RacChannel ---RacChannelTerminal+++UIControl
RacKVOChannel ------RACChannelTo
RacObserve -------RacDelegateProxy

1, RACKVOChannel
双向绑定+继承于RACChannel

代理人模式；实现KVO；
底层还是依赖于RACKVO

    [RACKVOProxy.sharedProxy addObserver:self forContext:(__bridge void *)self];
    [strongTarget addObserver:RACKVOProxy.sharedProxy forKeyPath:self.keyPath options:options context:(__bridge void *)self];

RACChannelTo

 [[RACKVOChannel alloc] initWithTarget:(TARGET) keyPath:@keypath(TARGET, KEYPATH) nilValue:(NILVALUE)][@keypath(RACKVOChannel.new, followingTerminal)]

管道------ A[0] = 111; ///实现中括号下标操作符

- (RACChannelTerminal *)objectForKeyedSubscript:(NSString *)key {
    NSCParameterAssert(key != nil);

    RACChannelTerminal *terminal = [self valueForKey:key];
    NSCAssert([terminal isKindOfClass:RACChannelTerminal.class], @"Key \"%@\" does not identify a channel terminal", key);

    return terminal;
}

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

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

2,
使用KVC实现+ RACChannel

- (RACChannelTerminal *)rac_channelForControlEvents:(UIControlEvents)controlEvents key:(NSString *)key nilValue:(nullable id)nilValue;



    [[self
        rac_liftSelector:@selector(valueForKey:) withSignals:eventSignal, nil]
        subscribe:channel.followingTerminal];


    [self rac_liftSelector:@selector(setValue:forKey:) withSignals:valuesSignal, [RACSignal return:key], nil];