AFNetworking - _AFURLSessionTaskSwizzling源码解析

_AFURLSessionTaskSwizzling的作用就是为了解决不同iOS版本的NSURLSessionTask的继承链不一样，对resume和suspend方法作用的类也不一样而导致可能崩溃的问题

原理是在+load方法中对各个层次的类只要实现了resume和suspend都进行方法交换，并发送出相应的通知以便我们进行处理

+ (void)load {
    /**
     WARNING: Trouble Ahead
     https://github.com/AFNetworking/AFNetworking/pull/2702
     */
    if (NSClassFromString(@"NSURLSessionTask")) {
        NSURLSessionConfiguration *configuration = [NSURLSessionConfiguration ephemeralSessionConfiguration];
        NSURLSession *session = [NSURLSession sessionWithConfiguration:configuration];
#pragma GCC diagnostic push
#pragma GCC diagnostic ignored "-Wnonnull"
        NSURLSessionDataTask *localDataTask = [session dataTaskWithURL:nil];
#pragma clang diagnostic pop
        //获取af_resume的实现指针
        IMP originalAFResumeIMP = method_getImplementation(class_getInstanceMethod([self class], @selector(af_resume)));
        Class currentClass = [localDataTask class];
        
        //检查当前类是否有resume方法的实现
        while (class_getInstanceMethod(currentClass, @selector(resume))) {
            //获取当前类的父类
            Class superClass = [currentClass superclass];
            //当前类resume的实现
            IMP classResumeIMP = method_getImplementation(class_getInstanceMethod(currentClass, @selector(resume)));
            //父类resume的实现
            IMP superclassResumeIMP = method_getImplementation(class_getInstanceMethod(superClass, @selector(resume)));
            //如果当前类的resume实现和父类不一样，并且af_resume的方法实现和当前类的resume实现不一样（没有被交换过方法），才交换方法
            if (classResumeIMP != superclassResumeIMP &&
                originalAFResumeIMP != classResumeIMP) {
                [self swizzleResumeAndSuspendMethodForClass:currentClass];
            }
            currentClass = [currentClass superclass];
        }
        
        [localDataTask cancel];
        [session finishTasksAndInvalidate];
    }
}

利用runtime进行方法交换的方法

+ (void)swizzleResumeAndSuspendMethodForClass:(Class)theClass {
    Method afResumeMethod = class_getInstanceMethod(self, @selector(af_resume));
    Method afSuspendMethod = class_getInstanceMethod(self, @selector(af_suspend));

    // 给theClass添加一个名为af_resume的方法，使用@selector(af_resume)获取方法名，使用afResumeMethod作为方法实现
    if (af_addMethod(theClass, @selector(af_resume), afResumeMethod)) {
        af_swizzleSelector(theClass, @selector(resume), @selector(af_resume));
    }
    // 给theClass添加一个名为af_suspend的方法，使用@selector(af_suspend)获取方法名，使用afSuspendMethod作为方法实现
    if (af_addMethod(theClass, @selector(af_suspend), afSuspendMethod)) {
        af_swizzleSelector(theClass, @selector(suspend), @selector(af_suspend));
    }
}

相关的两个静态内联函数，一个是为类添加方法，一个是实现方法交换

static inline void af_swizzleSelector(Class theClass, SEL originalSelector, SEL swizzledSelector) {
    Method originalMethod = class_getInstanceMethod(theClass, originalSelector);
    Method swizzledMethod = class_getInstanceMethod(theClass, swizzledSelector);
    method_exchangeImplementations(originalMethod, swizzledMethod);
}

static inline BOOL af_addMethod(Class theClass, SEL selector, Method method) {
    return class_addMethod(theClass, selector,  method_getImplementation(method),  method_getTypeEncoding(method));
}

自己实现的af_resume和af_suspend方法

- (void)af_resume {
    NSAssert([self respondsToSelector:@selector(state)], @"Does not respond to state");
    NSURLSessionTaskState state = [self state];
    //经过method swizzling后，af_resume已经变成之前的resume，所以此处调用af_resume就是调用系统的resume
    [self af_resume];
    
    if (state != NSURLSessionTaskStateRunning) {
        [[NSNotificationCenter defaultCenter] postNotificationName:AFNSURLSessionTaskDidResumeNotification object:self];
    }
}

- (void)af_suspend {
    NSAssert([self respondsToSelector:@selector(state)], @"Does not respond to state");
    NSURLSessionTaskState state = [self state];
    //经过method swizzling后，af_suspend已经变成之前的suspend，所以此处调用af_suspend就是调用系统的suspend
    [self af_suspend];
    
    if (state != NSURLSessionTaskStateSuspended) {
        [[NSNotificationCenter defaultCenter] postNotificationName:AFNSURLSessionTaskDidSuspendNotification object:self];
    }
}