Since ReactiveCocoa runs on the thread where it's been emitted, which means if you catch an asynchronous reulst from server and emit a signal, which will be run in the thread other than main thread, so to update UI will have a big problem.

Use the method "deliverOn" in RACScheduler class can put this signal run in the main thread.

[[[[[[self requestAccessToTwitterSignal]
  then:^RACSignal *{
    @strongify(self)
    return self.searchText.rac_textSignal;
  }]
  filter:^BOOL(NSString *text) {
    @strongify(self)
    return [self isValidSearchText:text];
  }]
  flattenMap:^RACStream *(NSString *text) {
    @strongify(self)
    return [self signalForSearchWithText:text];
  }]
  deliverOn:[RACScheduler mainThreadScheduler]]
  subscribeNext:^(id x) {
    NSLog(@"%@", x);
  } error:^(NSError *error) {
    NSLog(@"An error occurred: %@", error);
  }];

利用RACScheduler来异步加载图片

创建一个信号, 并将该下载任务放入background thread进行

- (RACSignal *)signalForLoadingImage:(NSString *)imageUrl {

  RACScheduler *scheduler = [RACScheduler
                         schedulerWithPriority:RACSchedulerPriorityBackground];

  return [[RACSignal createSignal:^RACDisposable *(id<RACSubscriber> subscriber) {
    NSData *data = [NSData dataWithContentsOfURL:[NSURL URLWithString:imageUrl]];
    UIImage *image = [UIImage imageWithData:data];
    [subscriber sendNext:image];
    [subscriber sendCompleted];
    return nil;
  }] subscribeOn:scheduler];

}

订阅该信号

[[[self signalForLoadingImage:tweet.profileImageUrl] // 创建信号
  deliverOn:[RACScheduler mainThreadScheduler]] // 信号丢入主线程
  subscribeNext:^(UIImage *image) {
   cell.twitterAvatarView.image = image; // 也会在主线程触发
  }];