在iOS開發中，用UINavigationController可以push出一個界面（UIViewController)，但不能push出一個UINavigationController。
如果嘗試下這么做，應用會拋出異常，并給出log提示：

reason: 'Pushing a navigation controller is not supported'

因為一些奇怪的產品、交互需求，或者是舊有的界面層級問題，非要在一個已有的UINavigationController中以從左到右的動畫方式（即系統默認的push動畫樣式）展示一個UINavigationController，該如何實現呢？

解決思路：

UINavigationController雖然不可以push另一個UINavigationController，但是可以通過以下兩種方式“展示”另一個UINavigationController：

1. addChildViewController直接add一個UINavigationController，并將其view也add到原有的UINavigationController的view層級上；
2. presentViewController方式present出一個完整的UINavigationController結構；

上面兩種方式都可以作為解決思路，進行自定義動畫，實現這個“Pushing a navigation controller”的需求。
第一種方式沒嘗試過，可能會在UINavigationBar顯示等方面出現坑。
第二種方式的presentViewController已有成熟的自定義動畫、手勢API支持，實現起來更方便，本文將以第二種方式實現該效果。

實現要求：

• 將present動畫自定義為系統原生push樣式的動畫；
• 支持手指跟隨的右滑返回手勢；
• present后，兩個UINavigationController原有返回手勢不受影響；

實現效果：

實現步驟：

自定義present動畫

• 分別為present和dismiss創建實現了UIViewControllerAnimatedTransitioning協議的動畫類：PPTransitionPresenPushStyleAnimator及PPTransitionDismissPopStyleAnimator。

- (void)animateTransition:(id <UIViewControllerContextTransitioning>)transitionContext

在UIViewControllerAnimatedTransitioning協議的上面這個回調中，從transitionContext對象可獲取present和dissmiss相關的兩個UIViewController，以及動畫的畫布——containerView，在回調內完成自定義的動畫，PPTransitionDismissPopStyleAnimator的實現如下：

#define kPPTransitionDismissPopStyleDuration 0.3

@implementation PPTransitionDismissPopStyleAnimator

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

- (void)animateTransition:(id<UIViewControllerContextTransitioning>)transitionContext {
    UIViewController *fromVC = [transitionContext viewControllerForKey:UITransitionContextFromViewControllerKey];
    UIViewController *toVC = [transitionContext viewControllerForKey:UITransitionContextToViewControllerKey];
    UIView *container = [transitionContext containerView];
    
    CGFloat screenWidth = [UIScreen mainScreen].bounds.size.width;
    CGRect fromVCRect = fromVC.view.frame;
    fromVCRect.origin.x = 0;
    fromVC.view.frame = fromVCRect;
    
    [container addSubview:toVC.view];
    CGRect toVCRect = toVC.view.frame;
    toVCRect.origin.x = -screenWidth;
    toVC.view.frame = toVCRect;
    
    fromVCRect.origin.x = screenWidth;
    toVCRect.origin.x = 0;
    
    [UIView animateWithDuration:[self transitionDuration:transitionContext] animations:^{
        fromVC.view.frame = fromVCRect;
        toVC.view.frame = toVCRect;
    } completion:^(BOOL finished){
        [transitionContext completeTransition:![transitionContext transitionWasCancelled]];//動畫結束、取消必須調用
    }];
}
@end

PPTransitionDismissPopStyleAnimator實現也與之類似，具體見源碼。

• 調用presentViewController:animated:completion:消息的類，需要實現UIViewControllerTransitioningDelegate協議，以返回剛才定義的自定義動畫對象。

- (nullable id <UIViewControllerAnimatedTransitioning>)animationControllerForPresentedController:(UIViewController *)presented
                                                                            presentingController:(UIViewController *)presenting
                                                                                sourceController:(UIViewController *)source {
    return [PPTransitionPresenPushStyleAnimator new];
}

- (nullable id <UIViewControllerAnimatedTransitioning>)animationControllerForDismissedController:(UIViewController *)dismissed {
    return [PPTransitionDismissPopStyleAnimator new];
}

自定義返回手勢

• 為即將被present出來的view添加UIScreenEdgePanGestureRecognizer手勢作為返回操作手勢。
  該類型手勢與UINavigationController自帶的返回手勢interactivePopGestureRecognizer是同一類型，二者應該是互斥的，同一時間只識別其中一個。所以添加時需增加requireGestureRecognizerToFail的邏輯：

UIScreenEdgePanGestureRecognizer *screenGesture = [[UIScreenEdgePanGestureRecognizer alloc] initWithTarget:self action:@selector(onPanGesture:)];
screenGesture.delegate = self;
screenGesture.edges = UIRectEdgeLeft;
[viewControllerToPresent.view addGestureRecognizer:screenGesture];
if ([viewControllerToPresent isKindOfClass:[UINavigationController class]]) {
        [screenGesture requireGestureRecognizerToFail:((UINavigationController*)viewControllerToPresent).interactivePopGestureRecognizer];
}

• 實現pan手勢手指跟隨，即手指從左向右邊滑動后，又向左邊屏幕滑動，則需要取消此次的dismiss操作。
  UIViewControllerTransitioningDelegate的回調接口已有對這種情況的支持，只需要返回在下面兩個回調中，返回一個實現UIViewControllerInteractiveTransitioning協議的對象即可。

- (nullable id <UIViewControllerInteractiveTransitioning>)interactionControllerForPresentation:(id <UIViewControllerAnimatedTransitioning>)animator;

- (nullable id <UIViewControllerInteractiveTransitioning>)interactionControllerForDismissal:(id <UIViewControllerAnimatedTransitioning>)animator;

而系統自帶的UIPercentDrivenInteractiveTransition類已實現該協議，它的功能描述如下：

A percent-driven interactive transition object drives the custom animation between the disappearance of one view controller and the appearance of another.

所以此處使用UIPercentDrivenInteractiveTransition類完成這個進度相關的判斷，將UIPercentDrivenInteractiveTransition對象以屬性方式聲明在ViewController中。

@property (nonatomic, strong) UIPercentDrivenInteractiveTransition *percentDrivenTransition;

在pan手勢處理的方法中完成對該屬性的初始化和邏輯處理：

- (void)onPanGesture:(UIScreenEdgePanGestureRecognizer *)gesture {
    float progress = [gesture translationInView:self.view].x / [UIScreen mainScreen].bounds.size.width;
    if (gesture.state == UIGestureRecognizerStateBegan) {
        self.percentDrivenTransition = [UIPercentDrivenInteractiveTransition new];
        [self dismissViewControllerAnimated:YES completion:NULL];
    } else if (gesture.state == UIGestureRecognizerStateChanged) {
        [self.percentDrivenTransition updateInteractiveTransition:progress];
    } else if (gesture.state == UIGestureRecognizerStateCancelled ||
               gesture.state == UIGestureRecognizerStateEnded) {
        if (progress > 0.5) {
            [self.percentDrivenTransition finishInteractiveTransition];
        } else {
            [self.percentDrivenTransition cancelInteractiveTransition];
        }
        self.percentDrivenTransition = nil;
    }
}

至此，就完成了自定義push樣式的present動畫。
Demo下載