iOS开发中的ViewController转场切换效果实现简介

  UITabBarControllerDelegate delegate
- (id <UIViewControllerInteractiveTransitioning>)tabBarController:(UITabBarController *)tabBarController
interactionControllerForAnimationController: (id <UIViewControllerAnimatedTransitioning>)animationController NS_AVAILABLE_IOS(7_0);
- (id <UIViewControllerAnimatedTransitioning>)tabBarController:(UITabBarController *)tabBarController
animationControllerForTransitionFromViewController:(UIViewController *)fromVC
toViewController:(UIViewController *)toVC NS_AVAILABLE_IOS(7_0);

/**
 *  自定义的动画类
 *  实现协议------>@protocol UIViewControllerAnimatedTransitioning
 *  这个接口负责切换的具体内容，也即“切换中应该发生什么”
 */ 
@interface MTHCustomAnimator : NSObject <UIViewControllerAnimatedTransitioning> 
 
@end 
 
@implementation MTHCustomAnimator 
 
// 系统给出一个切换上下文，我们根据上下文环境返回这个切换所需要的花费时间 
- (NSTimeInterval)transitionDuration:(id<UIViewControllerContextTransitioning>)transitionContext 
{ 
    return 1.0; 
} 
 
// 完成容器转场动画的主要方法，我们对于切换时的UIView的设置和动画都在这个方法中完成 
- (void)animateTransition:(id<UIViewControllerContextTransitioning>)transitionContext 
{ 
    // 可以看做为destination ViewController 
    UIViewController *toViewController = [transitionContext viewControllerForKey:UITransitionContextToViewControllerKey]; 
    // 可以看做为source ViewController 
    UIViewController *fromViewController = [transitionContext viewControllerForKey:UITransitionContextFromViewControllerKey]; 
    // 添加toView到容器上 
        // 如果是XCode6 就可以用这段 
    if ([[[UIDevice currentDevice] systemVersion] floatValue] >= 8.0) 
    { 
        // iOS8 SDK 新API 
        UIView *toView = [transitionContext viewForKey:UITransitionContextToViewKey]; 
        //UIView *fromView = [transitionContext viewForKey:UITransitionContextFromViewKey]; 
        [[transitionContext containerView] addSubview:toView]; 
    }else{ 
        // 添加toView到容器上 
        [[transitionContext containerView] addSubview:toViewController.view]; 
    } 
     
    // 如果是XCode5 就是用这段 
    [[transitionContext containerView] addSubview:toViewController.view]; 
    toViewController.view.alpha = 0.0; 
    [UIView animateWithDuration:[self transitionDuration:transitionContext] animations:^{ 
        // 动画效果有很多,这里就展示个左偏移 
        fromViewController.view.transform = CGAffineTransformMakeTranslation(-320, 0); 
        toViewController.view.alpha = 1.0; 
    } completion:^(BOOL finished) { 
        fromViewController.view.transform = CGAffineTransformIdentity; 
        // 声明过渡结束-->记住，一定别忘了在过渡结束时调用 completeTransition: 这个方法 
        [transitionContext completeTransition:![transitionContext transitionWasCancelled]]; 
    }]; 
} 

     
// MainViewController 
@interface MTHMainViewController () <UINavigationControllerDelegate,UIViewControllerTransitioningDelegate> 
 
@property (nonatomic,strong) MTHCustomAnimator *customAnimator; 
@property (nonatomic,strong) PDTransitionAnimator *minToMaxAnimator; 
@property (nonatomic,strong) MTHNextViewController *nextVC; 
// 交互控制器 (Interaction Controllers) 通过遵从 UIViewControllerInteractiveTransitioning 协议来控制可交互式的转场。 
@property (strong, nonatomic) UIPercentDrivenInteractiveTransition* interactionController; 
@end 
 
@implementation MTHMainViewController 
 
- (id)initWithNibName:(NSString *)nibNameOrNil bundle:(NSBundle *)nibBundleOrNil 
{ 
    self = [super initWithNibName:nibNameOrNil bundle:nibBundleOrNil]; 
    if (self) { 
        // Custom initialization 
    } 
    return self; 
} 
 
- (void)viewDidLoad 
{ 
    [super viewDidLoad]; 
    // Do any additional setup after loading the view. 
    self.navigationItem.title = @"Demo"; 
    self.view.backgroundColor = [UIColor yellowColor]; 
    // 设置代理 
    self.navigationController.delegate = self; 
    // 设置转场动画 
    self.customAnimator = [[MTHCustomAnimator alloc] init]; 
    self.minToMaxAnimator = [PDTransitionAnimator new]; 
 
    self.nextVC = [[MTHNextViewController alloc] init]; 
    // Present的代理和自定义设置 
    _nextVC.transitioningDelegate = self; 
    _nextVC.modalPresentationStyle = UIModalPresentationCustom; (貌似有BUG)换成modalTransitionStyle = UIModalPresentationCustom 
     
    // Push 
    UIButton *pushButton = [UIButton buttonWithType:UIButtonTypeSystem]; 
    pushButton.frame = CGRectMake(140, 200, 40, 40); 
    [pushButton setTitle:@"Push" forState:UIControlStateNormal]; 
    [pushButton addTarget:self action:@selector(push) forControlEvents:UIControlEventTouchUpInside]; 
    [self.view addSubview:pushButton]; 
     
    // Present 
    UIButton *modalButton = [UIButton buttonWithType:UIButtonTypeSystem]; 
    modalButton.frame = CGRectMake(265, 500, 50, 50); 
    [modalButton setTitle:@"Modal" forState:UIControlStateNormal]; 
    [modalButton addTarget:self action:@selector(modal) forControlEvents:UIControlEventTouchUpInside]; 
    [self.view addSubview:modalButton]; 
     
    // 实现交互操作的手势 
    UIPanGestureRecognizer *panRecognizer = [[UIPanGestureRecognizer alloc] initWithTarget:self action:@selector(didClickPanGestureRecognizer:)]; 
    [self.navigationController.view addGestureRecognizer:panRecognizer]; 
} 
 
 
- (void)push 
{ 
    [self.navigationController pushViewController:_nextVC animated:YES]; 
} 
 
- (void)modal 
{ 
    [self presentViewController:_nextVC animated:YES completion:nil]; 
} 
 
#pragma mark - UINavigationControllerDelegate iOS7新增的2个方法 
// 动画特效 
- (id<UIViewControllerAnimatedTransitioning>) navigationController:(UINavigationController *)navigationController animationControllerForOperation:(UINavigationControllerOperation)operation fromViewController:(UIViewController *)fromVC toViewController:(UIViewController *)toVC 
{ 
    /**
     *  typedef NS_ENUM(NSInteger, UINavigationControllerOperation) {
     *     UINavigationControllerOperationNone,
     *     UINavigationControllerOperationPush,
     *     UINavigationControllerOperationPop,
     *  };
     */ 
    if (operation == UINavigationControllerOperationPush) { 
        return self.customAnimator; 
    }else{ 
        return nil; 
    } 
} 
 
// 交互 
- (id <UIViewControllerInteractiveTransitioning>)navigationController:(UINavigationController*)navigationController                           interactionControllerForAnimationController:(id <UIViewControllerAnimatedTransitioning>)animationController 
{ 
    /**
     *  在非交互式动画效果中，该方法返回 nil
     *  交互式转场,自我理解意思是,用户能通过自己的动作来(常见:手势)控制,不同于系统缺省给定的push或者pop(非交互式)
     */ 
    return _interactionController; 
}  
 
#pragma mark - Transitioning Delegate (Modal) 
// 前2个用于动画 
-(id<UIViewControllerAnimatedTransitioning>)animationControllerForPresentedController:(UIViewController *)presented presentingController:(UIViewController *)presenting sourceController:(UIViewController *)source 
{ 
    self.minToMaxAnimator.animationType = AnimationTypePresent; 
    return _minToMaxAnimator; 
} 
 
-(id<UIViewControllerAnimatedTransitioning>)animationControllerForDismissedController:(UIViewController *)dismissed 
{ 
    self.minToMaxAnimator.animationType = AnimationTypeDismiss; 
    return _minToMaxAnimator; 
} 
 
// 后2个用于交互 
- (id <UIViewControllerInteractiveTransitioning>)interactionControllerForPresentation:(id <UIViewControllerAnimatedTransitioning>)animator 
{ 
    return _interactionController; 
} 
 
- (id <UIViewControllerInteractiveTransitioning>)interactionControllerForDismissal:(id <UIViewControllerAnimatedTransitioning>)animator 
{ 
    return nil; 
} 

#pragma mark - 手势交互的主要实现--->UIPercentDrivenInteractiveTransition 
- (void)didClickPanGestureRecognizer:(UIPanGestureRecognizer*)recognizer 
{ 
    UIView* view = self.view; 
    if (recognizer.state == UIGestureRecognizerStateBegan) { 
        // 获取手势的触摸点坐标 
        CGPoint location = [recognizer locationInView:view]; 
        // 判断,用户从右半边滑动的时候,推出下一个VC(根据实际需要是推进还是推出) 
        if (location.x > CGRectGetMidX(view.bounds) && self.navigationController.viewControllers.count == 1){ 
            self.interactionController = [[UIPercentDrivenInteractiveTransition alloc] init]; 
            // 
            [self presentViewController:_nextVC animated:YES completion:nil]; 
        } 
    } else if (recognizer.state == UIGestureRecognizerStateChanged) { 
        // 获取手势在视图上偏移的坐标 
        CGPoint translation = [recognizer translationInView:view]; 
        // 根据手指拖动的距离计算一个百分比，切换的动画效果也随着这个百分比来走 
        CGFloat distance = fabs(translation.x / CGRectGetWidth(view.bounds)); 
        // 交互控制器控制动画的进度 
        [self.interactionController updateInteractiveTransition:distance]; 
    } else if (recognizer.state == UIGestureRecognizerStateEnded) { 
        CGPoint translation = [recognizer translationInView:view]; 
        // 根据手指拖动的距离计算一个百分比，切换的动画效果也随着这个百分比来走 
        CGFloat distance = fabs(translation.x / CGRectGetWidth(view.bounds)); 
        // 移动超过一半就强制完成 
        if (distance > 0.5) { 
            [self.interactionController finishInteractiveTransition]; 
        } else { 
            [self.interactionController cancelInteractiveTransition]; 
        } 
        // 结束后一定要置为nil 
        self.interactionController = nil; 
    } 
} 

@implementation PDTransitionAnimator 
 
#define Switch_Time 1.2 
- (NSTimeInterval)transitionDuration:(id <UIViewControllerContextTransitioning>)transitionContext { 
    return Switch_Time; 
} 
 
#define Button_Width 50.f 
#define Button_Space 10.f 
- (void)animateTransition:(id <UIViewControllerContextTransitioning>)transitionContext { 
    UIViewController* toViewController = [transitionContext viewControllerForKey:UITransitionContextToViewControllerKey]; 
    UIViewController* fromViewController = [transitionContext viewControllerForKey:UITransitionContextFromViewControllerKey]; 
 
    UIView * toView = toViewController.view; 
    UIView * fromView = fromViewController.view; 
     
    if (self.animationType == AnimationTypeDismiss) { 
        // 这个方法能够高效的将当前显示的view截取成一个新的view.你可以用这个截取的view用来显示.例如,也许你只想用一张截图来做动画,毕竟用原始的view做动画代价太高.因为是截取了已经存在的内容,这个方法只能反应出这个被截取的view当前的状态信息,而不能反应这个被截取的view以后要显示的信息.然而,不管怎么样,调用这个方法都会比将view做成截图来加载效率更高. 
        UIView * snap = [toView snapshotViewAfterScreenUpdates:YES]; 
        [transitionContext.containerView addSubview:snap]; 
        [snap setFrame:CGRectMake([UIScreen mainScreen].bounds.size.width - Button_Width - Button_Space, [UIScreen mainScreen].bounds.size.height - Button_Width - Button_Space, Button_Width, Button_Width)]; 
         
        [UIView animateWithDuration:[self transitionDuration:transitionContext] animations:^{ 
            [snap setFrame:[UIScreen mainScreen].bounds]; 
        } completion:^(BOOL finished) { 
            [UIView animateWithDuration:0.5 animations:^{ 
                [[transitionContext containerView] addSubview:toView]; 
                snap.alpha = 0;  
            } completion:^(BOOL finished) { 
                [snap removeFromSuperview]; 
                [transitionContext completeTransition:![transitionContext transitionWasCancelled]]; 
            }]; 
        }]; 
    } else { 
        UIView * snap2 = [toView snapshotViewAfterScreenUpdates:YES]; 
        [transitionContext.containerView addSubview:snap2]; 
        UIView * snap = [fromView snapshotViewAfterScreenUpdates:YES]; 
        [transitionContext.containerView addSubview:snap]; 
         
        [UIView animateWithDuration:[self transitionDuration:transitionContext] animations:^{ 
            [snap setFrame:CGRectMake([UIScreen mainScreen].bounds.size.width - Button_Width - Button_Space+ (Button_Width/2), [UIScreen mainScreen].bounds.size.height - Button_Width - Button_Space + (Button_Width/2), 0, 0)]; 
        } completion:^(BOOL finished) { 
            [UIView animateWithDuration:0.5 animations:^{ 
                //snap.alpha = 0; 
            } completion:^(BOOL finished) { 
                [snap removeFromSuperview]; 
                [snap2 removeFromSuperview]; 
                [[transitionContext containerView] addSubview:toView]; 
                // 切记不要忘记了噢 
                [transitionContext completeTransition:![transitionContext transitionWasCancelled]]; 
            }]; 
        }]; 
         
    } 
} 

[view snapshotViewAfterScreenUpdates:NO];

[view snapshotViewAfterScreenUpdates:YES];
[view setAlpha:0.0];

// ContainerViewController 
@interface FTContainerViewController ()  
 
@property (strong, nonatomic) FTPhotoSenderViewController       *photoSenderViewController; 
@property (strong, nonatomic) FTVideoSenderViewController       *videoSenderViewController; 
@property (strong, nonatomic) FTFileSenderViewController        *fileSenderViewController; 
@property (strong, nonatomic) FTContactSenderViewController     *contactSenderViewController; 
@property (strong, nonatomic) FTClipboardSenderViewController   *clipboardSenderViewController; 
@property (strong, nonatomic) UIViewController                  *selectedViewController;        // 当前选择的VC 
@property (strong, nonatomic) NSArray                           *viewControllers;               // childVC数组 
@property (assign, nonatomic) NSInteger                         currentControllerIndex;         // 当前选择的VC的数组下标号 
 
 
@end 
 
@implementation FTContainerViewController 
 
#pragma mark - ViewLifecycle Methods 
 
- (void)viewDidLoad 
{ 
    [super viewDidLoad]; 
    // childVC 
    self.photoSenderViewController = [self.storyboard instantiateViewControllerWithIdentifier:@"FTPhotoSenderViewController"]; 
    self.videoSenderViewController = [self.storyboard instantiateViewControllerWithIdentifier:@"FTVideoSenderViewController"]; 
    self.fileSenderViewController = [self.storyboard instantiateViewControllerWithIdentifier:@"FTFileSenderViewController"]; 
    self.contactSenderViewController = [self.storyboard instantiateViewControllerWithIdentifier:@"FTContactSenderViewController"]; 
    self.clipboardSenderViewController = [self.storyboard instantiateViewControllerWithIdentifier:@"FTClipboardSenderViewController"]; 
    // 存储childVC的数组 
    self.viewControllers = @[_photoSenderViewController,_videoSenderViewController,_fileSenderViewController,_contactSenderViewController,_clipboardSenderViewController]; 
    // 缺省为下标为0的VC 
    self.selectedViewController = self.selectedViewController ?: self.viewControllers[0]; 
    self.currentControllerIndex = 0; 
} 

#import "FTMthTransitionAnimator.h" 
 
@implementation FTMthTransitionAnimator 
 
static CGFloat const kChildViewPadding = 16; 
static CGFloat const kDamping = 0.5;    // damping参数代表弹性阻尼，随着阻尼值越来越接近0.0，动画的弹性效果会越来越明显，而如果设置阻尼值为1.0，则视图动画不会有弹性效果 
static CGFloat const kInitialSpringVelocity = 0.5;  // 初始化弹簧速率 
 
- (NSTimeInterval)transitionDuration:(id<UIViewControllerContextTransitioning>)transitionContext 
{ 
    return 1.0; 
} 
 
- (void)animateTransition:(id<UIViewControllerContextTransitioning>)transitionContext 
{ 
    /**
     *  - viewControllerForKey：我们可以通过他访问过渡的两个 ViewController。
     *  - containerView：两个 ViewController 的 containerView。
     *  - initialFrameForViewController 和 finalFrameForViewController 是过渡开始和结束时每个 ViewController 的 frame。
     */ 
    UIViewController *fromViewController = [transitionContext viewControllerForKey:UITransitionContextFromViewControllerKey]; 
    UIViewController *toViewController = [transitionContext viewControllerForKey:UITransitionContextToViewControllerKey]; 
     
    [[transitionContext containerView] addSubview:toViewController.view]; 
    toViewController.view.alpha = 0; 
    BOOL goingRight = ([transitionContext initialFrameForViewController:toViewController].origin.x < [transitionContext finalFrameForViewController:toViewController].origin.x); 
    CGFloat transDistance = [transitionContext containerView].bounds.size.width + kChildViewPadding; 
    CGAffineTransform transform = CGAffineTransformMakeTranslation(goingRight ? transDistance : -transDistance, 0); 
    // CGAffineTransformInvert 反转 
    toViewController.view.transform = CGAffineTransformInvert(transform); 
//    toViewController.view.transform = CGAffineTransformTranslate(toViewController.view.transform, (goingRight ? transDistance : -transDistance), 0); 
     
    /**
     *   ----------弹簧动画.....-------
     *  使用由弹簧的运动描述的时序曲线` animations` 。当` dampingRatio`为1时，动画将平稳减速到其最终的模型值不会振荡。阻尼比小于1来完全停止前将振荡越来越多。可以使用弹簧的初始速度，以指定的速度在模拟弹簧的端部的物体被移动它附着之前。这是一个单元坐标系，其中1是指行驶总距离的动画在第二。所以，如果你改变一个物体的位置由200PT在这个动画，以及你想要的动画表现得好像物体在动，在100PT /秒的动画开始之前，你会通过0.5 。你通常会想通过0的速度。
     */ 
    [UIView animateWithDuration:[self transitionDuration:transitionContext] delay:0 usingSpringWithDamping:kDamping initialSpringVelocity:kInitialSpringVelocity options:0x00 animations:^{ 
        fromViewController.view.transform = transform; 
        fromViewController.view.alpha = 0; 
        // CGAffineTransformIdentity  重置,初始化 
        toViewController.view.transform = CGAffineTransformIdentity; 
        toViewController.view.alpha = 1; 
    } completion:^(BOOL finished) { 
        fromViewController.view.transform = CGAffineTransformIdentity; 
        // 声明过渡结束-->记住，一定别忘了在过渡结束时调用 completeTransition: 这个方法。 
        [transitionContext completeTransition:![transitionContext transitionWasCancelled]]; 
    }]; 
} 
 
@end 

@interface FTMthTransitionContext : NSObject <UIViewControllerContextTransitioning> 
 
- (instancetype)initWithFromViewController:(UIViewController *)fromViewController toViewController:(UIViewController *)toViewController goingRight:(BOOL)goingRight; 
@property (nonatomic, copy) void (^completionBlock)(BOOL didComplete); 
@property (nonatomic, assign, getter=isAnimated) BOOL animated; 
@property (nonatomic, assign, getter=isInteractive) BOOL interactive; // 是否交互式 
 
@property (nonatomic, strong) NSDictionary *privateViewControllers; 
@property (nonatomic, assign) CGRect privateDisappearingFromRect; 
@property (nonatomic, assign) CGRect privateAppearingFromRect; 
@property (nonatomic, assign) CGRect privateDisappearingToRect; 
@property (nonatomic, assign) CGRect privateAppearingToRect; 
@property (nonatomic, weak) UIView *containerView; 
@property (nonatomic, assign) UIModalPresentationStyle presentationStyle; 
 
@end 
 
@implementation FTMthTransitionContext 
 
- (instancetype)initWithFromViewController:(UIViewController *)fromViewController toViewController:(UIViewController *)toViewController goingRight:(BOOL)goingRight { 
     
    if ((self = [super init])) { 
        self.presentationStyle = UIModalPresentationCustom; 
        self.containerView = fromViewController.view.superview; 
        self.privateViewControllers = @{ 
                                        UITransitionContextFromViewControllerKey:fromViewController, 
                                        UITransitionContextToViewControllerKey:toViewController, 
                                        }; 
         
        // Set the view frame properties which make sense in our specialized ContainerViewController context. Views appear from and disappear to the sides, corresponding to where the icon buttons are positioned. So tapping a button to the right of the currently selected, makes the view disappear to the left and the new view appear from the right. The animator object can choose to use this to determine whether the transition should be going left to right, or right to left, for example. 
        CGFloat travelDistance = (goingRight ? -self.containerView.bounds.size.width : self.containerView.bounds.size.width); 
        self.privateDisappearingFromRect = self.privateAppearingToRect = self.containerView.bounds; 
        self.privateDisappearingToRect = CGRectOffset (self.containerView.bounds, travelDistance, 0); 
        self.privateAppearingFromRect = CGRectOffset (self.containerView.bounds, -travelDistance, 0); 
    } 
     
    return self; 
} 
 
- (CGRect)initialFrameForViewController:(UIViewController *)viewController { 
    if (viewController == [self viewControllerForKey:UITransitionContextFromViewControllerKey]) { 
        return self.privateDisappearingFromRect; 
    } else { 
        return self.privateAppearingFromRect; 
    } 
} 
 
- (CGRect)finalFrameForViewController:(UIViewController *)viewController { 
    if (viewController == [self viewControllerForKey:UITransitionContextFromViewControllerKey]) { 
        return self.privateDisappearingToRect; 
    } else { 
        return self.privateAppearingToRect; 
    } 
} 
 
- (UIViewController *)viewControllerForKey:(NSString *)key { 
    return self.privateViewControllers[key]; 
} 
 
- (void)completeTransition:(BOOL)didComplete { 
    if (self.completionBlock) { 
        self.completionBlock (didComplete); 
    } 
} 
 
// 非交互式,直接返回NO,因为不允许交互当然也就无法操作进度取消 
- (BOOL)transitionWasCancelled { return NO; } 
 
// 非交互式,直接不进行操作,只有进行交互,下面3个协议方法才有意义,可参照系统给我们定义好的交互控制器 
// @interface UIPercentDrivenInteractiveTransition : NSObject <UIViewControllerInteractiveTransitioning> 
- (void)updateInteractiveTransition:(CGFloat)percentComplete {} 
- (void)finishInteractiveTransition {} 
- (void)cancelInteractiveTransition {} 
 
@end 

UISwipeGestureRecognizer *leftGesture = [[UISwipeGestureRecognizer alloc] initWithTarget:self action:@selector(swapController:)]; 
[leftGesture setDirection:UISwipeGestureRecognizerDirectionLeft]; 
[self.view addGestureRecognizer:leftGesture]; 
 
UISwipeGestureRecognizer *rightGesture = [[UISwipeGestureRecognizer alloc] initWithTarget:self action:@selector(swapController:)]; 
[rightGesture setDirection:UISwipeGestureRecognizerDirectionRight]; 
[self.view addGestureRecognizer:rightGesture]; 
[objc] view plaincopy
// 响应手势的方法 
- (void)swapViewControllers:(UISwipeGestureRecognizer *)swipeGestureRecognizer 
{ 
    if (swipeGestureRecognizer.direction == UISwipeGestureRecognizerDirectionLeft) { 
        if (_currentControllerIndex < 4) { 
            _currentControllerIndex++; 
        } 
        NSLog(@"_currentControllerIndex = %ld",(long)_currentControllerIndex); 
        UIViewController *selectedViewController = self.viewControllers[_currentControllerIndex]; 
         NSLog(@"%s__%d__|%@",__FUNCTION__,__LINE__,@"右边"); 
         self.selectedViewController = selectedViewController; 
    } else if (swipeGestureRecognizer.direction == UISwipeGestureRecognizerDirectionRight){ 
         NSLog(@"%s__%d__|%@",__FUNCTION__,__LINE__,@"左边"); 
        if (_currentControllerIndex > 0) { 
            _currentControllerIndex--; 
        } 
        UIViewController *selectedViewController = self.viewControllers[_currentControllerIndex]; 
        self.selectedViewController = selectedViewController; 
    } 
} 
 
// 重写selectedViewController的setter 
- (void)setSelectedViewController:(UIViewController *)selectedViewController 
{ 
    NSParameterAssert (selectedViewController); 
    [self _transitionToChildViewController:selectedViewController]; 
    _selectedViewController = selectedViewController; 
} 
 
// 切换操作(自定义的,联想我在前面文章网易标签栏切换中,系统给的transitionFromViewController,是一个道理) 
- (void)_transitionToChildViewController:(UIViewController *)toViewController 
{ 
    UIViewController *fromViewController = self.childViewControllers.count > 0 ? self.childViewControllers[0] : nil; 
    if (toViewController == fromViewController) { 
        return; 
    } 
    UIView *toView = toViewController.view; 
    [toView setTranslatesAutoresizingMaskIntoConstraints:YES]; 
    toView.autoresizingMask = UIViewAutoresizingFlexibleWidth | UIViewAutoresizingFlexibleHeight; 
    toView.frame = self.view.bounds; 
     
    // 自定义容器的切换,addChildViewController是关键,它保证了你想要显示的VC能够加载到容器中 
    // 而所谓的动画和上下文,只是为了转场的动画效果 
    // 因此,就算用UIScrollView切换,也不能缺少addChildViewController,切记!切记! 
    [fromViewController willMoveToParentViewController:nil]; 
    [self addChildViewController:toViewController]; 
     
    if (!fromViewController) { 
        [self.view addSubview:toViewController.view]; 
        [toViewController didMoveToParentViewController:self]; 
        return; 
    } 
     
    // Animator 
    FTMthTransitionAnimator *transitionAnimator = [[FTMthTransitionAnimator alloc] init]; 
    NSUInteger fromIndex = [self.viewControllers indexOfObject:fromViewController]; 
    NSUInteger toIndex = [self.viewControllers indexOfObject:toViewController]; 
     
    // Context 
    FTMthTransitionContext *transitionContext = [[FTMthTransitionContext alloc] initWithFromViewController:fromViewController toViewController:toViewController goingRight:(toIndex > fromIndex)]; 
    transitionContext.animated = YES; 
    transitionContext.interactive = NO; 
    transitionContext.completionBlock = ^(BOOL didComplete) { 
        // 因为是非交互式,所以fromVC可以直接直接remove出its parent's children controllers array 
        [fromViewController.view removeFromSuperview]; 
        [fromViewController removeFromParentViewController]; 
        [toViewController didMoveToParentViewController:self]; 
        if ([transitionAnimator respondsToSelector:@selector (animationEnded:)]) { 
            [transitionAnimator animationEnded:didComplete]; 
        } 
    }; 
    // 转场动画需要以转场上下文为依托,因为我们是自定义的Context,所以要手动设置 
    [transitionAnimator animateTransition:transitionContext]; 
}