项目中实际需要是三种操作依次执行，顺序相互依赖。如果没有依赖的处理方式很多，但相互依赖时需要对信号进行 FlattenMap处理。

1. 依次创建信号

    RACSignal *signal1 = [RACSignal createSignal:^RACDisposable *(id<RACSubscriber> subscriber) {
        
        NSLog(@"打蛋液");
        [subscriber sendNext:@"蛋液"];
        [subscriber sendCompleted];
        return nil;
        
    }] ;
      
    RACSignal  *signal2 = [[signal1 flattenMap:^__kindof RACSignal * _Nullable(id  _Nullable value) {
        
        NSLog(@"把%@倒进锅里面煎",value);
        return [RACSignal createSignal:^RACDisposable *(id<RACSubscriber> subscriber) {
          
            //[subscriber sendNext:@"煎蛋"];
            NSError *error = [NSError errorWithDomain:@"访问权限" code:1000 userInfo:nil];
            [subscriber sendError:error];
            [subscriber sendCompleted];

            return nil;
        }];
        
    }] ;

    RACSignal  *signal3 = [signal2 flattenMap:^__kindof RACSignal * _Nullable(id  _Nullable value) {
        
        NSLog(@"把%@装载盘里",value);
        return [RACSignal createSignal:^RACDisposable *(id<RACSubscriber> subscriber) {
                
                [subscriber sendNext:@"上菜"];
                [subscriber sendCompleted];
            return nil;
        }];
        
    }]  ;
      

2. 观察最终结果

    [signal3 subscribeNext:^(id  _Nullable x) {
        
        NSLog(@"====%@",x);
        
    } error:^(NSError * _Nullable error) {
        
        # 任何一个步骤错误都会进入这个Block
        NSLog(@"%@",error);
    }];