iOS 转场动画：以Push方式实现Present跳转

在iOS开发中，页面跳转一般有两种方式：

• navigation：push & pop，动画是从右到左
• modal（模态）：presnet & dismiss，动画是从下到上

在SDK开发中，由于需要减小侵入性，通常会使用modal方式弹出SDK的页面，那么默认唤起的动画就是从下到上。现在有一个需求，需要从右到左唤起SDK的页面，所以我们需要用到转场动画，将present的动画改成从右到左，也就是以Push方式实现Present跳转。

开始前

实现模态的转场动画的步骤，大概分以下几步：

1. 自定义一个遵循的<UIViewControllerAnimatedTransitioning>协议的动画转场管理对象，并实现两个必须实现的方法：

   //返回动画时间  
   - (NSTimeInterval)transitionDuration:(nullable id <UIViewControllerContextTransitioning>)transitionContext;
   //所有的转场动画事务都在这个方法里面完成
   - (void)animateTransition:(id <UIViewControllerContextTransitioning>)transitionContext;

2. 成为相应的代理，实现相应的代理方法，返回上面的自定义协议对象

   //返回一个管理prenent动画转场的对象
    - (nullable id <UIViewControllerAnimatedTransitioning>)animationControllerForPresentedController:(UIViewController *)presented presentingController:(UIViewController *)presenting sourceController:(UIViewController *)source;
    //返回一个管理dismiss动画转场的对象
    - (nullable id <UIViewControllerAnimatedTransitioning>)animationControllerForDismissedController:(UIViewController *)dismissed;

3. vc调用presentViewController、dismissViewControllerAnimated时开启 animated

下面按照这个步骤来实现功能

ZDPushAnimatedTransition

创建一个类 ZDPushAnimatedTransition 来封装转场动画

typedef NS_ENUM(NSUInteger, ZDPushAnimatedTransitionType) {
    ZDPushAnimatedTransitionTypePresent, // present动画
    ZDPushAnimatedTransitionTypeDismiss  // dismiss动画
};

/// push动画
@interface ZDPushAnimatedTransition : NSObject<UIViewControllerAnimatedTransitioning>

- (instancetype)initWithTransitionType:(ZDPushAnimatedTransitionType)type;

@end

实现系统的 UIViewControllerAnimatedTransitioning 协议

- (NSTimeInterval)transitionDuration:(id<UIViewControllerContextTransitioning>)transitionContext {
    return 0.25;
}

- (void)animateTransition:(id<UIViewControllerContextTransitioning>)transitionContext {
    switch (_type) {
        case ZDPushAnimatedTransitionTypePresent:
            [self presentAnimation:transitionContext];
            break;
        case ZDPushAnimatedTransitionTypeDismiss:
            [self dismissAnimation:transitionContext];
            break;
    }
}

实现present动画，实际就是使用UIView动画来改变 fromVC 和 toVC 的 view 的位置

/**
 A present to B. fromVC = A, toVC = B
 */
- (void)presentAnimation:(id<UIViewControllerContextTransitioning>)transitionContext {
    
    UIViewController *fromVC = [transitionContext viewControllerForKey:UITransitionContextFromViewControllerKey];
    UIViewController *toVC = [transitionContext viewControllerForKey:UITransitionContextToViewControllerKey];

    UIView *containerView = [transitionContext containerView];
    [containerView addSubview:fromVC.view];
    [containerView addSubview:toVC.view];
    
    toVC.view.frame = CGRectMake(containerView.bounds.size.width, 0, containerView.bounds.size.width, containerView.bounds.size.height);
    [UIView animateWithDuration:[self transitionDuration:transitionContext] delay:0 options:UIViewAnimationOptionCurveEaseInOut animations:^{
        fromVC.view.transform = CGAffineTransformMakeTranslation(-containerView.bounds.size.width / 3, 0);
        toVC.view.transform = CGAffineTransformMakeTranslation(-containerView.bounds.size.width, 0);
    } completion:^(BOOL finished) {
        [transitionContext completeTransition:![transitionContext transitionWasCancelled]];
    }];
}

实现dismiss动画，和上面的present动画相反，将fromVC 和 toVC 的 view 的位置还原

/**
  B dismiss to A. fromVC = B, toVC = A
  */
- (void)dismissAnimation:(id<UIViewControllerContextTransitioning>)transitionContext {
  
    UIViewController *fromVC = [transitionContext viewControllerForKey:UITransitionContextFromViewControllerKey];
    UIViewController *toVC = [transitionContext viewControllerForKey:UITransitionContextToViewControllerKey];
    
    UIView *containerView = [transitionContext containerView];
    [containerView addSubview:toVC.view];
    [containerView addSubview:fromVC.view];
    
    [UIView animateWithDuration:[self transitionDuration:transitionContext] delay:0 options:UIViewAnimationOptionCurveEaseInOut animations:^{
        fromVC.view.transform = CGAffineTransformIdentity;
        toVC.view.transform = CGAffineTransformIdentity;
    } completion:^(BOOL finished) {
        [transitionContext completeTransition:![transitionContext transitionWasCancelled]];
    }];
}

这样，整个转场动画的封装就完成了，下面来实现代理

实现代理

在要present出来的vc中实现系统代理 UIViewControllerTransitioningDelegate

// 设置代理
self.transitioningDelegate = self;


#pragma mark - UIViewControllerTransitioningDelegate
- (nullable id <UIViewControllerAnimatedTransitioning>)animationControllerForPresentedController:(UIViewController *)presented presentingController:(UIViewController *)presenting sourceController:(UIViewController *)source {
    return [[ZDPushAnimatedTransition alloc] initWithTransitionType:ZDPushAnimatedTransitionTypePresent];
}

- (nullable id <UIViewControllerAnimatedTransitioning>)animationControllerForDismissedController:(UIViewController *)dismissed {
    return [[ZDPushAnimatedTransition alloc] initWithTransitionType:ZDPushAnimatedTransitionTypeDismiss];
}

这样的话，整个功能就基本实现了

但是，体验上还缺少一点：没有侧滑返回。所以，接下来，我们实现侧滑返回的交互转场

ZDPopInteractiveTransition

创建一个继承自 UIPercentDrivenInteractiveTransition 的类 ZDPopInteractiveTransition 来封装交互转场

@interface ZDPopInteractiveTransition : UIPercentDrivenInteractiveTransition

@property (nonatomic, assign) BOOL isTransitioning;

- (instancetype)initWithViewController:(UIViewController *)vc;

@end

为传入的vc添加拖动手势

- (instancetype)initWithViewController:(UIViewController *)vc {
    self = [super init];
    if (self) {
        _viewControllerDismissing = vc;
        [self addPanGestureForViewController:viewController];
    }
    return self;
}

- (void)addPanGestureForViewController:(UIViewController *)vc {
    UIPanGestureRecognizer *pan = [[UIPanGestureRecognizer alloc] initWithTarget:self action:@selector(handleGesture:)];
    pan.delegate = self;
    [vc.view addGestureRecognizer:pan];
}

拖动手势是加在全屏上的，我们需要将它限制到屏幕左侧，这样才是侧滑返回，否则是全屏返回

#pragma mark - UIGestureRecognizerDelegate
/// 侧滑返回
- (BOOL)gestureRecognizerShouldBegin:(UIGestureRecognizer *)gestureRecognizer {
    CGPoint location = [gestureRecognizer locationInView:gestureRecognizer.view];
    return location.x < 50;
}

核心是处理手势交互转场的过程，我们需要更新转场百分比 updateInteractiveTransition ，标记 完成转场 finishInteractiveTransition 和取消转场 cancelInteractiveTransition

/// 手势过渡的过程
- (void)handleGesture:(UIPanGestureRecognizer *)panGesture {
    // 手势百分比
    CGFloat transitionX = [panGesture translationInView:panGesture.view].x;
    CGFloat percent = transitionX / panGesture.view.frame.size.width;
    // 侧滑速度，速度大于某值，判断完成
    CGFloat velocityX = [panGesture velocityInView:panGesture.view].x;

    switch (panGesture.state) {
        case UIGestureRecognizerStateBegan:
            // 手势开始的时候标记手势状态，并开始相应的事件
            self.isTransitioning = YES;
            [self startGesture];
            break;
        case UIGestureRecognizerStateChanged:
            // 手势过程中，通过updateInteractiveTransition设置转场的百分比
            [self updateInteractiveTransition:percent];
            break;
        case UIGestureRecognizerStateEnded:
            // 手势完成后结束标记并且判断移动距离是否过半，过则finishInteractiveTransition完成转场操作，否者取消转场操作
            self.isTransitioning = NO;
            if (percent > 0.3 || velocityX > 300) {
                [self finishInteractiveTransition];
            } else {
                [self cancelInteractiveTransition];
            }
            break;
        default:
            break;
    }
}

- (void)startGesture {
    [_viewControllerDismissing dismissViewControllerAnimated:YES completion:nil];
}

这样，整个交互转场的封装就完成了，下面来实现代理

实现交互代理

在要dismiss消失的vc中实现系统代理 UIViewControllerTransitioningDelegate

// 创建交互转场对象
self.interactiveDismissing = [[ZDPopInteractiveTransition alloc] initWithViewController:self.visibleViewController];


#pragma mark - UIViewControllerTransitioningDelegate
- (nullable id <UIViewControllerInteractiveTransitioning>)interactionControllerForDismissal:(id <UIViewControllerAnimatedTransitioning>)animator {
    return _interactiveDismiss.isTransitioning ? _interactiveDismissing : nil;
}

这样的话，整个侧滑返回的功能就实现了

至此，我们基本上就实现了以Push方式实现Present跳转，同时可以像原生导航一样进行侧滑返回