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App开发流程之右滑返回手势功能
iOS7以后，导航控制器，自带了从屏幕左边缘右滑返回的手势功能。
但是，如果自定义了导航栏返回按钮，这项功能就失效了，需要自行实现。又如果需要修改手势触发范围，还是需要自行实现。
广泛应用的一种实现方案是，采用私有变量和Api，完成手势交互和返回功能，自定义手势触发条件和额外功能。
另一种实现方案是，采用UINavigationController的代理方法实现交互和动画：
- (nullable id <UIViewControllerInteractiveTransitioning>)navigationController:(UINavigationController *)navigationController
interactionControllerForAnimationController:(id <UIViewControllerAnimatedTransitioning>) animationController NS_AVAILABLE_IOS(7_0);
- (nullable id <UIViewControllerAnimatedTransitioning>)navigationController:(UINavigationController *)navigationController
animationControllerForOperation:(UINavigationControllerOperation)operation
fromViewController:(UIViewController *)fromVC
toViewController:(UIViewController *)toVC NS_AVAILABLE_IOS(7_0);
前者，特点是便捷，但是只能使用系统定义的交互和动画；后者，特点是高度自定义，但是需要额外实现交互协议和动画协议。
采用私有变量和Api，实现右滑返回手势功能。
先看最核心的逻辑：
- (void)base_pushViewController:(UIViewController *)viewController animated:(BOOL)animated{ if (![self.interactivePopGestureRecognizer.view.gestureRecognizers containsObject:self.base_panGestureRecognizer]) { [self.interactivePopGestureRecognizer.view addGestureRecognizer:self.base_panGestureRecognizer]; //使用KVC获取私有变量和Api，实现系统原生的pop手势效果 NSArray *internalTargets = [self.interactivePopGestureRecognizer valueForKey:@"targets"]; id internalTarget = [internalTargets.firstObject valueForKey:@"target"]; SEL internalAction = NSSelectorFromString(@"handleNavigationTransition:"); self.base_panGestureRecognizer.delegate = [self base_panGestureRecognizerDelegateObject]; [self.base_panGestureRecognizer addTarget:internalTarget action:internalAction]; self.interactivePopGestureRecognizer.enabled = NO; } [self base_setupViewControllerBasedNavigationBarAppearanceIfNeeded:viewController]; if (![self.viewControllers containsObject:viewController]) { [self base_pushViewController:viewController animated:animated]; }}
UINavigationController的interactivePopGestureRecognizer属性，是系统专用于将viewController弹出导航栈的手势识别对象，有一个私有变量名为“targets”，类似为NSArray；该数组第一个元素对象，有一个私有变量名为“target”，即为实现预期交互的对象；该对象有一个私有方法名为“handleNavigationTransition:”，即为目标方法。
在返回手势交互的UIView（self.interactivePopGestureRecognizer.view）上添加一个自定义的UIPanGestureRecognizer，利用其delegate对象实现的代理方法gestureRecognizerShouldBegin来控制手势生效条件。最后禁用系统的返回手势识别对象，就可以用自定义实现的pan手势来调用系统的pop交互和动画。
判断pan手势是否能触发返回操作的代码如下：
-(BOOL)gestureRecognizerShouldBegin:(UIGestureRecognizer *)gestureRecognizer{ //正在转场 if ([[self.navigationController valueForKey:@"_isTransitioning"] boolValue]) { return NO; } //在导航控制器的根控制器界面 if (self.navigationController.viewControllers.count <= 1) { return NO; } UIViewController *popedController = [self.navigationController.viewControllers lastObject]; if (popedController.base_popGestureDisabled) { return NO; } //满足有效手势范围 CGPoint beginningLocation = [gestureRecognizer locationInView:gestureRecognizer.view]; CGFloat popGestureEffectiveDistanceFromLeftEdge = popedController.base_popGestureEffectiveDistanceFromLeftEdge; if (popGestureEffectiveDistanceFromLeftEdge > 0 && beginningLocation.x > popGestureEffectiveDistanceFromLeftEdge) { return NO; } //右滑手势 UIPanGestureRecognizer *panGesture = (UIPanGestureRecognizer *)gestureRecognizer; CGPoint transition = [panGesture translationInView:panGesture.view]; if (transition.x <= 0) { return NO; } return YES;}
其中navigationController还使用了私有变量“_isTransitioning”，用于判断交互是否正在进行中。
为了使过场动画过程中，导航栏的交互动画自然，需要在UIViewController的viewWillAppear方法中，通过swizzle方法调用导航栏显示或隐藏的动画方法，所以需要增加一个延迟执行的代码块：
- (void)base_setupViewControllerBasedNavigationBarAppearanceIfNeeded:(UIViewController *)appearingViewController{ //如果navigationController不显示导航栏，直接return if (self.navigationBarHidden) { return; } __weak typeof(self) weakSelf = self; ViewControllerViewWillAppearDelayBlock block = ^(UIViewController *viewController, BOOL animated) { __strong typeof(weakSelf) strongSelf = weakSelf; if (strongSelf) { [strongSelf setNavigationBarHidden:viewController.base_currentNavigationBarHidden animated:animated]; } }; appearingViewController.viewWillAppearDelayBlock = block; UIViewController *disappearingViewController = self.viewControllers.lastObject; if (disappearingViewController && !disappearingViewController.viewWillAppearDelayBlock) { disappearingViewController.viewWillAppearDelayBlock = block; }}
实现完整的分类的过程中，使用了一些运行时类型和方法。
1.引用头文件#import <objc/runtime.h>
2.为分类增加属性，涉及到了如下方法：
void objc_setAssociatedObject(id object, const void *key, id value, objc_AssociationPolicy policy)
id objc_getAssociatedObject(id object, const void *key)
例如UIViewController (PopGesture)中增加的属性base_currentNavigationBarHidden的get/set方法：
- (BOOL)base_currentNavigationBarHidden{ NSNumber *number = objc_getAssociatedObject(self, _cmd); if (number) { return number.boolValue; } self.base_currentNavigationBarHidden = NO; return NO;}- (void)setBase_currentNavigationBarHidden:(BOOL)hidden{ self.canUseViewWillAppearDelayBlock = YES; objc_setAssociatedObject(self, @selector(base_currentNavigationBarHidden), @(hidden), OBJC_ASSOCIATION_RETAIN_NONATOMIC);}
第一个参数一般为self。
第二个参数const void *key，要求传入一个地址。
可以声明一个static char *key;或者static NSString *key;，赋值与否并不重要，因为需要的只是地址，参数为&key。
而上述代码中使用了_cmd和@selector，作用是一样的。_cmd返回的是当前方法的SEL，@selector也是返回目标方法的SEL，即是函数地址。
第三个参数即是关联的值。
第四个参数policy，为枚举类型，基本对应属性引用相关的关键字：
typedef OBJC_ENUM(uintptr_t, objc_AssociationPolicy) { OBJC_ASSOCIATION_ASSIGN = 0, /**< Specifies a weak reference to the associated object. */ OBJC_ASSOCIATION_RETAIN_NONATOMIC = 1, /**< Specifies a strong reference to the associated object. * The association is not made atomically. */ OBJC_ASSOCIATION_COPY_NONATOMIC = 3, /**< Specifies that the associated object is copied. * The association is not made atomically. */ OBJC_ASSOCIATION_RETAIN = 01401, /**< Specifies a strong reference to the associated object. * The association is made atomically. */ OBJC_ASSOCIATION_COPY = 01403 /**< Specifies that the associated object is copied. * The association is made atomically. */};
3.理解其他的一些运行时类型或方法
typedef struct objc_method *Method;//An opaque type that represents a method in a class definition.
Method class_getInstanceMethod(Class cls, SEL name) //返回实例方法
Method class_getClassMethod(Class cls, SEL name) //返回类方法
IMP method_getImplementation(Method m) //Returns the implementation of a method.
const char *method_getTypeEncoding(Method m) //Returns a string describing a method‘s parameter and return types.
BOOL class_addMethod(Class cls, SEL name, IMP imp, const char *types) //Adds a new method to a class with a given name and implementation.
IMP class_replaceMethod(Class cls, SEL name, IMP imp, const char *types) //Replaces the implementation of a method for a given class.
void method_exchangeImplementations(Method m1, Method m2) //Exchanges the implementations of two methods.
以上方法，可以达到swizzle方法的目的，将分类中新增方法与已有旧的方法交换函数地址，可以作为完全替换（因为运行时，执行的方法名称仍然为viewWillAppear:，但是指向新增方法地址），也可以在新增方法代码中调用当前的方法名称（交换后，当前的方法名称指向旧方法地址）。例如UIViewController中的下列代码：
+(void)load{ __weak typeof(self) weakSelf = self; static dispatch_once_t once; dispatch_once(&once, ^{ [weakSelf swizzleOriginalSelector:@selector(viewWillAppear:) withNewSelector:@selector(base_viewWillAppear:)]; [weakSelf swizzleOriginalSelector:@selector(viewDidDisappear:) withNewSelector:@selector(base_viewDidDisappear:)]; });}+(void)swizzleOriginalSelector:(SEL)originalSelector withNewSelector:(SEL)newSelector{ Class selfClass = [self class]; Method originalMethod = class_getInstanceMethod(selfClass, originalSelector); Method newMethod = class_getInstanceMethod(selfClass, newSelector); IMP originalIMP = method_getImplementation(originalMethod); IMP newIMP = method_getImplementation(newMethod); //先用新的IMP加到原始SEL中 BOOL addSuccess = class_addMethod(selfClass, originalSelector, newIMP, method_getTypeEncoding(newMethod)); if (addSuccess) { class_replaceMethod(selfClass, newSelector, originalIMP, method_getTypeEncoding(originalMethod)); }else{ method_exchangeImplementations(originalMethod, newMethod); }}-(void)base_viewWillAppear:(BOOL)animated{ [self base_viewWillAppear:animated]; if (self.canUseViewWillAppearDelayBlock && self.viewWillAppearDelayBlock) { self.viewWillAppearDelayBlock(self, animated); } if (self.transitionCoordinator) { [self.transitionCoordinator notifyWhenInteractionEndsUsingBlock:^(id<UIViewControllerTransitionCoordinatorContext> _Nonnull context) { if ([context isCancelled]) { self.base_isBeingPoped = NO; } }]; }}
特别说明：
A.+load静态方法将在此分类加入运行时调用（Invoked whenever a class or category is added to the Objective-C runtime），执行顺序在该类自己的+load方法之后。
B.如果在使用中未明确设置base_currentNavigationBarHidden，canUseViewWillAppearDelayBlock则为NO，因为我封装的父类中提供了类似功能，所以不需要开启分类中同样的功能。该功能目的是提供给直接集成分类的朋友。
C.UIViewController的transitionCoordinator属性，在当前界面有过场交互时候，该属性有值。并且在交互结束时候，可以回调一个block，以告知过场交互的状态和相关属性。这里声明了一个属性base_isBeingPoped，用于标记当前视图控制器是否正在被pop出导航栈，如果交互取消了，置为NO，最终可以在viewDidDisappear:方法中判断并执行一些操作。
使用UINavigationController的代理方法来实现高度自定义的方案，下次再更新记录。
base项目已更新：git@github.com:ALongWay/base.git
App开发流程之右滑返回手势功能