变更记录

方案名称

Runtime - 使用 Aspects 实现 Method Swizzling 和 AOP 实践

关键字

Runtime \ Method Swizzling \ Aspect Oriented Programming \ 面向切面编程

需求场景

1. 利用 Method Swizzling 对系统方法进行替换和追加行为

参考链接

1. 顾 鹏 - Method Swizzling 和 AOP 实践(推荐)
2. GitHub - steipete/Aspects

详细内容

在 Objective-C 中， Aspect Oriented Programming (面向切面编程) 就是利用 Runtime 特性给指定的方法添加自定义代码。有很多方式可以实现 AOP ，Method Swizzling 就是其中之一。而且幸运的是，目前已经有一些第三方库可以让你不需要了解 Runtime ，就能直接开始使用 AOP 。

Aspects 就是一个不错的 AOP 库，封装了 Runtime ， Method Swizzling 这些黑色技巧，只提供两个简单的API：

+ (id<AspectToken>)aspect_hookSelector:(SEL)selector
                          withOptions:(AspectOptions)options
                       usingBlock:(id)block
                            error:(NSError **)error;

- (id<AspectToken>)aspect_hookSelector:(SEL)selector
                      withOptions:(AspectOptions)options
                       usingBlock:(id)block
                            error:(NSError **)error;

示例 ： 用户看到某个 View 的时候，就把这个事件记下来

1. 通过 Runtime 提供的 Method Swizzling 方法实现

@implementation UIViewController (Logging)

void swizzleMethod(Class class, SEL originalSelector, SEL swizzledSelector)  
{
    // the method might not exist in the class, but in its superclass
    Method originalMethod = class_getInstanceMethod(class, originalSelector);
    Method swizzledMethod = class_getInstanceMethod(class, swizzledSelector);

    // class_addMethod will fail if original method already exists
    BOOL didAddMethod = class_addMethod(class, originalSelector, method_getImplementation(swizzledMethod), method_getTypeEncoding(swizzledMethod));

    // the method doesn’t exist and we just added one
    if (didAddMethod) {
        class_replaceMethod(class, swizzledSelector, method_getImplementation(originalMethod), method_getTypeEncoding(originalMethod));
    } 
    else {
        method_exchangeImplementations(originalMethod, swizzledMethod);
    }
}

+ (void)load
{
    swizzleMethod([self class], @selector(viewDidAppear:), @selector(swizzled_viewDidAppear:));
}

- (void)swizzled_viewDidAppear:(BOOL)animated
{
    // call original implementation
    [self swizzled_viewDidAppear:animated];

    // Logging
    [Logging logWithEventName:NSStringFromClass([self class])];
}

@end

要先尝试添加原 selector 是为了做一层保护，因为如果这个类没有实现 originalSelector ，但其父类实现了，那 class_getInstanceMethod 会返回父类的方法。这样 method_exchangeImplementations 替换的是父类的那个方法，这当然不是你想要的。所以我们先尝试添加 orginalSelector ，如果已经存在，再用 method_exchangeImplementations 把原方法的实现跟新的方法实现给交换掉

2. 通过 Runtime 提供的替换 IMP 方法直接用新的 IMP 取代原 IMP 来实现

void (*gOriginalViewDidAppear) (id,SEL,BOOL);

@implementation UIViewController (Logging)

void newViewDidAppear(UIViewController *self, SEL _cmd, BOOL animated)
{
    // call original implementation
    gOriginalViewDidAppear(self, _cmd, animated);

    // Logging
    [Logging logWithEventName:NSStringFromClass([self class])];
}

+ (void)load
{
    Method originalMethod = class_getInstanceMethod(self, @selector(viewDidAppear:));
    gOriginalViewDidAppear = (void *)method_getImplementation(originalMethod);

    if(!class_addMethod(self, @selector(viewDidAppear:), (IMP) newViewDidAppear, method_getTypeEncoding(originalMethod))) {
        method_setImplementation(originalMethod, (IMP) newViewDidAppear);
    }
}

@end

2. 通过 Aspects 提供接口实现

@implementation UIViewController (Logging)

+ (void)load
{
    [UIViewController aspect_hookSelector:@selector(viewDidAppear:)
                              withOptions:AspectPositionAfter
                               usingBlock:^(id<AspectInfo> aspectInfo) {

                                   NSString *className = NSStringFromClass([[aspectInfo instance] class]);
                                   [Logging logWithEventName:className];
                               }
                                    error:NULL];
}

@end

效果图

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备注

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