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_objc_msgforward_demo's Introduction

_objc_msgForward_demo用来测试消息转发机制

对象查找selector时，先查找cachelist，如果没有则查找methodlist，如果还没有就查找父类的methodlist

都没有是还有三次机会可以处理这次selector访问

1. + (BOOL)resolveInstanceMethod:(SEL)sel; 在此方法中添加相应selector以及IMP即可

2. - (id)forwardingTargetForSelector:(SEL)aSelector 在此方法中可以指定处理的对象，在该对象中实现selector即可

3. - (NSMethodSignature *)methodSignatureForSelector:(SEL)aSelector; - (void)forwardInvocation:(NSInvocation *)anInvocation; 在第一个方法中reture signal，这样runtime就会产生一个NSInvocation给第二个方法，anInvocation中有target和selector，可以修改，然后让target进行调用，不调用也可以，只是什么都不发生

如果以上三次机会都不把握，则会调用- (void)doesNotRecognizeSelector:(SEL)aSelector; 默认的实现是抛出异常

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