Runtime底层原理总结--反汇编分析消息转发

消息转发：发送一个消息，也就是sel查找imp，当没有找到imp，接下来进入动态方法解析，如果开发者并没有处理，会进入消息转发。

消息转发

前几篇文章介绍了Runtime底层原理和动态方法解析总结

，我们知道如果前面的动态方法解析也没有解决问题的话，那么就会进入消息转发_objc_msgForward_impcache方法，会有快速消息转发和慢速消息转发。
_objc_msgForward_impcache方法会从C转换到汇编部分__objc_msgForward_impcache进行快速消息转发，执行闭源__objc_msgForward。

如果我们的方法没有查找到会报错_forwarding_prep_0

但是我们在源代码中找不到该方法，除了前面文章—Runtime底层原理—动态方法解析、消息转发源码分析提到的方法外，我们可以用反汇编分析消息转发。

首先进入/Applications/Xcode.app/Contents/Developer/Platforms/iPhoneOS.platform/Developer/Library/CoreSimulator/Profiles/Runtimes/iOS.simruntime/Contents/Resources/RuntimeRoot/System/Library/Frameworks/CoreFoundation.framework中，找到可执行文件CoreFoundation，将该文件拖入hopper中，找到CFInitialze可以看到了___forwarding_prep_0___

进入___forwarding_prep_0___内部，

从崩溃堆栈信息中看到有执行forwarding，进入forwarding内部，里面判断了_objc_msgSend_stret、_objc_msgSend、taggedpointer之后有个forwardingTargetForSelector:，判断forwardingTargetForSelector:没有实现，没有实现跳转到loc_126fc7，

进入loc_126fc7，判断僵尸对象之后执行方法签名methodSignatureForSelector:，如果有值，获取Name、是否有效的位移等，之后会响应_forwardStackInvocation:，

如果没有响应_forwardStackInvocation:，则会响应forwardInvocation:，给rdi发送rax消息，rdi就是NSInvocation，rax就是selforwardInvocation:，这就是消息转发的流程

对_objc_msgForward部分进行反汇编成OC代码：

int __forwarding__(void *frameStackPointer, int isStret) {
    id receiver = *(id *)frameStackPointer;
    SEL sel = *(SEL *)(frameStackPointer + 8);
    const char *selName = sel_getName(sel);
    Class receiverClass = object_getClass(receiver);
    
    // 调用 forwardingTargetForSelector:
    if (class_respondsToSelector(receiverClass, @selector(forwardingTargetForSelector:))) {
        id forwardingTarget = [receiver forwardingTargetForSelector:sel];
        if (forwardingTarget && forwarding != receiver) {
            if (isStret == 1) {
                int ret;
                objc_msgSend_stret(&ret,forwardingTarget, sel, ...);
                return ret;
            }
            return objc_msgSend(forwardingTarget, sel, ...);
        }
    }
    
    // 僵尸对象
    const char *className = class_getName(receiverClass);
    const char *zombiePrefix = "_NSZombie_";
    size_t prefixLen = strlen(zombiePrefix); // 0xa
    if (strncmp(className, zombiePrefix, prefixLen) == 0) {
        CFLog(kCFLogLevelError,
              @"*** -[%s %s]: message sent to deallocated instance %p",
              className + prefixLen,
              selName,
              receiver);
        <breakpoint-interrupt>
    }
    
    // 调用 methodSignatureForSelector 获取方法签名后再调用 forwardInvocation
    if (class_respondsToSelector(receiverClass, @selector(methodSignatureForSelector:))) {
        NSMethodSignature *methodSignature = [receiver methodSignatureForSelector:sel];
        if (methodSignature) {
            BOOL signatureIsStret = [methodSignature _frameDescriptor]->returnArgInfo.flags.isStruct;
            if (signatureIsStret != isStret) {
                CFLog(kCFLogLevelWarning ,
                      @"*** NSForwarding: warning: method signature and compiler disagree on struct-return-edness of '%s'.  Signature thinks it does%s return a struct, and compiler thinks it does%s.",
                      selName,
                      signatureIsStret ? "" : not,
                      isStret ? "" : not);
            }
            if (class_respondsToSelector(receiverClass, @selector(forwardInvocation:))) {
                NSInvocation *invocation = [NSInvocation _invocationWithMethodSignature:methodSignature frame:frameStackPointer];
                
                [receiver forwardInvocation:invocation];
                
                void *returnValue = NULL;
                [invocation getReturnValue:&value];
                return returnValue;
            } else {
                CFLog(kCFLogLevelWarning ,
                      @"*** NSForwarding: warning: object %p of class '%s' does not implement forwardInvocation: -- dropping message",
                      receiver,
                      className);
                return 0;
            }
        }
    }
    
    SEL *registeredSel = sel_getUid(selName);
    
    // selector 是否已经在 Runtime 注册过
    if (sel != registeredSel) {
        CFLog(kCFLogLevelWarning ,
              @"*** NSForwarding: warning: selector (%p) for message '%s' does not match selector known to Objective C runtime (%p)-- abort",
              sel,
              selName,
              registeredSel);
    } // doesNotRecognizeSelector
    else if (class_respondsToSelector(receiverClass,@selector(doesNotRecognizeSelector:))) {
        [receiver doesNotRecognizeSelector:sel];
    }
    else {
        CFLog(kCFLogLevelWarning ,
              @"*** NSForwarding: warning: object %p of class '%s' does not implement doesNotRecognizeSelector: -- abort",
              receiver,
              className);
    }
    
    // The point of no return.
    kill(getpid(), 9);
}

该文章为记录本人的学习路程，希望能够帮助大家！！！