由于forward_imp的实现不开源，也涉及到反汇编，存在知识盲区，这篇文章主要是参考别人的博客，原作者博客（http://yulingtianxia.com/blog/2016/06/15/Objective-C-Message-Sending-and-Forwarding/），如下

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);
}

objc_msgForwad执行流程如下

未命名文件-2.jpg

结合上一篇文章，方法调用的执行流程如下

未命名文件-4.jpg