ReactiveCocoa操作方法-秩序

doNext:

     执行Next之前，会先执行这个Block

doCompleted:

     执行sendCompleted之前，会先执行这个Block

- (void)doNext
{
    [[[[RACSignal createSignal:^RACDisposable *(id<RACSubscriber> subscriber) {
        [subscriber sendNext:@1];
        [subscriber sendCompleted];
        return nil;
    }] doNext:^(id x) {
        //执行[subscriber sendNext:@1];前,会调用这个block
        NSLog(@"doNext");
    }] doCompleted:^{
        //执行[subscriber sendCompleted];前,会调用这个block
        NSLog(@"doCompleted");
    }] subscribeNext:^(id x) {
        NSLog(@"%@", x);
    }];
}