Objc_msgSend流程(二)之方法慢速查找

上篇文章Objc_msgSend流程 说到objc_msg_Send快速查找流程，最后调用_lookUpImpOrForward，但是_lookUpImpOrForward在哪里呢？

查找_lookUpImpOrForward

在objc源码工程调用一个方法并在该处新增一个断点，运行工程

断点调试.png

开启菜单栏Debug->Debug Workflow->Always Show Disassembly

Always Show Disassembly.png

在objc_msgSend处新增一个断点，让程序运行至断点处

objc_msgSend断点调试.png

按住ctrl，点击step Into，进入objc_msgSend方法调试

step Into.png

在objc_msgSend调试中找到_objc_msgSend_uncached,让程序运行至断点处，同样的按住ctrl，点击step Into;

_objc_msgSend_uncached.png

在_objc_msgSend_uncached找到发现lookUpImpOrForward位于objc-runtime-new.mm文件

lookUpImpOrForward.png

探索 lookupImpOrForward

找到lookupImpOrForward源码，lookupImpOrForward开始objc_msgSend的慢速查找

在if (fastpath(behavior & LOOKUP_CACHE))，增加断点，使代码运行到断点处，并输出参数的值

调试lookupImpOrForward.png

慢速查找流程

查看lookupImpOrForward

1. 首先查看当前cls是否有SEL的缓存

    if (fastpath(behavior & LOOKUP_CACHE)) {
           imp = cache_getImp(cls, sel);
           if (imp) goto done_nolock;
       }
   

2. 继续查看是否当前类已知

   checkIsKnownClass(cls);
   //
   static void
   checkIsKnownClass(Class cls)
   {
       if (slowpath(!isKnownClass(cls))) {
           _objc_fatal("Attempt to use unknown class %p.", cls);
       }
   }
   

3. 查看当前类是否已经实现，若未实现，则对class进行创建，并递归寻找父类及元类的继承链

   realizeClassMaybeSwiftAndLeaveLocked 调用realizeClassMaybeSwiftMaybeRelock调用

   realizeClassWithoutSwift

   if (slowpath(!cls->isRealized())) {
           cls = realizeClassMaybeSwiftAndLeaveLocked(cls, runtimeLock);
           // runtimeLock may have been dropped but is now locked again
   }
   

4. 查看当前类是否已经初始化，若未初始化，则对class进行初始化，并递归寻找父类及元类的继承链；

   if (slowpath((behavior & LOOKUP_INITIALIZE) && !cls->isInitialized())) {
           cls = initializeAndLeaveLocked(cls, inst, runtimeLock);
           // runtimeLock may have been dropped but is now locked again
   
           // If sel == initialize, class_initialize will send +initialize and 
           // then the messenger will send +initialize again after this 
           // procedure finishes. Of course, if this is not being called 
           // from the messenger then it won't happen. 2778172
   }
   

5. 进行查找

   for (unsigned attempts = unreasonableClassCount();;) {
          // curClass method list.
          Method meth = getMethodNoSuper_nolock(curClass, sel);
          if (meth) {
              imp = meth->imp;
              goto done;
          }
   
          if (slowpath((curClass = curClass->superclass) == nil)) {
              // No implementation found, and method resolver didn't help.
              // Use forwarding.
              imp = forward_imp;
              break;
          }
   
          // Halt if there is a cycle in the superclass chain.
          if (slowpath(--attempts == 0)) {
              _objc_fatal("Memory corruption in class list.");
          }
   
          // Superclass cache.
          imp = cache_getImp(curClass, sel);
          if (slowpath(imp == forward_imp)) {
              // Found a forward:: entry in a superclass.
              // Stop searching, but don't cache yet; call method
              // resolver for this class first.
              break;
          }
          if (fastpath(imp)) {
              // Found the method in a superclass. Cache it in this class.
              goto done;
          }
      }
   

   • Method meth = getMethodNoSuper_nolock(curClass, sel);

     调用search_method_list_inline

     调用findMethodInSortedMethodList(SEL key, const method_list_t *list)进行折半查找。

     ALWAYS_INLINE static method_t *
     findMethodInSortedMethodList(SEL key, const method_list_t *list)
     {
        ASSERT(list);
     
        const method_t * const first = &list->first;
        const method_t *base = first;
        const method_t *probe;
        uintptr_t keyValue = (uintptr_t)key;
        uint32_t count;
     
        for (count = list->count; count != 0; count >>= 1) {
            probe = base + (count >> 1);
     
            uintptr_t probeValue = (uintptr_t)probe->name;
     
            if (keyValue == probeValue) {
                // `probe` is a match.
                // Rewind looking for the *first* occurrence of this value.
                // This is required for correct category overrides.
                while (probe > first && keyValue == (uintptr_t)probe[-1].name) {
                    probe--;
                }
                return (method_t *)probe;
            }
     
            if (keyValue > probeValue) {
                base = probe + 1;
                count--;
            }
        }
     
        return nil;
     }
     

     • 如果找到方法直接调用log_and_fill_cache，缓存该方法cache_fill(cls, sel, imp, receiver);方便下次直接在缓存中快速查找;
     • 如果未找到，则将superClass赋值给当前的class

   • 从父类中的缓存中查找，

     • 若找到该方法调用log_and_fill_cache，缓存该方法cache_fill(cls, sel, imp, receiver);方便下次直接在缓存中快速查找
     • 如果未找到，则会再次调用_lookupImpOrForward进行递归

   • 若根类未找到该方法，将forward_imp赋值给imp

     if (slowpath((curClass = curClass->superclass) == nil)) {
                // No implementation found, and method resolver didn't help.
                // Use forwarding.
                imp = forward_imp;
                break;
     }
     

   • 若imp等于forward_imp，则调用break退出循环

     if (slowpath(imp == forward_imp)) {
                // Found a forward:: entry in a superclass.
                // Stop searching, but don't cache yet; call method
                // resolver for this class first.
                break;
     }
     

6. forward_imp是_objc_msgForward_impcache返回的，那么forward_imp是什么呢？全局搜索找到__objc_msgForward_impcache，找到进入消息转发流程__objc_msgForward，消息转发__objc_forward_handler

   STATIC_ENTRY __objc_msgForward_impcache
   // No stret specialization.
   b    __objc_msgForward
   END_ENTRY __objc_msgForward_impcache
   
   ENTRY __objc_msgForward
   adrp x17, __objc_forward_handler@PAGE
   ldr  p17, [x17, __objc_forward_handler@PAGEOFF]
   TailCallFunctionPointer x17
   END_ENTRY __objc_msgForward
   

7. 那么__objc_forward_handler是什么呢？我们找到__objc_forward_handler是objc_defaultForwardHandler，而objc_defaultForwardHandler就是我们平常看到的未找到方法的提示语

   __attribute__((noreturn, cold)) void
   objc_defaultForwardHandler(id self, SEL sel)
   {
       _objc_fatal("%c[%s %s]: unrecognized selector sent to instance %p "
                   "(no message forward handler is installed)", 
                   class_isMetaClass(object_getClass(self)) ? '+' : '-', 
                   object_getClassName(self), sel_getName(sel), self);
   }
   void *_objc_forward_handler = (void*)objc_defaultForwardHandler;