+(RACSignal *)requestPhotoData{    NSURLRequest *request = [self popularURLRequest];        return [[NSURLConnection rac_sendAsynchronousRequest:request] reduceEach:^id(NSURLResponse *response, NSData *data){        return data;    }];}复制代码

创建一个URLConnection请求之后返回的数据信号。

+(RACSignal *)importPhotos {    return [[[[[self requestPhotoData]            deliverOn:[RACScheduler mainThreadScheduler]]               map:^id(NSData *data) {                      id results = [NSJSONSerialization JSONObjectWithData:data options:0 error:nil];                return [[[results[@"photos"] rac_sequence] map:^id(NSDictionary *photoDictionary) {            FRPPhotoModel *model = [FRPPhotoModel new];                        [self configurePhotoModel:model withDictionary:photoDictionary];            [self downloadThumbnailForPhotoModel:model];                        return model;        }] array];    }] publish] autoconnect];}复制代码

deliverOn用来操作线程； rac_sequence用来序列化，通常用在数组上； configurePhotoModel：withDictionary用来把NSDictionary转成Model； array是把序列化重新转成数组 publish和autoconnect是为了解决多次请求，假如一个信号中发送请求，那么每次订阅都会发送请求，publish保存订阅到数组，当调用连接autoconnect，就会调用所有的订阅者的sendNext。