• 自定义转场动画,需要遵守 UIViewControllerAnimatedTransitioning 协议,该协议有两个方法:

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

第一个方法返回转场执行的时间(动画执行的时间),第二个方法是处理转场动画(想要实现什么样的转场动画,在这个方法中实现便可).
这里介绍几个背景知识:fromVC,toVC,fromView,toView,containerView
如果 A 视图控制器present到 B ,则 A 为 fromVC , A.view 为 fromView , B 为 toVC , B.view 为 toView ; 如果 B 视图控制器dismiss到 A , 则 B 为 fromVC , B.view 为 fromView , A 为 toVC , A.view 为 toView ; containerView:容器视图,所有的动画都要在容器视图上完成.
注:动画执行完成必须执行[transitionContext completeTransition:YES];

下面介绍转场动画的实现步骤:

1. 创建继承自 NSObject 的类,并遵守 UIViewControllerAnimatedTransitioning 协议,并实现协议中的两个方法.
2. 让presened视图控制器 遵守 UIViewControllerTransitioningDelegate 协议,设置代理为自己(self.transitioningDelegate = self;),并实现其中的两个方法:

- (nullable id < UIViewControllerAnimatedTransitioning >)animationControllerForPresentedController:(UIViewController *)presented presentingController:(UIViewController *)presenting sourceController:(UIViewController *)source;

- (nullable id < UIViewControllerAnimatedTransitioning >)animationControllerForDismissedController:(UIViewController *)dismissed;

两个方法的返回值类型都为 id < UIViewControllerAnimatedTransitioning >,只需要返回自定义动画类对应的方法即可.
第一个方法返回的是present时执行的动画,第二个方法返回的是dismiss时执行的动画.

代码如下:
.h 文件如下:

typedef NS_ENUM(NSInteger, DDPresentOneTransitionType) {
    DDPresentOneTransitionTypePresent = 0,
    DDPresentOneTransitionTypeDismiss
};

@interface DDPresentOneTransition : NSObject<UIViewControllerAnimatedTransitioning>

+ (instancetype)transitionWithTransitionType:(DDPresentOneTransitionType)type;

- (instancetype)initWithTransitionType:(DDPresentOneTransitionType)type;

@end
定义枚举,用来区分是presen还是dismiss,定义初始化方法,一个类方法,一个实例方法.

.m 文件如下:
```objective-c
@interface DDPresentOneTransition ()
@property (nonatomic, assign)DDPresentOneTransitionType type;
@property (nonatomic, strong) UIImageView *originalIV;//存储点击位置图片的ImageView
@property (nonatomic, strong) UIImageView *targetIV;//存储将要显示位置图片的ImageView

@end

@implementation DDPresentOneTransition

+ (instancetype _Nonnull )transitionWithTransitionType:(DDPresentOneTransitionType)type imageView:(UIImageView *_Nonnull)imageView
{
    return [[self alloc] initWithTransitionType:type imageView:imageView];
}

- (instancetype _Nonnull )initWithTransitionType:(DDPresentOneTransitionType)type imageView:(UIImageView *_Nonnull)imageView
{
    self = [super init];
    if (self) {
        _type = type;
        _originalIV = imageView;
    }
    return self;
}

- (NSTimeInterval)transitionDuration:(id<UIViewControllerContextTransitioning>)transitionContext
{
    if (_type == DDPresentOneTransitionTypePresent) {
        return 0.25;
    }else{
        return 0.25;
    }
}

- (void)animateTransition:(id<UIViewControllerContextTransitioning>)transitionContext
{
    switch (_type) {
        case DDPresentOneTransitionTypePresent:
            [self presentAnimateTransition:transitionContext];
            break;
        case DDPresentOneTransitionTypeDismiss:
            [self dismissAnimateTransition:transitionContext];
            break;
        default:
            break;
    }
}

- (void)presentAnimateTransition:(id<UIViewControllerContextTransitioning>)transitionContext
{
    UIViewController *fromVC = [transitionContext viewControllerForKey:UITransitionContextFromViewControllerKey];
    UIViewController *toVC = [transitionContext viewControllerForKey:UITransitionContextToViewControllerKey];

    UIView *containerView = [transitionContext containerView];
    containerView.backgroundColor = [UIColor whiteColor];

    UIView *tempView = [self.originalIV snapshotViewAfterScreenUpdates:YES];
    tempView.frame = [self.originalIV convertRect:self.originalIV.bounds toView:containerView];

    self.originalIV.hidden = YES;
    toVC.view.alpha = 0;

    [containerView addSubview:toVC.view];
    [containerView addSubview:tempView];

    //290*121

    UIImage *image = self.originalIV.image;
    CGFloat imageH = ceil(image.size.height);
    CGFloat imageW = ceil(image.size.width);
    CGFloat height = ceil(kScreenWidth*imageH/imageW);

    [UIView animateWithDuration:[self transitionDuration:transitionContext] animations:^{
        tempView.frame = CGRectMake(0, 0, kScreenWidth, height);

        fromVC.view.alpha = 0;
    } completion:^(BOOL finished) {
        toVC.view.alpha = 1;
        tempView.hidden = YES;
        [transitionContext completeTransition:YES];
    }];

}

- (void)dismissAnimateTransition:(id<UIViewControllerContextTransitioning>)transitionContext
{
    UIViewController *fromVC = [transitionContext viewControllerForKey:UITransitionContextFromViewControllerKey];
    UIViewController *toVC = [transitionContext viewControllerForKey:UITransitionContextToViewControllerKey];
    
    UIView *containerView = [transitionContext containerView];
    containerView.backgroundColor = [UIColor whiteColor];
    UIView *tempView = containerView.subviews.lastObject;
    
    self.originalIV.hidden = YES;
    fromVC.view.hidden = YES;
    tempView.hidden = NO;
    toVC.view.alpha = 0;
    
    [containerView insertSubview:toVC.view atIndex:0];
    
    [UIView animateWithDuration:[self transitionDuration:transitionContext] animations:^{
        tempView.frame = [self.originalIV convertRect:self.originalIV.bounds toView:containerView];
        toVC.view.alpha = 1;
    } completion:^(BOOL finished) {
        self.originalIV.hidden = NO;
        tempView.hidden = YES;
        [tempView removeFromSuperview];
        [transitionContext completeTransition:YES];
    }];
}