【Preprocess】
在使用forwarding机制前，会先经历2个步骤，只有当这2个步骤均失败的情况下，才会激活forwarding。
1、+(BOOL)resolveInstanceMethod:(SEL)selector、resolveClassMethod。
当第一次没找到SEL时，调用上述两方法之一。如果在上述2方法中加入了方法，并返回YES，则会尝试重新解析此SEL。
2、-(id)forwardingTargetForSelector:(SEL)selector，当上述第1条机制失败后，本条机制会被激活。此方法返回能够处理SEL的对象。如果返回非nil，则会调用此对象的SEL。此种机制无法定制参数。
当上述2个机制均失败后，会起用forwarding机制，如下。
【Message Forwarding】
If you send a message to an object that does not handle that message, before announcing an error the runtime sends the object a forwardInvocation: message with an NSInvocation object as its sole argument—the NSInvocation object encapsulates the original message and the arguments that were passed with it.
forwardInvocation方法可以帮助我们轻松的实现完美转发。
- (void)forwardInvocation:(NSInvocation *)anInvocation
{
if ([someOtherObject respondsToSelector:
[anInvocation selector]])
[anInvocation invokeWithTarget:someOtherObject];
else
[super forwardInvocation:anInvocation];
}
forwarding实际上提供了一种多生继承的变相实现。返回值被存储在NSInvocation中，NSObject框架会把NSInvocation中的返回值返回给原调用者。
Although forwarding mimics inheritance, the NSObject class never confuses the two. Methods like respondsToSelector: and isKindOfClass: look only at the inheritance hierarchy, never at the forwarding chain. If, for example, a Warrior object is asked whether it responds to a negotiate message,
if ( [aWarrior respondsToSelector:@selector(negotiate)] )
...
the answer is NO, even though it can receive negotiate messages without error and respond to them.
如果想让respondsToSelector对代理的方法也返回YES，可以按如下实现：
- (BOOL)respondsToSelector:(SEL)aSelector
{
if ( [super respondsToSelector:aSelector] )
return YES;
else {
/* Here, test whether the aSelector message can *
* be forwarded to another object and whether that *
* object can respond to it. Return YES if it can. */
}
return NO;
}
if an object forwards any remote messages it receives, it should have a version of methodSignatureForSelector: that can return accurate descriptions of the methods that ultimately respond to the forwarded messages
- (NSMethodSignature*)methodSignatureForSelector:(SEL)selector
{
NSMethodSignature* signature = [super methodSignatureForSelector:selector];
if (!signature) {
signature = [surrogate methodSignatureForSelector:selector];
}
return signature;
}