RACChannelTo() = RACChannelTo()的

RACChannel 可以被理解为一个双向的连接，这个连接的两端都是 RACSignal 实例，它们可以向彼此发送消息，如果我们在视图和模型之间通过 RACChannel 建立这样的连接：

RACChannelTo(self, text) = RACChannelTo(self.ViewModel, text);

那么从模型发出的消息，最后会发送到视图上；反之，用户对视图进行的操作最后也会体现在模型上。这种通信方式的实现是基于信号的， RACChannel 内部封装了两个 RACChannelTerminal 对象，它们都是 RACSignal 的子类.

    RACReplaySubject *leadingSubject = [[RACReplaySubject replaySubjectWithCapacity:0] setNameWithFormat:@"leadingSubject"];
    RACReplaySubject *followingSubject = [[RACReplaySubject replaySubjectWithCapacity:1] setNameWithFormat:@"followingSubject"];

    // Propagate errors and completion to everything.
    [[leadingSubject ignoreValues] subscribe:followingSubject];
    [[followingSubject ignoreValues] subscribe:leadingSubject];

    _leadingTerminal = [[[RACChannelTerminal alloc] initWithValues:leadingSubject otherTerminal:followingSubject] setNameWithFormat:@"leadingTerminal"];
    _followingTerminal = [[[RACChannelTerminal alloc] initWithValues:followingSubject otherTerminal:leadingSubject] setNameWithFormat:@"followingTerminal"];

事实上，RACChannel中一共包含了四个信号，而其中的_leadingTerminal、_followingTerminal相当于socket，代表了两个端点，主要进行转发将消息及订阅者转发到内部的leadingSubject、followingSubject，为了方便区分将_leadingTerminal、_followingTerminal分别称为L终端，F终端。

1. 实质是就是Channel订阅leadingSubject用来接收消息，而发送消息则通过followingSubject
2. 由于RACChannel通过informal-protocol实现了自定义的下标访问方法，所以RACChannelTo() = RACChannelTo()表达式实质上调用了

- (void)setObject:(RACChannelTerminal *)otherTerminal forKeyedSubscript:(NSString *)key {
    NSCParameterAssert(otherTerminal != nil);

    RACChannelTerminal *selfTerminal = [self objectForKeyedSubscript:key];
    [otherTerminal subscribe:selfTerminal];
    [[selfTerminal skip:1] subscribe:otherTerminal];
}

其实就是实现了分别订阅对方的followingSubject

3. 所以发送消息时通过followingSubject发送给了对方的F终端，而F终端又会将消息转发给leadingSubject

4. RACChannel实现两个信号，分别用来发送和接收消息，然后通过两个自定义的终端信号将两个信号进行转发，实现双向数据绑定。

   
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