Modal

modal轉場方式即使用 presentViewController() 方法推出的方式，默認情況下，第二個視圖從屏幕下方彈出。下面就來介紹下 modal 方式轉場動畫的自定義。

present

還是先來看一下完成的效果

準備

1. 創建一個新的工程，刪掉Main，在AppDelegate中創建自定義UIWindow，設置rootVC為ViewController。
2. 在ViewController中創建一個全屏的ImageView，并且指定一張圖片；新建一個按鈕用于present。
3. 新建一個SecondViewController。同樣創建一個全屏的ImageView，指定一張和ViewController不同的圖片，新建一個同于dismiss的按鈕

開始

1. 創建一個文件繼承自 NSObject，取名為PresentTransition并在.h中遵守 UIViewControllerAnimatedTransitioning協議。
2. 實現協議的兩個方法，并在其中編寫 Push 的動畫。 具體的動畫實現過程都在代碼的注釋里：

// 返回動畫的時間
- (NSTimeInterval)transitionDuration:(nullable id <UIViewControllerContextTransitioning>)transitionContext{
    return 0.8;
}
- (void)animateTransition:(id <UIViewControllerContextTransitioning>)transitionContext{
    ViewController * fromVC = [transitionContext viewControllerForKey:UITransitionContextFromViewControllerKey];
    SecondViewController * toVC = [transitionContext viewControllerForKey:UITransitionContextToViewControllerKey];
    UIView * container = [transitionContext containerView];
    
    [container addSubview:toVC.view];
    [container bringSubviewToFront:fromVC.view];
    
    // 改變m34
    CATransform3D transfrom = CATransform3DIdentity;
    transfrom.m34 = -0.002;
    container.layer.sublayerTransform = transfrom;
    
    // 設置archPoint和position
    CGRect initalFrame = [transitionContext initialFrameForViewController:fromVC];
    toVC.view.frame = initalFrame;
    fromVC.view.frame = initalFrame;
    fromVC.view.layer.anchorPoint = CGPointMake(0, 0.5);
    fromVC.view.layer.position = CGPointMake(0, initalFrame.size.height / 2.0);
    
    // 添加陰影效果
    CAGradientLayer * shadowLayer = [[CAGradientLayer alloc] init];
    shadowLayer.colors =@[
                         [UIColor colorWithWhite:0 alpha:1],
                         [UIColor colorWithWhite:0 alpha:0.5],
                         [UIColor colorWithWhite:1 alpha:0.5]
                         ];
    shadowLayer.startPoint = CGPointMake(0, 0.5);
    shadowLayer.endPoint = CGPointMake(1, 0.5);
    shadowLayer.frame = initalFrame;
    
    UIView * shadow = [[UIView alloc] initWithFrame:initalFrame];
    shadow.backgroundColor = [UIColor clearColor];
    [shadow.layer addSublayer:shadowLayer];
    [fromVC.view addSubview:shadow];
    shadow.alpha = 0;
    
    // 動畫
    [UIView animateKeyframesWithDuration:[self transitionDuration:transitionContext] delay:0 options:2 animations:^{
        fromVC.view.layer.transform = CATransform3DMakeRotation(-M_PI_2, 0, 1, 0);
        shadow.alpha = 1.0;
    } completion:^(BOOL finished) {
        fromVC.view.layer.anchorPoint = CGPointMake(0.5, 0.5);
        fromVC.view.layer.position = CGPointMake(CGRectGetMidX(initalFrame), CGRectGetMidY(initalFrame));
        fromVC.view.layer.transform = CATransform3DIdentity;
        [shadow removeFromSuperview];
        
        [transitionContext completeTransition:YES];
    }];
}

使用動畫

1. 讓 FirstViewController 遵守 UIViewControllerTransitioningDelegate 協議，并將 self.transitioningDelegate 設置為 self。

self.transitioningDelegate = self;

2. 實現 UIViewControllerTransitioningDelegate 協議的兩個方法，用來指定動畫類。

// present動畫
-(id<UIViewControllerAnimatedTransitioning>)animationControllerForPresentedController:(UIViewController *)presented presentingController:(UIViewController *)presenting sourceController:(UIViewController *)source{
    return [[PresentTransition alloc] init];
}

3. 在present按鈕的點擊方法中，必須把SecondViewController的transitioningDelegate也設置為self，讓ViewController管理SecondViewController的動畫

-(void)presentClick{
    SecondViewController * secondVC = [[SecondViewController alloc] init];
    secondVC.transitioningDelegate = self; // 必須second同樣設置delegate才有動畫
    [self presentViewController:secondVC animated:YES completion:^{
    }];
}