FDFullScreenPopGesture僅300行不到的代碼就完美實現(xiàn)了絲滑的全屏滑動匿沛，如果是我們自己實現(xiàn)一個全屏滑動而且還能保證UINavigationBar良好的切換效果你會怎么做呢？

1.最簡單的則是在返回的VC中寫下如下代碼：

- (void)viewWillAppear:(BOOL)animated {
    [super viewWillAppear:animated];
    [self.navigationController setNavigationBarHidden:YES animated:animated];
}

當然，如果場景不多還是可取，需要將其隱藏之后，在合適的時機又顯示出來夷家，場景一多可能就容易出亂子了

2.直接隱藏系統(tǒng)的UINavigationBar，自己去實現(xiàn)一個View放在頂部敏释，這種就是完全可自定義库快，不過花費代價也是稍大一些

3.使用截圖將前一個界面的視圖保存起來，自定義手勢钥顽，在滑動時判斷并顯示缺谴，這個需要處理的邏輯和情況也相對復雜，因為push和pop操作需要可以pop到上一級或者指定或者根視圖控制器等

4.實現(xiàn)UIViewControllerAnimatedTransitioning協(xié)議耳鸯，自定義轉場動畫湿蛔，在熟悉的情況下是可行的

5.今天的主角，F(xiàn)DFullScreenPopGesture县爬，完全解耦合阳啥，只需要拖入工程中就能實現(xiàn)該效果。那么這么好的東西他是如何實現(xiàn)的呢财喳？下面我們來看一下??

工程結構：

• UINavigationController (FDFullscreenPopGesture)：pushViewController:animated的hook

• UIViewController (FDFullscreenPopGesture)：主要進行viewWillAppear的hook

• _FDFullscreenPopGestureRecognizerDelegate：負責管理UIGestureRecognizerDelegate的代理

• UIViewController (FDFullscreenPopGesture)：通過runtime添加幾個設置屬性

  ?

代碼解析：

_FDFullscreenPopGestureRecognizerDelegate

//實現(xiàn)了UIGestureRecognizerDelegate的gestureRecognizerShouldBegin代理方法察迟，對手勢的操作進行管理
- (BOOL)gestureRecognizerShouldBegin:(UIPanGestureRecognizer *)gestureRecognizer
{
    //當navigationController中只有一個ViewController時返回NO，該手勢不生效
    if (self.navigationController.viewControllers.count <= 1) {
        return NO;
    }
    
    //當前的 ViewController 禁用了 fd_interactivePopDisabled耳高，fd_interactivePopDisabled是使用objc_getAssociatedObject在類別中添加的一個屬性扎瓶，用戶可設置是否禁用
  UIViewController *topViewController = self.navigationController.viewControllers.lastObject;
    if (topViewController.fd_interactivePopDisabled) {
        return NO;
    }
    //同理設置了一個fd_interactivePopMaxAllowedInitialDistanceToLeftEdge屬性用于設置最大左邊距，當滑動的x坐標大于他時也是無效的
    CGPoint beginningLocation = [gestureRecognizer locationInView:gestureRecognizer.view];
    CGFloat maxAllowedInitialDistance = topViewController.fd_interactivePopMaxAllowedInitialDistanceToLeftEdge;
    if (maxAllowedInitialDistance > 0 && beginningLocation.x > maxAllowedInitialDistance) {
        return NO;
    }

    //當前是否在轉場過程中泌枪。這里通過 KVC 拿到了 NavigationController 中的私有 _isTransitioning 屬性
    if ([[self.navigationController valueForKey:@"_isTransitioning"] boolValue]) {
        return NO;
    }
    
    // 從右往左滑動也是無效的
    CGPoint translation = [gestureRecognizer translationInView:gestureRecognizer.view];
    if (translation.x <= 0) {
        return NO;
    }
    
    return YES;
}

UIViewController (FDFullscreenPopGesturePrivate)

typedef void (^_FDViewControllerWillAppearInjectBlock)(UIViewController *viewController, BOOL animated);//定義了一個block用于在Swizzling的fd_viewWillAppear方法中回調

//在整個文件被加載到運行時概荷，在 main 函數(shù)調用之前被 ObjC 運行時調用的鉤子方法
+ (void)load
{
    static dispatch_once_t onceToken;
    dispatch_once(&onceToken, ^{
        Class class = [self class];
        
        SEL originalSelector = @selector(viewWillAppear:);
        SEL swizzledSelector = @selector(fd_viewWillAppear:);
        
        Method originalMethod = class_getInstanceMethod(class, originalSelector);
        Method swizzledMethod = class_getInstanceMethod(class, swizzledSelector);
        
        BOOL success = class_addMethod(class, originalSelector, method_getImplementation(swizzledMethod), method_getTypeEncoding(swizzledMethod));
        if (success) {
          //主類本身沒有實現(xiàn)需要替換的方法，而是繼承了父類的實現(xiàn)碌燕，即 class_addMethod 方法返回 YES 误证。這時使用 class_getInstanceMethod 函數(shù)獲取到的 originalSelector 指向的就是父類的方法，我們再通過執(zhí)行 class_replaceMethod(class, swizzledSelector, method_getImplementation(originalMethod), method_getTypeEncoding(originalMethod)); 將父類的實現(xiàn)替換到我們自定義的 mrc_viewWillAppear 方法中修壕。這樣就達到了在 mrc_viewWillAppear 方法的實現(xiàn)中調用父類實現(xiàn)的目的愈捅。
            class_replaceMethod(class, swizzledSelector, method_getImplementation(originalMethod), method_getTypeEncoding(originalMethod));
        } else {
          //主類本身有實現(xiàn)需要替換的方法，也就是 class_addMethod 方法返回 NO 慈鸠。這種情況的處理比較簡單蓝谨，直接交換兩個方法的實現(xiàn)就可以了
            method_exchangeImplementations(originalMethod, swizzledMethod);
        }
    });
}

- (void)fd_viewWillAppear:(BOOL)animated
{
    // Method Swizzling之后, 調用fd_viewWillAppear:實際執(zhí)行的代碼已經(jīng)是原來viewWillAppear中的代碼了
    [self fd_viewWillAppear:animated];
    
    if (self.fd_willAppearInjectBlock) {
        self.fd_willAppearInjectBlock(self, animated);
    }
}

- (_FDViewControllerWillAppearInjectBlock)fd_willAppearInjectBlock
{
  //_cmd在Objective-C的方法中表示當前方法的selector，正如同self表示當前方法調用的對象實例
    return objc_getAssociatedObject(self, _cmd);
}

- (void)setFd_willAppearInjectBlock:(_FDViewControllerWillAppearInjectBlock)block
{
    objc_setAssociatedObject(self, @selector(fd_willAppearInjectBlock), block, OBJC_ASSOCIATION_COPY_NONATOMIC);
}

UINavigationController (FDFullscreenPopGesture)

//將此方法替換了pushViewController:animated:
- (void)fd_pushViewController:(UIViewController *)viewController animated:(BOOL)animated
{
  
    if (![self.interactivePopGestureRecognizer.view.gestureRecognizers containsObject:self.fd_fullscreenPopGestureRecognizer]) {
        
        // 將自定義的UIPanGestureRecognizer添加到本來interactivePopGestureRecognizer所在的view上
        [self.interactivePopGestureRecognizer.view addGestureRecognizer:self.fd_fullscreenPopGestureRecognizer];

        // 使用kvc拿到內部的targets數(shù)組，并找到他的target和action SEL譬巫，將fd_fullscreenPopGestureRecognizer的target設置為internalTarget咖楣，action設置為handleNavigationTransition.實際上就是將系統(tǒng)的手勢事件轉發(fā)為自定義的手勢，觸發(fā)的事件不變缕题，厲害吧截歉，能找到這些屬性也是牛逼炸了
        NSArray *internalTargets = [self.interactivePopGestureRecognizer valueForKey:@"targets"];
        id internalTarget = [internalTargets.firstObject valueForKey:@"target"];
        SEL internalAction = NSSelectorFromString(@"handleNavigationTransition:");
        self.fd_fullscreenPopGestureRecognizer.delegate = self.fd_popGestureRecognizerDelegate;
        [self.fd_fullscreenPopGestureRecognizer addTarget:internalTarget action:internalAction];

        // 將原來的手勢禁用
        self.interactivePopGestureRecognizer.enabled = NO;
    }
    
    // 通過 fd_prefersNavigationBarHidden 來顯示和隱藏 NavigationBar
    [self fd_setupViewControllerBasedNavigationBarAppearanceIfNeeded:viewController];
    
    //調用父類pushViewController:animated
    if (![self.viewControllers containsObject:viewController]) {
        [self fd_pushViewController:viewController animated:animated];
    }
}

- (void)fd_setupViewControllerBasedNavigationBarAppearanceIfNeeded:(UIViewController *)appearingViewController
{
    //如果設置屬性為NO，即為隱藏
    if (!self.fd_viewControllerBasedNavigationBarAppearanceEnabled) {
        return;
    }
    
    //viewWillAppear的時候將會調用該方法烟零，實際上內部也是通過調用setNavigationBarHidden:animated:來設置NavigationBar的顯示
    __weak typeof(self) weakSelf = self;
    _FDViewControllerWillAppearInjectBlock block = ^(UIViewController *viewController, BOOL animated) {
        __strong typeof(weakSelf) strongSelf = weakSelf;
        if (strongSelf) {
            [strongSelf setNavigationBarHidden:viewController.fd_prefersNavigationBarHidden animated:animated];
        }
    };
    
    // 將fd_willAppearInjectBlock注入到新的的view controller中
    // 將棧頂?shù)膙iewController拿出來并且判斷是否已經(jīng)注入了fd_willAppearInjectBlock瘪松，沒有則添加，因為并不一定每個vc都是通過push加入到棧的锨阿，也有可能通過"-setViewControllers:"
    appearingViewController.fd_willAppearInjectBlock = block;
    UIViewController *disappearingViewController = self.viewControllers.lastObject;
    if (disappearingViewController && !disappearingViewController.fd_willAppearInjectBlock) {
        disappearingViewController.fd_willAppearInjectBlock = block;
    }
}