Rac集合操作与绑定

第一大类：

merge，for/merge
concat，for/concat

zip，join/zipWith 打包元组+reduce解包
combineLatest，join/combineLatestWith 打包元祖+reduce解包

第二大类：
map --> 自动对block返回的值加上class return 即升高维度的操作。
flattenMap --> 不对block返回值的附加操作。仅判断如果是空转化为class empty其实就是降维了
flatten: --> 降到指定维度
flatten --> 降维

bind

    // mapReplace->  map (class return:block(value))->  flattenMap(block(value)) -> bind
         flatten (value) ->   



- (__kindof RACStream *)flattenMap:(__kindof RACStream * (^)(id value))block {
    Class class = self.class;

    return [[self bind:^{
        return ^(id value, BOOL *stop) {
            id stream = block(value) ?: [class empty];
            NSCAssert([stream isKindOfClass:RACStream.class], @"Value returned from -flattenMap: is not a stream: %@", stream);

            return stream;
        };
    }] setNameWithFormat:@"[%@] -flattenMap:", self.name];
}

- (__kindof RACStream *)flatten {
    return [[self flattenMap:^(id value) {
        return value;
    }] setNameWithFormat:@"[%@] -flatten", self.name];
}

- (__kindof RACStream *)map:(id (^)(id value))block {
    NSCParameterAssert(block != nil);

    Class class = self.class;
    
    return [[self flattenMap:^(id value) {
        return [class return:block(value)];
    }] setNameWithFormat:@"[%@] -map:", self.name];
}

- (__kindof RACStream *)mapReplace:(id)object {
    return [[self map:^(id _) {
        return object;
    }] setNameWithFormat:@"[%@] -mapReplace: %@", self.name, RACDescription(object)];
}