AFN 中的 Method swizzling

最终在研究AFN3.0 源码中无意发现了 “Method swizzling”。AFN3.0 源码中知识还是值得研究的。

Method swizzling是什么？

Method Swizzling是objective-c中的黑魔法，算是runtime中的一种实战使用模式，它允许我们动态的替换方法，实现Hook功能。

Method swizzling可以干什么？

Method swizzling字面意思就是方法交换。那它可以运用在那些场景呢？

1. 按钮的重复点击问题
2. 界面点击次数
   …

Method Swizzling 原理

在Method方法中，有两个关键的成员变量：SEL和IMP
SEL就是我们平时看到的方法的名称，比如 @selector(resume)。
IMP是一个函数指针，指向的是方法的实现。

原则上 方法名SEL 和IMP是一一对应的。但是我们可以通过Method Swizzling来改变它们的对应关系。从而改变方法的实现。

Method Swizzling 实现

实现之前我们先来介绍Method Swizzling 用到的一些函数

class_getInstanceMethod 获取一个方法的名称SEL；
method_getImplementation 获取一个方法的实现 IMP；
method_getTypeEncoding 获取一个实现编码类型;
class_addMethod 给方法添加实现;
class_replaceMethod 用一个方法实现替换另一个方法的实现；
method_exchangeImplementations 交换两个方法的实现;

请看下面AFN中的源码：

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));
}

static NSString * const AFNSURLSessionTaskDidResumeNotification  = @"com.alamofire.networking.nsurlsessiontask.resume";
static NSString * const AFNSURLSessionTaskDidSuspendNotification = @"com.alamofire.networking.nsurlsessiontask.suspend";

@interface _AFURLSessionTaskSwizzling : NSObject

@end

@implementation _AFURLSessionTaskSwizzling

+ (void)load {
    /**
     WARNING: Trouble Ahead
     https://github.com/AFNetworking/AFNetworking/pull/2702
     */

    if (NSClassFromString(@"NSURLSessionTask")) {
        /**
         iOS 7 and iOS 8 differ in NSURLSessionTask implementation, which makes the next bit of code a bit tricky.
         Many Unit Tests have been built to validate as much of this behavior has possible.
         Here is what we know:
            - NSURLSessionTasks are implemented with class clusters, meaning the class you request from the API isn't actually the type of class you will get back.
            - Simply referencing `[NSURLSessionTask class]` will not work. You need to ask an `NSURLSession` to actually create an object, and grab the class from there.
            - On iOS 7, `localDataTask` is a `__NSCFLocalDataTask`, which inherits from `__NSCFLocalSessionTask`, which inherits from `__NSCFURLSessionTask`.
            - On iOS 8, `localDataTask` is a `__NSCFLocalDataTask`, which inherits from `__NSCFLocalSessionTask`, which inherits from `NSURLSessionTask`.
            - On iOS 7, `__NSCFLocalSessionTask` and `__NSCFURLSessionTask` are the only two classes that have their own implementations of `resume` and `suspend`, and `__NSCFLocalSessionTask` DOES NOT CALL SUPER. This means both classes need to be swizzled.
            - On iOS 8, `NSURLSessionTask` is the only class that implements `resume` and `suspend`. This means this is the only class that needs to be swizzled.
            - Because `NSURLSessionTask` is not involved in the class hierarchy for every version of iOS, its easier to add the swizzled methods to a dummy class and manage them there.
        
         Some Assumptions:
            - No implementations of `resume` or `suspend` call super. If this were to change in a future version of iOS, we'd need to handle it.
            - No background task classes override `resume` or `suspend`
         
         The current solution:
            1) Grab an instance of `__NSCFLocalDataTask` by asking an instance of `NSURLSession` for a data task.
            2) Grab a pointer to the original implementation of `af_resume`
            3) Check to see if the current class has an implementation of resume. If so, continue to step 4.
            4) Grab the super class of the current class.
            5) Grab a pointer for the current class to the current implementation of `resume`.
            6) Grab a pointer for the super class to the current implementation of `resume`.
            7) If the current class implementation of `resume` is not equal to the super class implementation of `resume` AND the current implementation of `resume` is not equal to the original implementation of `af_resume`, THEN swizzle the methods
            8) Set the current class to the super class, and repeat steps 3-8
         */
        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
        IMP originalAFResumeIMP = method_getImplementation(class_getInstanceMethod([self class], @selector(af_resume)));
        Class currentClass = [localDataTask class];
        
        while (class_getInstanceMethod(currentClass, @selector(resume))) {
            Class superClass = [currentClass superclass];
            IMP classResumeIMP = method_getImplementation(class_getInstanceMethod(currentClass, @selector(resume)));
            IMP superclassResumeIMP = method_getImplementation(class_getInstanceMethod(superClass, @selector(resume)));
            if (classResumeIMP != superclassResumeIMP &&
                originalAFResumeIMP != classResumeIMP) {
                [self swizzleResumeAndSuspendMethodForClass:currentClass];
            }
            currentClass = [currentClass superclass];
        }
        
        [localDataTask cancel];
        [session finishTasksAndInvalidate];
    }
}

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

    if (af_addMethod(theClass, @selector(af_resume), afResumeMethod)) {
        af_swizzleSelector(theClass, @selector(resume), @selector(af_resume));
    }

    if (af_addMethod(theClass, @selector(af_suspend), afSuspendMethod)) {
        af_swizzleSelector(theClass, @selector(suspend), @selector(af_suspend));
    }
}

- (NSURLSessionTaskState)state {
    NSAssert(NO, @"State method should never be called in the actual dummy class");
    return NSURLSessionTaskStateCanceling;
}

- (void)af_resume {
    NSAssert([self respondsToSelector:@selector(state)], @"Does not respond to state");
    NSURLSessionTaskState state = [self state];
    [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];
    [self af_suspend];
    
    if (state != NSURLSessionTaskStateSuspended) {
        [[NSNotificationCenter defaultCenter] postNotificationName:AFNSURLSessionTaskDidSuspendNotification object:self];
    }
}
@end

Method Swizzling 注意

1、在+load实现的原因？
因为+load 方法是在类被加载在的时候调用的。+initialize在第一次给某个类发送消息时调用（比如实例化一个对象），并且只会调用一次，是懒加载模式，如果这个类一直没有使用，就不回调用到+initialize方法。
2、af_resume 方法里面调用af_resume，不会死循环吗？
因为Method Swizzling的原理为方法互换，所以这时候用执行的[self af_resume]方法，实际上是执行的resume方法。