iOS：探究視圖控制器的轉場動畫

一、介紹

在iOS開發中，轉場動畫的使用無處不見，不只是我們自己更多的使用UIViewblock動畫實現一個轉場動畫，其實，在我們實現VC控制器跳轉的時候都是轉場動畫的實現，例如標簽欄控制器的切換、模態動畫present和dismiss、導航控制器的push和pop。實現它們的轉場動畫，只需要實現它們的動畫協議即可，說起來有點太籠統，不如看下麵的圖吧：

二、分析

對於上面的三種類型的控制器，系統都會為它們設置一個代理，通過這個代理方法去監測它們切換VC的過程，這個過程僅僅是出現和消失的過程，至於這個過程是什麼過渡效果，這個代理是不管的。要想這個過程是有動畫的，那麼在這些過程中，也就是代理函數中，需要另外再返回一個實現動畫的對象，這個對象必須遵循實現動畫的協議，在這個協議中開發者可以重寫自定義轉場動畫。下麵會慢慢演示這三種類型控制器的自定義轉場動畫。

重寫不可交互轉場動畫的核心協議內容：

//重寫動畫協議
@protocol UIViewControllerAnimatedTransitioning <NSObject>

//動畫執行時間
- (NSTimeInterval)transitionDuration:(nullable id <UIViewControllerContextTransitioning>)transitionContext;

//自定義動畫效果
- (void)animateTransition:(id <UIViewControllerContextTransitioning>)transitionContext;

@end

重寫可交互轉場動畫的核心協議內容：

//重寫動畫協議
@protocol UIViewControllerInteractiveTransitioning <NSObject>

//自定義動畫效果
- (void)startInteractiveTransition:(id <UIViewControllerContextTransitioning>)transitionContext;

@end

系統提供的一個百分比可交互轉場動畫核心類內容：

//系統提供的百分比動畫類，已經遵循了可交互協議
@interface UIPercentDrivenInteractiveTransition : NSObject <UIViewControllerInteractiveTransitioning>

- (void)pauseInteractiveTransition;
- (void)updateInteractiveTransition:(CGFloat)percentComplete;
- (void)cancelInteractiveTransition;
- (void)finishInteractiveTransition;

@end

三、轉場動畫View之間的切換

四、實現一個自定義的模態動畫

1、概述

正如我們所知，系統為我們提供的模態動畫預設是從底部present出，然後dismiss回到底部。 雖然說這個基本能夠滿足使用，但是如果我們還想使用其他形式的模態動畫例如從頂部present出dismiss回到頂部，這個時候就需要對系統預設的轉場動畫進行自定義了。

2、詳解

（1）要自定義模態轉場動畫，首先需要給被模態的控制器設置一個實現了UIViewControllerAnimatedTransitioning協議的代理，這些協議方法可以監測動畫執行的過程，代理和協議如下：

//代理
@protocol UIViewControllerTransitioningDelegate;
@interface UIViewController(UIViewControllerTransitioning)
@property (nullable, nonatomic, weak) id <UIViewControllerTransitioningDelegate> transitioningDelegate API_AVAILABLE(ios(7.0));
@end

//協議
@protocol UIViewControllerTransitioningDelegate <NSObject>
@optional
//present時調用，返回一個實現了不可交互轉場動畫協議的代理
- (nullable id <UIViewControllerAnimatedTransitioning>)animationControllerForPresentedController:(UIViewController *)presented presentingController:(UIViewController *)presenting sourceController:(UIViewController *)source;

//dismiss時調用，返回一個實現了不可交互轉場動畫協議的代理
- (nullable id <UIViewControllerAnimatedTransitioning>)animationControllerForDismissedController:(UIViewController *)dismissed;

//presnt過程中交互時調用，返回一個實現了可交互的轉場動畫協議的代理
- (nullable id <UIViewControllerInteractiveTransitioning>)interactionControllerForPresentation:(id <UIViewControllerAnimatedTransitioning>)animator;

//dismiss過程中交互時調用，返回一個實現了可交互的轉場動畫協議的代理
- (nullable id <UIViewControllerInteractiveTransitioning>)interactionControllerForDismissal:(id <UIViewControllerAnimatedTransitioning>)animator;

//返回新的模態彈框控制器（這個是對模態風格進行自定義時調用，後面會說到）
- (nullable UIPresentationController *)presentationControllerForPresentedViewController:(UIViewController *)presented presentingViewController:(nullable UIViewController *)presenting sourceViewController:(UIViewController *)source API_AVAILABLE(ios(8.0));
@end

（2）然後在上面的協議方法中返回一個實現了UIViewControllerAnimatedTransitioning協議的代理，在這個代理的協議方法中可以真正重寫轉場動畫了，協議如下：

@protocol UIViewControllerAnimatedTransitioning <NSObject>
//動畫執行時間
- (NSTimeInterval)transitionDuration:(nullable id <UIViewControllerContextTransitioning>)transitionContext;
//自定義轉場動畫
- (void)animateTransition:(id <UIViewControllerContextTransitioning>)transitionContext;
@optional

（3）自定義轉場動畫實現如下【註意：Singleton單例類和UIView+Extesion分類需要自己去拷貝引入】

• 設置UIViewControllerAnimatedTransitioning代理對象TransitionDelegate，監測動畫執行過程，將其設置為單例

  #import <UIKit/UIKit.h>
  #import "Singleton.h"
  @interface TransitionDelegate : NSObject<UIViewControllerTransitioningDelegate>
  SingletonH(TransitionDelegate);
  @end

  #import "TransitionDelegate.h"
  #import "CustomAnimationTransition.h"
  
  @implementation TransitionDelegate
  SingletonM(TransitionDelegate);
  
  #pragma mark - <UIViewControllerTransitioningDelegate>
  
  //展示的動畫
  - (id <UIViewControllerAnimatedTransitioning>)animationControllerForPresentedController:(UIViewController *)presented presentingController:(UIViewController *)presenting sourceController:(UIViewController *)source
  {
      CustomAnimationTransition *animation = [[CustomAnimationTransition alloc]init];
      animation.presented = YES;
      return animation;
  }
  
  //關閉的動畫
  - (id <UIViewControllerAnimatedTransitioning>)animationControllerForDismissedController:(UIViewController *)dismissed
  {
      CustomAnimationTransition *animation = [[CustomAnimationTransition alloc]init];
      animation.presented = NO;
      return animation;
  }
  @end

• 設置UIViewControllerAnimatedTransitioning代理對象CustomTransitionAnimationTransition，重寫動畫效果

  #import <UIKit/UIKit.h>
  
  @interface CustomAnimationTransition : NSObject<UIViewControllerAnimatedTransitioning>
  //判斷是present還是dismiss, YES:present  NO:dismisss
  @property (assign,nonatomic)BOOL presented;
  @end

• //設置過渡動畫（modal和dismiss的動畫都需要在這裡處理）
  - (void)animateTransition:(id <UIViewControllerContextTransitioning>)transitionContext
  {
      // UITransitionContextToViewKey,
      // UITransitionContextFromViewKey.
      
      //出來的動畫
      if (self.presented) {
          
          //獲取並添加轉場視圖
          UIView *toView = [transitionContext viewForKey:UITransitionContextToViewKey];
          [transitionContext.containerView addSubview:toView];
           
          //設置動畫從上往下出來
          toView.y = -toView.height;
          
          [UIView animateWithDuration:duration animations:^{
              
              toView.y = 0;
              
          } completion:^(BOOL finished) {
              
              //移除視圖
              BOOL cancle = [transitionContext transitionWasCancelled];
              if (cancle) {
                  [toView removeFromSuperview];
              }
              
              //動畫完成後,視圖上的事件才能處理
              [transitionContext completeTransition:!cancle];
          }];
      }
      //銷毀的動畫
      else
      {
          //獲取轉場視圖
          UIView *fromView = [transitionContext viewForKey:UITransitionContextFromViewKey];
          
          [UIView animateWithDuration:duration animations:^{
              
              fromView.y = -fromView.height;
  
          } completion:^(BOOL finished) {
              
              //移除視圖
              BOOL cancle = [transitionContext transitionWasCancelled];
              if (!cancle) {
                  [fromView removeFromSuperview];
              }
              
              //動畫完成後,視圖上的事件才能處理
              [transitionContext completeTransition:!cancle];
          }];
      }
  }

• 開始執行，結果如gif圖 

  //present
  UINavigationController *nav = [[UINavigationController alloc] initWithRootViewController:[[SecondViewController alloc] init]];
  nav.transitioningDelegate = [TransitionDelegate sharedTransitionDelegate];//自定義轉場動畫
  nav.modalPresentationStyle = UIModalPresentationFullScreen;
  [self presentViewController:nav animated:YES completion:nil];

• 

 （4）我們已經實現了一個簡單的自定義模態不可交互的轉場動畫，其實，在模態控制器的時候，我們還可以自定義可交互的轉場動畫以及設置自定義的模態風格。可交互的轉場動畫一會兒再討論，先來討論一下模態風格，系統在iOS13之前預設都是滿屏模式的UIModalPresentationFullScreen，但是iOS13之後，預設是UIModalPresentationPageSheet。系統提供的模態風格如下：

//模態風格枚舉
typedef NS_ENUM(NSInteger, UIModalPresentationStyle) {
    UIModalPresentationFullScreen = 0,
    UIModalPresentationPageSheet ,
    UIModalPresentationFormSheet ,
    UIModalPresentationCurrentContext ,
    UIModalPresentationCustom , //自定義
    UIModalPresentationOverFullScreen ,
    UIModalPresentationOverCurrentContext ),
    UIModalPresentationPopover ,
    UIModalPresentationBlurOverFullScreen ,
    UIModalPresentationNone,
    UIModalPresentationAutomatic , 
};

 （5）從上面的枚舉可以看到，系統是支持我們實現自己的風格的，也就是自定義。在實現自定義之前，一定得知道UIPresentationController這個類，這個是彈出框控制項，模態的控制器都是由它進行管理，主要代碼如下：

//重寫此方法可以在彈框即將顯示時執行所需要的操作
- (void)presentationTransitionWillBegin;

//重寫此方法可以在彈框顯示完畢時執行所需要的操作
- (void)presentationTransitionDidEnd:(BOOL)completed;

//重寫此方法可以在彈框即將消失時執行所需要的操作
- (void)dismissalTransitionWillBegin;

//重寫此方法可以在彈框消失之後執行所需要的操作
- (void)dismissalTransitionDidEnd:(BOOL)completed;

//重寫決定了彈出框的frame
- (CGRect)frameOfPresentedViewInContainerView;

//重寫對containerView進行佈局
- (void)containerViewWillLayoutSubviews;
- (void)containerViewDidLayoutSubviews;

//初始化方法
- (instancetype)initWithPresentedViewController:(UIViewController *)presentedViewController presentingViewController:(UIViewController *)presentingViewController;

（6）額外再提一個知識點，因為一會兒在自定義模態風格時會涉及到。在本文開篇結構圖中介紹了創建的轉場動畫都是在轉場動畫上下文UIViewControllerContextTransitioning協議中完成的，那麼這個轉場動畫的執行是誰管理呢？看結構圖如下，沒錯，是由UIViewControllerTransitionCoordinator這個代理協調器在協調器上下文中完成的，系統給UIViewController提供了一個分類，這個分類持有這個代理協調器，通過這個代理協調器可以拿到執行轉場動畫的方法。最終，我們可以自己添加一些操作與轉場動畫同步執行。

UIViewControllerContextTransitioning協議核心內容

@protocol UIViewControllerTransitionCoordinatorContext <NSObject>
// 執行的屬性
@property(nonatomic, readonly, getter=isAnimated) BOOL animated;
@property(nonatomic, readonly) UIModalPresentationStyle presentationStyle;
@property(nonatomic, readonly) NSTimeInterval transitionDuration;
@property(nonatomic, readonly) UIView *containerView;
@property(nonatomic, readonly) CGAffineTransform targetTransform 

// 參與控制器
// UITransitionContextToViewControllerKey、UITransitionContextFromViewControllerKey
- (nullable __kindof UIViewController *)viewControllerForKey:(UITransitionContextViewControllerKey)key;

// 參與的視圖
// UITransitionContextToViewKey、UITransitionContextFromViewKey
- (nullable __kindof UIView *)viewForKey:(UITransitionContextViewKey)key API_AVAILABLE(ios(8.0));
@end

UIViewControllerTransitionCoordinator協議核心內容

// 與動畫控制器中的轉場動畫同步，執行其他動畫
- (BOOL)animateAlongsideTransition:(void (^ __nullable)(id <UIViewControllerTransitionCoordinatorContext>context))animation
                        completion:(void (^ __nullable)(id <UIViewControllerTransitionCoordinatorContext>context))completion;
 
// 與動畫控制器中的轉場動畫同步，在指定的視圖內執行動畫
- (BOOL)animateAlongsideTransitionInView:(nullable UIView *)view
                               animation:(void (^ __nullable)(id <UIViewControllerTransitionCoordinatorContext>context))animation
                              completion:(void (^ __nullable)(id <UIViewControllerTransitionCoordinatorContext>context))completion;

UIViewController(UIViewControllerTransitionCoordinator) 分類核心內容

//持有轉場動畫執行協調器
@interface UIViewController(UIViewControllerTransitionCoordinator)
@property(nonatomic, readonly, nullable) id <UIViewControllerTransitionCoordinator> transitionCoordinator;
@end

（7）自定義模態風格實現如下【註意：Singleton單例類和UIView+Extesion分類需要自己去拷貝引入】

• 設置UIViewControllerAnimatedTransitioning代理對象TransitionDelegate，監測動畫執行過程並返回模態風格，將其設置為單例

  #import <UIKit/UIKit.h>
  #import "Singleton.h"
  
  @interface TransitionDelegate : NSObject<UIViewControllerTransitioningDelegate>
  SingletonH(TransitionDelegate);
  @end

  #import "TransitionDelegate.h"
  #import "CustomPresentationController.h"
  #import "CustomAnimationTransition.h"
  
  @implementation TransitionDelegate
  SingletonM(TransitionDelegate);
  
  #pragma mark - <UIViewControllerTransitioningDelegate>
  //返回模態風格
  -(UIPresentationController *)presentationControllerForPresentedViewController:(UIViewController *)presented presentingViewController:(UIViewController *)presenting sourceViewController:(UIViewController *)source
  {
      return [[CustomPresentationController alloc] initWithPresentedViewController:presented presentingViewController:presenting];
  }
  
  //展示的動畫
  - (id <UIViewControllerAnimatedTransitioning>)animationControllerForPresentedController:(UIViewController *)presented presentingController:(UIViewController *)presenting sourceController:(UIViewController *)source
  {
      CustomAnimationTransition *animation = [[CustomAnimationTransition alloc]init];
      animation.presented = YES;
      return animation;
  }
  
  //關閉的動畫
  - (id <UIViewControllerAnimatedTransitioning>)animationControllerForDismissedController:(UIViewController *)dismissed
  {
      CustomAnimationTransition *animation = [[CustomAnimationTransition alloc]init];
      animation.presented = NO;
      return animation;
  }
  @end

• 設置UIViewControllerAnimatedTransitioning代理對象CustomTransitionAnimationTransition，重寫動畫效果

  #import <UIKit/UIKit.h>
  
  @interface CustomAnimationTransition : NSObject<UIViewControllerAnimatedTransitioning>
  //判斷是present還是dismiss, YES:present  NO:dismisss
  @property (assign,nonatomic)BOOL presented;
  @end

  #import "CustomAnimationTransition.h"
  #import "UIView+Extension.h"
  
  const CGFloat duration = 0.5f;
  
  @implementation CustomAnimationTransition
  
  #pragma mark -<UIViewControllerAnimatedTransitioning>
  //動畫時間
  - (NSTimeInterval)transitionDuration:(id <UIViewControllerContextTransitioning>)transitionContext
  {
      return duration;
  }
  
  //設置過渡動畫（modal和dismiss的動畫都需要在這裡處理）
  - (void)animateTransition:(id <UIViewControllerContextTransitioning>)transitionContext
  {
      // UITransitionContextToViewKey,
      // UITransitionContextFromViewKey.    
      
  　　 //發現此處並沒有添加toView到containerView中以及從containerView中移除toView，與上面的有區別。
      //我把添加和移除toView的操作放到了下麵的自定義的模態風格類中完成的
      
      //出來的動畫
      if (self.presented) {
           UIView *toView = [transitionContext viewForKey:UITransitionContextToViewKey];
      
          //設置動畫從上往下出來
          toView.y = -toView.height;
          
          [UIView animateWithDuration:duration animations:^{
              
              toView.y = 0;
              
          } completion:^(BOOL finished) {
              
              //動畫完成後,視圖上的事件才能處理
              [transitionContext completeTransition:YES];
          }];
      }
      //銷毀的動畫
      else
      {
          [UIView animateWithDuration:duration animations:^{
              
              UIView *fromView = [transitionContext viewForKey:UITransitionContextFromViewKey];
              
              fromView.y = -fromView.height;
              
          } completion:^(BOOL finished) {
              
              //動畫完成後,視圖上的事件才能處理
              [transitionContext completeTransition:YES];
          }];
      }   
  }

• 設置自定義的模態風格類CustomPresenttationController

  #import "CustomPresentationController.h"
  
  @implementation CustomPresentationController
  
  //可以改變被模態的控制器視圖的尺寸
  - (CGRect)frameOfPresentedViewInContainerView
  {
  　　 //CGRectInset: 在containerView的frame基礎上，將width減小100，將height減小200
      //containerView是容納presentedView的一個容器
      return CGRectInset(self.containerView.bounds, 50, 100);
  }
  
  //將上面重置的frame完成佈局
  - (void)containerViewDidLayoutSubviews {
      self.presentedView.frame = self.frameOfPresentedViewInContainerView;
      [super containerViewDidLayoutSubviews];
  }
  
  //過渡即將展示時的處理
  //這個過程可以改變視圖屬性、或者添加視圖等
  - (void)presentationTransitionWillBegin
  {
      self.presentedView.frame = self.containerView.frame;
      [self.containerView addSubview:self.presentedView];
  }
  
  
  //過渡展示完成
  //做清理工作
  - (void)presentationTransitionDidEnd:(BOOL)completed
  {
       if (!completed) {
          [self.presentedView removeFromSuperview];
       }
  }
  
  //過渡即將消失時的處理
  //這個過程可以改變視圖屬性等
  - (void)dismissalTransitionWillBegin
  {
      //例如改變透明度，與轉場控制器中的轉場動畫同步執行
      [self.presentingViewController.transitionCoordinator animateAlongsideTransition:^(id<UIViewControllerTransitionCoordinatorContext>  _Nonnull context) {
          
          self.presentedView.alpha = 0.f;
          
      } completion:nil];
  }
  
  //過渡消失完成
  //做清理工作
  - (void)dismissalTransitionDidEnd:(BOOL)completed
  {
      if (completed) {
          [self.presentedView removeFromSuperview];
      }
  }
  @end

• 開始執行，結果如gif圖

  //present
  UINavigationController *nav = [[UINavigationController alloc] initWithRootViewController:[[SecondViewController alloc] init]];
  nav.transitioningDelegate = [TransitionDelegate sharedTransitionDelegate];//自定義轉場動畫
  nav.modalPresentationStyle = UIModalPresentationCustom; //自定義模態風格
  [self presentViewController:nav animated:YES completion:nil];

  

 （8）自定義模態轉場動畫和自定義模態風格我們都實現完了，但是上面的動畫過程中都是不可交互的，那麼要想實現可交互的動畫該怎麼做呢？如上面所說的，在dismiss時返回一個實現了UIViewControllerInteractiveTransitioning協議的代理或者直接是原生類UIPercentDrivenInteractiveTransition對象。其中，UIPercentDrivenInteractiveTransition是系統封裝好了百分比驅動，用起來很簡單，那麼真正的實現原理還是我們去實現一下。下麵咱們來實現導航模式的交互效果，如下：

• TransitioningDelegate

  #import <UIKit/UIKit.h>
  #import "Singleton.h"
  
  @interface TransitioningDelegate : NSObject<UIViewControllerTransitioningDelegate>
  SingletonH(TransitioningDelegate);
  @end

  #import "TransitioningDelegate.h"
  #import "CustomAnimationTransition.h"
  #import "CustomInteractiveTransition.h"
  
  @implementation TransitioningDelegate
  SingletonM(TransitioningDelegate);
  
  #pragma mark - UIViewControllerTransitioningDelegate
  
  //present
  - (nullable id <UIViewControllerAnimatedTransitioning>)animationControllerForPresentedController:(UIViewController *)presented presentingController:(UIViewController *)presenting sourceController:(UIViewController *)source {
      
       //這裡還是採用自定義的轉場動畫方式進行present，使其present時從屏幕右側滑入
       CustomAnimationTransition *animation = [[CustomAnimationTransition alloc]init];
       animation.presented = YES;
       return animation;
  }
  
  //dismiss,必須重寫
  - (nullable id <UIViewControllerAnimatedTransitioning>)animationControllerForDismissedController:(UIViewController *)dismissed {
      //這裡採用自定義的轉場動畫覆蓋系統的dismiss效果，在dismiss時，由於自定義了交互動畫，所以系統自己的dismiss動畫不會執行
      CustomAnimationTransition *animation = [[CustomAnimationTransition alloc] init];
      animation.presented = NO;
      return animation;
  }
  
  //將要dismiss時的交互行為，必須重寫
  - (nullable id <UIViewControllerInteractiveTransitioning>)interactionControllerForDismissal:(id <UIViewControllerAnimatedTransitioning>)animator {
      
      CustomInteractiveTransition *animation = [[CustomInteractiveTransition alloc] init];
      return animation;
  }
  
  @end

• CustomAnimationTransition

  #import <UIKit/UIKit.h>
  
  @interface CustomAnimationTransition : NSObject<UIViewControllerAnimatedTransitioning>
  //判斷是present還是dismiss, YES:present  NO:dismisss
  @property (assign,nonatomic)BOOL presented;
  @end

  #import "CustomAnimationTransition.h"
  #import "UIView+Extension.h"
  
  const CGFloat duration = 0.5f;
  
  @implementation CustomAnimationTransition
  
  #pragma mark -<UIViewControllerAnimatedTransitioning>
  //動畫時間
  - (NSTimeInterval)transitionDuration:(id <UIViewControllerContextTransitioning>)transitionContext
  {
      return duration;
  }
  
  //設置過渡動畫（modal和dismiss的動畫都需要在這裡處理）
  - (void)animateTransition:(id <UIViewControllerContextTransitioning>)transitionContext
  {
      // UITransitionContextToViewKey,
      // UITransitionContextFromViewKey.
      
      //出來的動畫
      if (self.presented) {
          
          //獲取並添加轉場視圖
          UIView *toView = [transitionContext viewForKey:UITransitionContextToViewKey];
          [transitionContext.containerView addSubview:toView];
           
          //設置動畫從右往左出來
          toView.x = toView.width;
  
          [UIView animateWithDuration:duration animations:^{
  
              toView.x = 0;
  
          } completion:^(BOOL finished) {
              
              //移除視圖
              BOOL cancle = [transitionContext transitionWasCancelled];
              if (cancle) {
                  [toView removeFromSuperview];
              }
              
              //動畫完成後,視圖上的事件才能處理
              [transitionContext completeTransition:!cancle];
          }];
      }
      //銷毀的動畫
      else
      {
          //不做處理，而是交給自定義的交互動畫去完成
      }
  }
  @end

• CustomInteractiveTransition

  #import <UIKit/UIKit.h>
  #import "Singleton.h"
  
  @interface CustomInteractiveTransition : NSObject<UIViewControllerInteractiveTransitioning>
  SingletonH(CustomInteractiveTransition); //採用單例的方式主要是為了保存交互上下文
  
  //動畫進度更新
  -(void)updateAnimationProgress:(CGFloat)progress;
  
  //動畫完成
  -(void)finish;
  
  //動畫取消
  -(void)cancel;
  
  @end

  #import "CustomInteractiveTransition.h"
  #import "UIView+Extension.h"
  
  @interface CustomInteractiveTransition ()
  @property (nonatomic, strong) id<UIViewControllerContextTransitioning> context;
  @end
  
  @implementation CustomInteractiveTransition
  SingletonM(CustomInteractiveTransition);
  
  #pragma mark - UIViewControllerInteractiveTransitioning
  
  //開始交互時調用
  - (void)startInteractiveTransition:(id <UIViewControllerContextTransitioning>)transitionContext {
      
      //保存上下文
      self.context = transitionContext;
      
      //更改視圖層級
      UIView *toView = [transitionContext viewForKey:UITransitionContextToViewKey];
      UIView *fromView = [transitionContext viewForKey:UITransitionContextFromViewKey];
      [transitionContext.containerView insertSubview:toView belowSubview:fromView];
  }
  
  
  //動畫進度更新
  -(void)updateAnimationProgress:(CGFloat)progress {
      
      UIView *fromView = [self.context viewForKey:UITransitionContextFromViewKey];
      fromView.x = self.context.containerView.width * progress;
      
  }
  
  //動畫完成
  -(void)finish {
      
      UIView *fromView = [self.context viewForKey:UITransitionContextFromViewKey]; 
      [UIView animateWithDuration:0.2 animations:^{
          
          fromView.x += self.context.containerView.width;
          
      } completion:^(BOOL finished) { 
          [fromView removeFromSuperView]; 
          [self.context completeTransition:finished];
      }];
  }
  
  //動畫取消
  -(void)cancel {
      
      UIView *fromView = [self.context viewForKey:UITransitionContextFromViewKey]; 
      [UIView animateWithDuration:0.2 animations:^{
  
          fromView.x = 0;
          
      } completion:^(BOOL finished) {
         [fromView removeFromSuperView]; 
         [self.context cancelInteractiveTransition];
      }];
  }
  @end

• 在被模態的控制器添加拖拽手勢

  #import "SecondViewController.h"
  #import "CustomInteractiveTransition.h"
  
  @interface SecondViewController ()
  
  @end
  
  @implementation SecondViewController
  
  - (void)viewDidLoad {
      [super viewDidLoad];
      
      self.title = @"secondVc";
      self.view.backgroundColor = [UIColor redColor];
      
      [self.view addGestureRecognizer:[[UIPanGestureRecognizer alloc] initWithTarget:self action:@selector(pan:)]];
  }
  
  -(void)pan:(UIPanGestureRecognizer *)pan {
      
      CGPoint translatedPoint = [pan translationInView:self.view];
      CGFloat progress = translatedPoint.x / [UIScreen mainScreen].bounds.size.width;
      if (progress < 0) {
          return;
      }
      //拖拽的距離進度比
      progress = fabs(progress);
      CustomInteractiveTransition *transition = [[CustomInteractiveTransition alloc] init];
      switch (pan.state) {
          case UIGestureRecognizerStateBegan:
              [self dismissViewControllerAnimated:YES completion:nil];
              break;
          case UIGestureRecognizerStateChanged:
              [transition updateAnimationProgress:progress];
              break;
          case UIGestureRecognizerStateEnded:
          {
              if (progress > 0.5) {
                  [transition finish];
              }else{
                  [transition cancel];
              }
              break;
          }
          default:
              break;
      }
  }
  @end

• 開始執行，結果如gif圖

  UINavigationController *nav = [[UINavigationController alloc] initWithRootViewController:[[SecondViewController alloc] init]];
  nav.transitioningDelegate = [TransitioningDelegate sharedTransitioningDelegate];//自定義可交互轉場動畫
  nav.modalPresentationStyle = UIModalPresentationFullScreen; //系統模態風格
  [self presentViewController:nav animated:YES completion:nil];  

五、實現一個自定義的導航動畫

1、重寫導航控制器的協議，返回自定義的導航轉場動畫，動畫實現的方式和modal思想一致，重寫的核心協議如下：

//重寫導航控制器協議
@protocol UINavigationControllerDelegate <NSObject>
@optional
................

//返回一個實現了自定義交互動畫的對象
- (nullable id <UIViewControllerInteractiveTransitioning>)navigationController:(UINavigationController *)navigationController
                          interactionControllerForAnimationController:(id <UIViewControllerAnimatedTransitioning>) animationController;


//返回一個實現了普通動畫的對象
- (nullable id <UIViewControllerAnimatedTransitioning>)navigationController:(UINavigationController *)navigationController
                                   animationControllerForOperation:(UINavigationControllerOperation)operation
                                                fromViewController:(UIViewController *)fromVC
                                                  toViewController:(UIViewController *)toVC;
@end

2、 現在就來自定義一個導航轉場動畫，步驟如下：

• 創建一個CustomNavigationTransition類，實現導航控制器的協議

  #import <UIKit/UIKit.h>
  #import "Singleton.h"
  
  NS_ASSUME_NONNULL_BEGIN
  
  @interface CustomNavigationTransition : NSObject<UINavigationControllerDelegate>
  SingletonH(CustomNavigationTransition);
  @end
  
  NS_ASSUME_NONNULL_END

  #import "CustomNavigationTransition.h"
  #import "CustomNavigationAnimation.h"
  
  @implementation CustomNavigationTransition
  SingletonM(CustomNavigationTransition);
  
  #pragma mark - UINavigationControllerDelegate
  
  //返回一個實現了普通動畫的對象
  - (nullable id <UIViewControllerAnimatedTransitioning>)navigationController:(UINavigationController *)navigationController
                                     animationControllerForOperation:(UINavigationControllerOperation)operation
                                                  fromViewController:(UIViewController *)fromVC
                                                             toViewController:(UIViewController *)toVC {
      
      CustomNavigationAnimation *animation = [[CustomNavigationAnimation alloc] init];
      animation.operation = operation;
      return animation;
  }
  
  @end

• 自定義一個CustomNavigationAnimation類，實現動畫協議，重寫動畫效果

  #import <UIKit/UIKit.h>
  
  NS_ASSUME_NONNULL_BEGIN
  
  @inte