+(RACSignal *)requestPhotoData
{
NSURLRequest *request = [self popularURLRequest];

return [[NSURLConnection rac_sendAsynchronousRequest:request] reduceEach:^id(NSURLResponse *response, NSData *data){
    return data;
}];

}
創(chuàng)建一個URLConnection請求之后返回的數(shù)據(jù)信號。

+(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];

}
deliverOn用來操作線程；
rac_sequence用來序列化，通常用在數(shù)組上；
configurePhotoModel：withDictionary用來把NSDictionary轉(zhuǎn)成Model；
array是把序列化重新轉(zhuǎn)成數(shù)組
publish和autoconnect是為了解決多次請求，假如一個信號中發(fā)送請求，那么每次訂閱都會發(fā)送請求，publish保存訂閱到數(shù)組，當(dāng)調(diào)用連接autoconnect，就會調(diào)用所有的訂閱者的sendNext。