RAC的Aggregate操作

数量操作——Aggregate：以及各种变种的操作

// 数量操作——Aggregate：
RACSignal *signalA = [RACSignal createSignal:^RACDisposable *(id<RACSubscriber> subscriber) {
  [subscriber sendNext:@1];
  [subscriber sendNext:@2];
  [subscriber sendNext:@4];
  [subscriber sendCompleted];
  return [RACDisposable disposableWithBlock:^{
    NSLog(@"完成");
  }];
}];

RACSignal *signalB = [signalA aggregateWithStart:@0 reduce:^id(NSNumber *running, NSNumber *next) {
  return @(running.integerValue + next.integerValue);
}];

[[signalB subscribeNext:^(id x) {
  NSLog(@"subscribeNext:%@",x);
}] dispose];
// 结果:7