext
{
     UIImage *image = getImageFromPhotoGraphy();
     UIImageView *animateImageView = [[UIImageView alloc] initWithImage:image];
     [animateImageView setFrame:getInitialFrameFromPhotoGraphy()];
     [containerView addSubview:animateImageView ];

     [UIView animateWithDuration:0.5 animations:^{
         [animateImageView setFrame:getFinalFrameFromPhotoGraphy()];
     } completion: ^(BOOL finished) {
         [animateImageView removeFromSuperView]
     }]
}

在具体实现的过程中，笔者还考虑了不同图片尺寸的修正来达到视觉上的流畅效果，感兴趣的小伙伴可以前往 JYCustomTransition 查看~

为了欺骗而欺骗－开门/关门的转场动画

首先直接上GIF图吧

door_effect-iloveimg-compressed.gif
依然是 “看起来” 是一张图片被切割、被旋转，实际上是 view controller 之间的转场。
(笔者在敲代码的时候想了想，非游戏类的 App 应该不会特意做这么花哨并且毫无意义的转场动画，不过最后还是决定做出来，一个是为了向小伙伴们展示 coder 们为了欺骗用户眼球可以做到什么地步，二是为了介绍转场过程中用到的干货 - CATransform 3D，有很多漂亮的转场效果都有小小的用到 3D Animation 哦)
具体的实现过程是用 UIKit 提供的 UIGraphicsGetImageFromCurrentImageContext() 给 presenting view controller（雪山背景那个）截屏生成 snapshotImage，通过对snapshotImage切割生成的 UIImageView 设置 transform 属性来实现旋转开门/关门效果，伪代码如下：

- (void)animateTransition:(id<UIViewControllerContextTransitioning>)transitionContext
{
    UIImage *snapshotImage = getSnapshotImageFromPresentingViewController();

    UIImageView *leftImageView = cropImageViewFromLeftSnapshotImage();
    [[leftImageView layer] setAnchorPoint:CGPointMake(0.0, 0.5)];
    [leftImageView setFrame:leftRect];
    [containerView insertSubview:leftImageView aboveSubview:toView];

    UIImageView *rightImageView = cropImageViewFromRightSnapshotImage();
    [[rightImageView layer] setAnchorPoint:CGPointMake(1.0, 0.5)];
    [rightImageView setFrame:rightRect];
    [containerView insertSubview:rightImageView aboveSubview:toView];

    CATransform3D leftRotateTransform = CATransform3DIdentity;
    leftRotateTransform.m34 = 4.5 / -2000;
    leftRotateTransform = CATransform3DRotate(leftRotateTransform, 90.0 * M_PI / 180.0f, 0, 1.0, 0);

    CATransform3D rightRotateTransform = CATransform3DIdentity;
    rightRotateTransform.m34 = 4.5 / -2000;
    rightRotateTransform = CATransform3DRotate(rightRotateTransform, -90.0 * M_PI / 180.0f, 0, 1.0, 0);

    [UIView animateWithDuration:0.5 delay:0.0 options:UIViewAnimationOptionCurveEaseOut animations:^{
      [[leftImageView layer] setTransform:leftRotateTransform];
      [[rightImageView layer] setTransform:rightRotateTransform];
    } completion:^(BOOL finished) {
       [leftImageView removeFromSuperview];
      [rightImageView removeFromSuperview];
    }];

}

笔者在具体实现过程遇到了多个 layer 相互遮挡的问题，后来发现是由于 [leftImageView layer] 和 [rightImageView layer] 的 z 坐标小于 presented view controller 的 layer 的 z 坐标的缘故，举一反三的小伙伴们可以停下来想想怎么解决这个问题，也可以空降到 JYCustomTransition 去看看具体的解决方法哦

终于用到了UIPresentationController－弹出便笺的转场动画

memo-compressed.gif
最后这个看起来最朴素的动画包含了 UIViewControllerAnimatedTransitioning 、UIPresentationController 两大主力， UIViewControllerAnimatedTransitioning 实现转场中的 frame 变化，伪代码如下：

- (void)animateTransition:(id<UIViewControllerContextTransitioning>)transitionContext
{
  UIViewController *fromViewController = [transitionContext viewControllerForKey:UITransitionContextFromViewControllerKey];
  UIViewController *toViewController = [transitionContext viewControllerForKey:UITransitionContextToViewControllerKey];
  UIView *fromView = [transitionContext viewForKey:UITransitionContextFromViewKey];
  UIView *toView = [transitionContext viewForKey:UITransitionContextToViewKey];
  UIView *containerView = [transitionContext containerView];

  const CGRect endFrame = [transitionContext finalFrameForViewController:toViewController];
  const CGRect startFrame = CGRectOffset(endFrame, 0.0, CGRectGetHeight(endFrame));

  [toView setFrame:startFrame];
  [toView setAlpha:0.0f];
  [containerView addSubview:toView];

  [UIView animateWithDuration:0.2 delay:0.0 options:UIViewAnimationOptionBeginFromCurrentState | UIViewAnimationOptionCurveEaseIn animations:^{
    [toView setFrame:endFrame];
    [toView setAlpha:1.0];
  } completion:^(BOOL finished) {
    if ([transitionContext transitionWasCancelled]) {
      [toView removeFromSuperview];
    }
    [transitionContext completeTransition:YES];
  }];
}

而展示的效果则是在 UIPresentationController 的子类实现，设置了 presented view controller 中的 view 的 frame（portrait 模式下固定高度 500，landscape 模式下占据全屏），并为 view 加了一个半透明的背景 backgroundView ，这样在 presented view controller 的初始化方法中就可以放心大胆的使用 [[self view] addSubview:subview] 而不用担心 subview 相对上边界的偏移，伪代码如下：

@implementation _JYMemoViewController_PresentationController

- (instancetype)initWithPresentedViewController:(UIViewController *)presentedViewController presentingViewController:(nullable UIViewController *)presentingViewController
{
  if ((self = [super initWithPresentedViewController:presentedViewController presentingViewController:presentingViewController]) != nil) {
    _backgroundView = [[UIView alloc] initWithFrame:CGRectZero];
    [_backgroundView setUserInteractionEnabled:NO];
    [_backgroundView setTranslatesAutoresizingMaskIntoConstraints:NO];

    _tapGestureRecognizer = [[UITapGestureRecognizer alloc] initWithTarget:self action:@selector(_handleTapGestureRecognizer:)];
    [_tapGestureRecognizer setDelegate:self];
  }

  return self;
}

- (CGRect)frameOfPresentedViewInContainerView
{
  CGFloat height = fmin(CGRectGetHeight([[self containerView] frame]), 500.0);
  return CGRectMake(0.0, CGRectGetMaxY([[self containerView] frame]) - height, CGRectGetWidth([[self containerView] frame]), height);
}

- (void)presentationTransitionWillBegin
{
  [super presentationTransitionWillBegin];

  [_backgroundView setBackgroundColor:[UIColor colorWithWhite:0.0 alpha:0.5]];
  [_backgroundView setAlpha:0.0];
  [[self containerView] addSubview:_backgroundView];

  [[[_backgroundView topAnchor] constraintEqualToAnchor:[[self containerView] topAnchor]] setActive:YES];
  [[[_backgroundView bottomAnchor] constraintEqualToAnchor:[[self containerView] bottomAnchor]] setActive:YES];
  [[[_backgroundView leadingAnchor] constraintEqualToAnchor:[[self containerView] leadingAnchor]] setActive:YES];
  [[[_backgroundView trailingAnchor] constraintEqualToAnchor:[[self containerView] trailingAnchor]] setActive:YES];

  [[[self presentingViewController] transitionCoordinator] animateAlongsideTransition:^(id <UIViewControllerTransitionCoordinatorContext>  context) {
    [_backgroundView setAlpha:1.0];
  } completion:^(id <UIViewControllerTransitionCoordinatorContext> context) {
    if ([context isCancelled]) {
      [[[self presentedView] layer] setShadowOpacity:0.0f];
      [_backgroundView setAlpha:0.0];
    }
  }];
}

- (void)presentationTransitionDidEnd:(BOOL)completed
{
  if (completed) {
    [[self containerView] addGestureRecognizer:_tapGestureRecognizer];
  }

  [super presentationTransitionDidEnd:completed];
}

- (void)dismissalTransitionWillBegin
{
  [super dismissalTransitionWillBegin];

  [[self containerView] removeGestureRecognizer:_tapGestureRecognizer];

  [[[self presentingViewController] transitionCoordinator] animateAlongsideTransition:^(id <UIViewControllerTransitionCoordinatorContext> context) {
    [[[self presentedView] layer] setShadowOpacity:0.0f];
    [_backgroundView setAlpha:0.0];
  } completion:^(id <UIViewControllerTransitionCoordinatorContext> context) {
    if ([context isCancelled]) {
      [[[self presentedView] layer] setShadowOpacity:1.0f];
      [_backgroundView setAlpha:1.0];
    }
    else {
      [_backgroundView removeFromSuperview];
    }
  }];
}

- (void)dismissalTransitionDidEnd:(BOOL)completed
{
  if (!completed) {
    [[self containerView] addGestureRecognizer:_tapGestureRecognizer];
  }

  [super dismissalTransitionDidEnd:completed];
}

- (void)viewWillTransitionToSize:(CGSize)size withTransitionCoordinator:(id <UIViewControllerTransitionCoordinator>)coordinator
{
  [super viewWillTransitionToSize:size withTransitionCoordinator:coordinator];

  [coordinator animateAlongsideTransition:^(id <UIViewControllerTransitionCoordinatorContext> context) {
    [[self presentedView] setFrame:CGRectMake(CGRectGetMinX([[self containerView] frame]),  CGRectGetMinY([[self containerView] frame]) + fmax(size.height - 500.0, 0.0), CGRectGetWidth([[self containerView] frame]), CGRectGetHeight([[self containerView] frame]) - fmax(size.height - 500.0, 0.0))];
  } completion:nil];
}

- (void)_handleTapGestureRecognizer:(UIGestureRecognizer *)gestureRecognizer
{
  [[self presentingViewController] dismissViewControllerAnimated:YES completion:nil];
}

#pragma mark - UIGestureRecognizerDelegate

- (BOOL)gestureRecognizer:(UIGestureRecognizer *)gestureRecognizer shouldReceiveTouch:(UITouch *)touch
{
  if (gestureRecognizer == _tapGestureRecognizer) {
    if ([touch view] != nil) {
      return [touch view] != [self presentedView] && ![[touch view] isDescendantOfView:[self presentedView]];
    }
  }
  
  return YES;
}

@end

依然可以前往 JYCustomTransition 看到全部代码哦