Chinese readers annotation:这个问题google,stackoverflow,github都还没有相关资料,所以把内容写成英文的,以便于帮助国外的朋友。
English readers annotation:This problem has not been found by google or stackoverflow or github yet. So I writed it at here so that you could get help from this.(By the way, I am a Chinese)

I want to use rac_signalForSelector to observe the UITableView's delegate so I could let the dataDelegate and delegate being reused from a independent class. So If they have the same cell, I would not need to implement them triples.

If you use the RAC(which means ReactiveCocoa), Firstly come into you head is a code sample like this:
self.rac_signalForSelector(Selector("scrollViewDidEndDecelerating:")).subscribeNext{ _ in
            self.setTabBarVisiable(false, animated:true)
scrollView.delegate = self

well...It's fine if you don't need this method:
self.rac_signalForSelector("scrollViewDidEndDragging:willDecelerate:").subscribeNext{ _ in
            self.setTabBarVisiable(false, animated:true)
But if you do, you will get a crash message(EXE_BAD_ACCESS) which caused by the RAC.It means RAC can't find a method form string '@' (which should be @"scrollViewDidEndDragging:willDecelerate:
"),  So I have work around with it,  and I guessed it's the reason of swift Protocol isn't a NSObject(you could see from UIScrollViewDelegate's declaration). 

By a chance I found there is a method named 'NSProtocolFromString' and RAC provided a method 
- (RACSignal *)rac_signalForSelector:(SEL)selector fromProtocol:(Protocol *)protocol;

So I combined them both
            fromProtocol:NSProtocolFromString("UITableViewDelegate")).subscribeNext{ _ in
            self.setTabBarVisiable(false, animated:true)

It doesn't crash any more and the function of my design worked fine. So, that's the way! done!
Hope this helps you out, it's really struggle.