FDFullscreenPopGesture实现原理解析

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最近在项目中处理了一个UIScrollView右滑返回上级页的问题，刚好项目中也使用了FDFullscreenPopGesture这个全屏返回的库，于是来分析它的实现吧。

这个库是sunngxx之前在百度的时候弄的，已经好久没有更新了，不过目前还能用，star也已经有5k多了，而且sunngxx这种runtime小王子，写的这个库几乎所有函数都是runtime实现，还是很有学习的价值的。

FDFullscreenPopGesture是以一个分类文件的形式提供给使用者，在内部重写了+(void)load;方法，只要把代码集成到项目中就可以使用全屏返回了。

先来看一下头文件中的内容，首先是UINavigationController的分类

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@interface UINavigationController (FDFullscreenPopGesture)
/// The gesture recognizer that actually handles interactive pop.
@property (nonatomic, strong, readonly) UIPanGestureRecognizer *fd_fullscreenPopGestureRecognizer;
/// A view controller is able to control navigation bar's appearance by itself,
/// rather than a global way, checking "fd_prefersNavigationBarHidden" property.
/// Default to YES, disable it if you don't want so.
@property (nonatomic, assign) BOOL fd_viewControllerBasedNavigationBarAppearanceEnabled;
@end

第一个属性是当前实际相应右滑的手势对象，这个对象替换了系统的手势对象，具体咋替换的后面在说。

第二个属性设置是否使用fd的方法去控制导航栏的显示隐藏。

然后我们看一下UIViewController的分类:

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/// Allows any view controller to disable interactive pop gesture, which might
/// be necessary when the view controller itself handles pan gesture in some
/// cases.
@interface UIViewController (FDFullscreenPopGesture)
/// Whether the interactive pop gesture is disabled when contained in a navigation
/// stack.
@property (nonatomic, assign) BOOL fd_interactivePopDisabled;
/// Indicate this view controller prefers its navigation bar hidden or not,
/// checked when view controller based navigation bar's appearance is enabled.
/// Default to NO, bars are more likely to show.
@property (nonatomic, assign) BOOL fd_prefersNavigationBarHidden;
/// Max allowed initial distance to left edge when you begin the interactive pop
/// gesture. 0 by default, which means it will ignore this limit.
@property (nonatomic, assign) CGFloat fd_interactivePopMaxAllowedInitialDistanceToLeftEdge;
@end
  • fd_interactivePopDisabled:当前页面是否支持右滑返回
  • fd_prefersNavigationBarHidden:控制当前页面导航栏的显示隐藏
  • fd_interactivePopMaxAllowedInitialDistanceToLeftEdge:右滑返回触发的位置，这里是距离左边距的距离。

全屏返回

我们先来看一下全屏返回的实现。

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@interface _FDFullscreenPopGestureRecognizerDelegate : NSObject <UIGestureRecognizerDelegate>
@property (nonatomic, weak) UINavigationController *navigationController;
@end
@implementation _FDFullscreenPopGestureRecognizerDelegate
//判断是否处理返回手势
- (BOOL)gestureRecognizerShouldBegin:(UIPanGestureRecognizer *)gestureRecognizer
{
    // Ignore when no view controller is pushed into the navigation stack.
    if (self.navigationController.viewControllers.count <= 1) {
        return NO;
    }
    
    // Ignore when the active view controller doesn't allow interactive pop.
    UIViewController *topViewController = self.navigationController.viewControllers.lastObject;
    if (topViewController.fd_interactivePopDisabled) {
        return NO;
    }
    
    // Ignore when the beginning location is beyond max allowed initial distance to left edge.
    CGPoint beginningLocation = [gestureRecognizer locationInView:gestureRecognizer.view];
    CGFloat maxAllowedInitialDistance = topViewController.fd_interactivePopMaxAllowedInitialDistanceToLeftEdge;
    if (maxAllowedInitialDistance > 0 && beginningLocation.x > maxAllowedInitialDistance) {
        return NO;
    }
    // Ignore pan gesture when the navigation controller is currently in transition.
    if ([[self.navigationController valueForKey:@"_isTransitioning"] boolValue]) {
        return NO;
    }
    
    // Prevent calling the handler when the gesture begins in an opposite direction.
    CGPoint translation = [gestureRecognizer translationInView:gestureRecognizer.view];
    BOOL isLeftToRight = [UIApplication sharedApplication].userInterfaceLayoutDirection == UIUserInterfaceLayoutDirectionLeftToRight;
    CGFloat multiplier = isLeftToRight ? 1 : - 1;
    if ((translation.x * multiplier) <= 0) {
        return NO;
    }
    
    return YES;
}
//初始化返回手势
- (UIPanGestureRecognizer *)fd_fullscreenPopGestureRecognizer
{
    UIPanGestureRecognizer *panGestureRecognizer = objc_getAssociatedObject(self, _cmd);
    if (!panGestureRecognizer) {
        panGestureRecognizer = [[UIPanGestureRecognizer alloc] init];
        panGestureRecognizer.maximumNumberOfTouches = 1;
        
        objc_setAssociatedObject(self, _cmd, panGestureRecognizer, OBJC_ASSOCIATION_RETAIN_NONATOMIC);
    }
    return panGestureRecognizer;
}
//初始化返回手势的代理
- (_FDFullscreenPopGestureRecognizerDelegate *)fd_popGestureRecognizerDelegate
{
    _FDFullscreenPopGestureRecognizerDelegate *delegate = objc_getAssociatedObject(self, _cmd);
    if (!delegate) {
        delegate = [[_FDFullscreenPopGestureRecognizerDelegate alloc] init];
        delegate.navigationController = self;
        
        objc_setAssociatedObject(self, _cmd, delegate, OBJC_ASSOCIATION_RETAIN_NONATOMIC);
    }
    return delegate;
}
//hook系统的pushViewController方法，每个vc在被push后都会调用fd的fd_pushViewController方法
+ (void)load
{
    // Inject "-pushViewController:animated:"
    static dispatch_once_t onceToken;
    dispatch_once(&onceToken, ^{
        Class class = [self class];
        
        SEL originalSelector = @selector(pushViewController:animated:);
        SEL swizzledSelector = @selector(fd_pushViewController:animated:);
        
        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) {
            class_replaceMethod(class, swizzledSelector, method_getImplementation(originalMethod), method_getTypeEncoding(originalMethod));
        } else {
            method_exchangeImplementations(originalMethod, swizzledMethod);
        }
    });
}
//重要代码部分，这里替换系统右滑手势为fd自己的对象，并给对象设置fd自定义的delegate
- (void)fd_pushViewController:(UIViewController *)viewController animated:(BOOL)animated
{
    if (![self.interactivePopGestureRecognizer.view.gestureRecognizers containsObject:self.fd_fullscreenPopGestureRecognizer]) {
        
        // Add our own gesture recognizer to where the onboard screen edge pan gesture recognizer is attached to.
        [self.interactivePopGestureRecognizer.view addGestureRecognizer:self.fd_fullscreenPopGestureRecognizer];
        // Forward the gesture events to the private handler of the onboard gesture recognizer.
        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];
        // Disable the onboard gesture recognizer.
        self.interactivePopGestureRecognizer.enabled = NO;
    }
    
    // Handle perferred navigation bar appearance.
    [self fd_setupViewControllerBasedNavigationBarAppearanceIfNeeded:viewController];
    
    // Forward to primary implementation.
    if (![self.viewControllers containsObject:viewController]) {
        [self fd_pushViewController:viewController animated:animated];
    }
}
@end

上面的代码展示了fd是怎么一步步的替换了系统的右滑手势的，其他部分比较好理解，这里我们来重点看一下替换代理的部分，如下：

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if (![self.interactivePopGestureRecognizer.view.gestureRecognizers containsObject:self.fd_fullscreenPopGestureRecognizer]) {
    
    // Add our own gesture recognizer to where the onboard screen edge pan gesture recognizer is attached to.
    [self.interactivePopGestureRecognizer.view addGestureRecognizer:self.fd_fullscreenPopGestureRecognizer];
    // Forward the gesture events to the private handler of the onboard gesture recognizer.
    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];
    // Disable the onboard gesture recognizer.
    self.interactivePopGestureRecognizer.enabled = NO;
}

这里，先判断当前view的手势列表中是否包含我们自定义的右滑手势，如果不包含，先将自定义手势add到手势列表中，然后重点来了，获取interactivePopGestureRecognizer这个系统右滑返回手势的所有target,然后取出该手势的第一响应者，然后生成系统返回手势处理函数（handleNavigationTransition）的SEL结构体，然后设置自定义手势的代理，最后将系统的target和SEL添加给自定义的手势对象，并将系统的返回手势关闭。这样，fd就可以通过自定义的代理控制右滑返回了。

隐藏导航栏

看完了右滑返回，再看一下fd是如何隐藏系统导航栏的。

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+ (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) {
            class_replaceMethod(class, swizzledSelector, method_getImplementation(originalMethod), method_getTypeEncoding(originalMethod));
        } else {
            method_exchangeImplementations(originalMethod, swizzledMethod);
        }
    });
}
- (void)fd_viewWillAppear:(BOOL)animated
{
    // Forward to primary implementation.
    [self fd_viewWillAppear:animated];
    
    if (self.fd_willAppearInjectBlock) {
        self.fd_willAppearInjectBlock(self, animated);
    }
}
- (_FDViewControllerWillAppearInjectBlock)fd_willAppearInjectBlock
{
    return objc_getAssociatedObject(self, _cmd);
}
- (void)setFd_willAppearInjectBlock:(_FDViewControllerWillAppearInjectBlock)block
{
    objc_setAssociatedObject(self, @selector(fd_willAppearInjectBlock), block, OBJC_ASSOCIATION_COPY_NONATOMIC);
}
- (void)fd_setupViewControllerBasedNavigationBarAppearanceIfNeeded:(UIViewController *)appearingViewController
{
    if (!self.fd_viewControllerBasedNavigationBarAppearanceEnabled) {
        return;
    }
    
    __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];
        }
    };
    
    // Setup will appear inject block to appearing view controller.
    // Setup disappearing view controller as well, because not every view controller is added into
    // stack by pushing, maybe by "-setViewControllers:".
    appearingViewController.fd_willAppearInjectBlock = block;
    UIViewController *disappearingViewController = self.viewControllers.lastObject;
    if (disappearingViewController && !disappearingViewController.fd_willAppearInjectBlock) {
        disappearingViewController.fd_willAppearInjectBlock = block;
    }
}

隐藏这个就比较简单了，也是在fd_pushViewController里面处理的，具体说一下，这里先生成一个用来隐藏导航栏的block，使用的方法还是系统的setNavigationBarHidden：animation 方式。然后将这个block赋值给self.fd_willAppearInjectBlock，然后再每次fd_viewWillAppear时，会调用一下这个block。

总结

可以看到，代码大量的使用runtime来编写，学习一遍下来对应runtime相关的内容会有更深刻的认识。然后就是fd替换系统方法的这种骚操作，如果有类似的需求的话也是可以参考的。这个库代码也不太多，内容也比较好理解，个人觉得有学习的价值。