一、系统的转场动画

@interface  UIViewController: NSObject
@property(nonatomic,assign) UIModalTransitionStyle modalTransitionStyle ;
@property(nonatomic,assign) UIModalPresentationStyle modalPresentationStyle ;
@end

• modalPresentationStyle :最后弹出的呈现风格。仅限于UIModalPresentationFullScreen 或 UIModalPresentationCustom 这两种模式;

• modalTransitionStyle：弹出时的过渡动画风格。默认是UIModalTransitionStyleCoverVertical.常用的：
  UIModalTransitionStyleFlipHorizontal；水平翻转动画。
  UIModalTransitionStylePartialCurl；翻页动画。

如果现在有A控制器present出B控制器。层级如图所示

屏幕快照 2019-03-22 下午4.08.59.png

A控制器present 出B,系统会自动生成一个UITransitionView添加window上，并将A控制器暂时从windows移除。其中UITransitionView是一个容器视图，转场动画中它扮演一个很重要的角色。A控制器presentB，其间的过渡动画其实就是在容器视图上完成的。过渡动画完成以后，先将容器视图的动画视图移除，随后再将B控制器的视图添加到容器视图上。

#pragma mark - UIViewControllerTransitioningDelegate
//1、弹出的动画由谁来做？
- (nullable id <UIViewControllerAnimatedTransitioning>)animationControllerForPresentedController:(UIViewController *)presented presentingController:(UIViewController *)presenting sourceController:(UIViewController *)source{
    return self;
}

/*消失的动画由谁来做？
- (nullable id <UIViewControllerAnimatedTransitioning>)animationControllerForDismissedController:(UIViewController *)dismissed{
    
    return self;
}
*/

#pragma mark - UIViewControllerAnimatedTransitioning
//2.1、动画持续的时长
- (NSTimeInterval)transitionDuration:(nullable id <UIViewControllerContextTransitioning>)transitionContext{
    
    return 3;
}

#pragma mark - UIViewControllerAnimatedTransitioning
//2.2、动画该怎么做
- (void)animateTransition:(id <UIViewControllerContextTransitioning>)transitionContext{
    UIView *containerView = transitionContext.containerView;//
  
    //2.2.1、将即将做动画的视图添加到容器view上
    UIView *anitionView = [[UIImageView  alloc]initWithImage:[UIImage imageNamed:@"timg.jpeg"]];
    [containerView addSubview:anitionView];
    
    
    //2.2.2、实现动画的细节
    anitionView.frame = CGRectMake(0, 0, 100, 100);//确定动画视图的起始位置
    
    [UIView animateWithDuration:3 animations:^{
     anitionView.frame = self.view.bounds;////确定动画视图的结束位置位置
    } completion:^(BOOL finished) {
        
        
        [transitionContext completeTransition:YES];//必须告诉系统过渡动画完成了，才可以继续交互。
        
        
        [anitionView removeFromSuperview];//将anitionView从容器View上移除
        
        //2.2.3、添加目标View到容器View上。默认情况下目标View不会被添加到containerView上，需要我们手动添加才可以。
        UIView *toView = [transitionContext viewForKey:UITransitionContextToViewKey];
        
        //UIViewController *vc = [transitionContext viewControllerForKey:UITransitionContextToViewControllerKey];
        //UIView *toView = vc.view;
     
        [containerView addSubview:toView];
    }];
    
}

关于dismiss原理一样就不再做赘述。