RAC的异步订阅

异步订阅：

// 异步订阅
RACSignal *signal = [RACSignal createSignal:^RACDisposable *(id<RACSubscriber> subscriber) {
  NSLog(@"111");
  [subscriber sendNext:@1];
  [subscriber sendCompleted];
  return nil;
}];
[[RACScheduler scheduler] schedule:^{
  NSLog(@"2222");
  [signal subscribeNext:^(id x) {
      NSLog(@"3333");
  }];
}];
NSLog(@"4444");
// 执行结果：

2021-04-17 09:10:32.088812+0800 NetSchool[4279:1292417] 4444

2021-04-17 09:10:32.088937+0800 NetSchool[4279:1292525] 2222

2021-04-17 09:10:32.091231+0800 NetSchool[4279:1292525] 111

2021-04-17 09:10:32.091260+0800 NetSchool[4279:1292525] 3333