"nil" should only be used in place of an "id", what we Java and C++ programmers would think of as a pointer to an object. Use NULL for non-object pointers.
Look at the declaration of that method:
-(void)observeValueForKeyPath:(NSString *)keyPath ofObject:(id)object
change:(NSDictionary *)change context:(void *)context
context is a "void *" (ie a C-style pointer), so you'd definitely use NULL (which is sometimes declared as "(void *)0") rather than nil (which is of type "id").
而且与null不同,可以发送消息给nil。返回值还是一个nil。