消息转发

在Objc_msgSend流程(二)之方法慢速查找的lookUpImpOrForward中，会调用log_and_fill_cache，其中会调用logMessageSend函数

• log_and_fill_cache

  static void
  log_and_fill_cache(Class cls, IMP imp, SEL sel, id receiver, Class implementer)
  {
  #if SUPPORT_MESSAGE_LOGGING
      if (slowpath(objcMsgLogEnabled && implementer)) {
          bool cacheIt = logMessageSend(implementer->isMetaClass(), 
                                        cls->nameForLogging(),
                                        implementer->nameForLogging(), 
                                        sel);
          if (!cacheIt) return;
      }
  #endif
      cache_fill(cls, sel, imp, receiver);
  }
  

• logMessageSend

  bool logMessageSend(bool isClassMethod,
                      const char *objectsClass,
                      const char *implementingClass,
                      SEL selector)
  {
      char    buf[ 1024 ];
  
      // Create/open the log file
      if (objcMsgLogFD == (-1))
      {
          snprintf (buf, sizeof(buf), "/tmp/msgSends-%d", (int) getpid ());
          objcMsgLogFD = secure_open (buf, O_WRONLY | O_CREAT, geteuid());
          if (objcMsgLogFD < 0) {
              // no log file - disable logging
              objcMsgLogEnabled = false;
              objcMsgLogFD = -1;
              return true;
          }
      }
      //...
  }
  

发现objcMsgLogEnabled为true的情况，可以调用logMessageSend，并输出在tmp路径下，搜索objcMsgLogEnabled，找到instrumentObjcMessageSends函数

• instrumentObjcMessageSends

  void instrumentObjcMessageSends(BOOL flag)
  {
      bool enable = flag;
  
      // Shortcut NOP
      if (objcMsgLogEnabled == enable)
          return;
  
      // If enabling, flush all method caches so we get some traces
      if (enable)
          _objc_flush_caches(Nil);
  
      // Sync our log file
      if (objcMsgLogFD != -1)
          fsync (objcMsgLogFD);
  
      objcMsgLogEnabled = enable;
  }
  

要想打印方法日志，需要声明extern void instrumentObjcMessageSends(BOOL flag);,并在执行方法前开启，执行方法后关闭;

instrumentObjcMessageSends(true);
[person say1];
instrumentObjcMessageSends(false);

在/tmp/路径下找到msgSends前缀的文件，打开查看

msgSends.png

  因此，在objc_msgSend中，如果查找流程未找到方法实现，并且未实现动态方法决议，就会调用

• 快速转发

  forwardingTargetForSelector

  可以在类中重新该方法forwardingTargetForSelector，返回一个接收类，实现消息转发

  - (id)forwardingTargetForSelector:(SEL)aSelector{
  
      return [super forwardingTargetForSelector:aSelector];
  }
  

• 慢速转发

  methodSignatureForSelector ，需要搭配forwardInvocation:使用，实现之后就不会再执行``doesNotRecognizeSelector:`

  - (NSMethodSignature *)methodSignatureForSelector:(SEL)aSelector{
      return [NSMethodSignature signatureWithObjCTypes:"v@:"];
  }
  
  - (void)forwardInvocation:(NSInvocation *)anInvocation{
  
  }
  

  resolveInstanceMethod

• 最终仍未找到，调用

  doesNotRecognizeSelector:

消息转发机制流程

消息转发流程.png