RACChannelTo

RACChannelTo(self, name) = RACChannelTo(self.model, name);
RACChannelTo(self, blurb) = RACChannelTo(self.model, blurb);
RACChannelTo(self, filmType, @(ASHRecipeFilmTypeColourNegative)) = RACChannelTo(self.model, filmType, @(ASHRecipeFilmTypeColourNegative));

这是一个双向绑定，self.name和self.model.name任何一个改变，另外一个也会改变。第三个参数是默认值，出现nil就用这个来代替。有了它，model和ViewModel就可以双向同步。

RACChannelTerminal *channelA = RACChannelTo(self, valueA);
    RACChannelTerminal *channelB = RACChannelTo(self, valueB);
    [[channelA map:^id(NSString *value) {
        if ([value isEqualToString:@"西"]) {
            return @"东";
        }
        return value;
    }] subscribe:channelB];
    [[channelB map:^id(NSString *value) {
        if ([value isEqualToString:@"左"]) {
            return @"右";
        }
        return value;
    }] subscribe:channelA];
    [[RACObserve(self, valueA) filter:^BOOL(id value) {
        return value ? YES : NO;
    }] subscribeNext:^(NSString* x) {
        NSLog(@"你向%@", x);
    }];
    [[RACObserve(self, valueB) filter:^BOOL(id value) {
        return value ? YES : NO;
    }] subscribeNext:^(NSString* x) {
        NSLog(@"他向%@", x);
    }];
    self.valueA = @"西";
    self.valueB = @"左";