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 <= ) {
        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 >
        && beginningLocation.x > popGestureEffectiveDistanceFromLeftEdge) {
        return NO;
    }

    //右滑手势
    UIPanGestureRecognizer *panGesture = (UIPanGestureRecognizer *)gestureRecognizer;
    CGPoint transition = [panGesture translationInView:panGesture.view];

    if (transition.x <= ) {
        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 = ,           /**< Specifies a weak reference to the associated object. */

    OBJC_ASSOCIATION_RETAIN_NONATOMIC = , /**< Specifies a strong reference to the associated object. 

                                            *   The association is not made atomically. */

    OBJC_ASSOCIATION_COPY_NONATOMIC = ,   /**< Specifies that the associated object is copied. 

                                            *   The association is not made atomically. */

    OBJC_ASSOCIATION_RETAIN = ,       /**< Specifies a strong reference to the associated object.

                                            *   The association is made atomically. */

    OBJC_ASSOCIATION_COPY =           /**< 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。

=================

已更新：http://www.cnblogs.com/ALongWay/p/5896982.html

base项目已更新：git@github.com:ALongWay/base.git