KVO valueForKey 源码解析

首先我是通过阅读GNUBase中的源码，来了解KVC的一些函数的调用过程，会跟苹果自身的Foundation有一些出入。

在KeyValueCoding文件中我们找到了如下的方法:

- (id) valueForKey: (NSString*)aKey
{
  unsigned	size = [aKey length] * 8; // 这里猜测应该是某些文字的解码可能会占用8个字节，这里给了一个足够大的空间
  char		key[size + 1];

  [aKey getCString: key
	 maxLength: size + 1
	  encoding: NSUTF8StringEncoding];
  size = strlen(key); // 这里重新计算size值。如如果key是一个中文字符串的话，那么 size = 3 * aKey.length
  return ValueForKey(self, key, size);
}

很巧的是，我们也能看到getCString方法的大致实现，他其实就是一个将NSString的数据给塞到char数组中的操作，并返回是否成功的结果。

- (BOOL) getCString: (char*)buffer
	  maxLength: (NSUInteger)maxLength
	   encoding: (NSStringEncoding)encoding
{
  if (0 == maxLength || 0 == buffer) return NO;
  if (encoding == NSUnicodeStringEncoding)
    {
      unsigned	length = [self length];

      if (maxLength > length * sizeof(unichar))
	{
	  unichar	*ptr = (unichar*)(void*)buffer;

	  maxLength = (maxLength - 1) / sizeof(unichar);
	  [self getCharacters: ptr
			range: NSMakeRange(0, maxLength)];
	  ptr[maxLength] = 0;
	  return YES;
	}
      return NO;
    }
  else
    {
      NSData	*d = [self dataUsingEncoding: encoding];
      unsigned	length = [d length];
      BOOL	result = (length < maxLength) ? YES : NO;

      if (d == nil)
        {
	  [NSException raise: NSCharacterConversionException
		      format: @"Can't convert to C string."];
	}
      if (length >= maxLength)
        {
          length = maxLength-1;
	}
      memcpy(buffer, [d bytes], length);
      buffer[length] = '\0';
      return result;
    }
}

接下来我们看ValueForKey方法的实现：

static id ValueForKey(NSObject *self, const char *key, unsigned size)
{
  SEL		sel = 0;
  int		off = 0;
  const char	*type = NULL;

  if (size > 0)
    {
      const char	*name;
      char		buf[size + 5];
      char		lo;
      char		hi;

      memcpy(buf, "_get", 4);
      memcpy(&buf[4], key, size);
      buf[size + 4] = '\0'; buff = _get + key + '\0'
      lo = buf[4];
      hi = islower(lo) ? toupper(lo) : lo;
      buf[4] = hi; //将key的首字母大写

      name = &buf[1];	// getKey
      sel = sel_getUid(name);  // 找到缓存的selector，具体实现可以往下看
      if (sel == 0 || [self respondsToSelector: sel] == NO) // 如果没有的话
	{
	  buf[4] = lo; // getKey -> getkey, 去除之前做的toupper(lo) 再做一遍查找的操作
	  name = &buf[4];	// key
	  sel = sel_getUid(name);
	  if (sel == 0 || [self respondsToSelector: sel] == NO)
	    {
              buf[4] = hi;
              buf[3] = 's';
              buf[2] = 'i';
              name = &buf[2];	// isKey
              sel = sel_getUid(name);
              if (sel == 0 || [self respondsToSelector: sel] == NO)
                {
                  sel = 0;
                }
	    }
	}

      if (sel == 0 && [[self class] accessInstanceVariablesDirectly] == YES)
	{
	  buf[4] = hi;
	  name = buf;	// _getKey
	  sel = sel_getUid(name);
	  if (sel == 0 || [self respondsToSelector: sel] == NO)
	    {
	      buf[4] = lo;
	      buf[3] = '_';
	      name = &buf[3];	// _key
	      sel = sel_getUid(name);
	      if (sel == 0 || [self respondsToSelector: sel] == NO)
		{
		  sel = 0;
		}
	    }
	  if (sel == 0)
	    {
	      if (GSObjCFindVariable(self, name, &type, &size, &off) == NO)
		{
                  buf[4] = hi;
                  buf[3] = 's';
                  buf[2] = 'i';
                  buf[1] = '_';
                  name = &buf[1];	// _isKey
		  if (!GSObjCFindVariable(self, name, &type, &size, &off))
                    {
                       buf[4] = lo;
                       name = &buf[4];		// key
		       if (!GSObjCFindVariable(self, name, &type, &size, &off))
                         {
                            buf[4] = hi;
                            buf[3] = 's';
                            buf[2] = 'i';
                            name = &buf[2];	// isKey
                            GSObjCFindVariable(self, name, &type, &size, &off);
                         }
                    }
		}
	    }
	}
    }
  return GSObjCGetVal(self, key, sel, type, size, off);
}

其实就是把key转换成了getKey,isKey,_isKey,_getKey,key,_key。

通过GSObjCFindVariable方法获取到对应的type（编码类型），size(大小)与offset（偏移量）

最后调用GSObjCGetVal拿到事例方法的值。

我们来看下sel_getUid方法的实现：

SEL sel_getUid(const char *name) {
    return __sel_registerName(name, 2, 1);  // YES lock, YES copy
}

static SEL __sel_registerName(const char *name, bool shouldLock, bool copy) 
{
    SEL result = 0;

    if (shouldLock) selLock.assertUnlocked();
    else selLock.assertLocked();

    if (!name) return (SEL)0;

    result = search_builtins(name); // 这里我理解是是否是内建的方法： _dyld_get_objc_selector
    if (result) return result;
    
    conditional_mutex_locker_t lock(selLock, shouldLock);
	auto it = namedSelectors.get().insert(name); // 然后在这个共享的数据结构中去找是否有这个SEL
	if (it.second) {
		// No match. Insert.
		*it.first = (const char *)sel_alloc(name, copy);
	}
	return (SEL)*it.first;
}

static SEL search_builtins(const char *name) 
{
#if SUPPORT_PREOPT
  if (SEL result = (SEL)_dyld_get_objc_selector(name))
    return result;
#endif
    return nil;
}