最近在学JS，发现使用JS的Promise写多个异步请求不需要嵌套，挺友好。所以就想研究下OC有没对应的方式。

OC的话可以通过ReactObj这个库来实现。ReactObj主要是为了实现MVVM而生的，通过信号控制一切。

1、先定义三个异步请求函数
接口2依赖接口的结果，接口3依赖接口2的结果。

//接口1
- (void)fetchData1:(void(^)(BOOL ret, NSString *userid))callBack{
    dispatch_async(dispatch_get_global_queue(0, 0), ^{
        [NSThread sleepForTimeInterval:2];
        dispatch_async(dispatch_get_main_queue(), ^{
            if(callBack) callBack(YES,@"65432");
        });
    });
}
//接口2依赖接口1返回的userid
- (void)fetchData2:(NSString *)userid callBack:(void(^)(BOOL ret,NSString *userid))callBack{
    
    dispatch_async(dispatch_get_global_queue(0, 0), ^{
        [NSThread sleepForTimeInterval:1];
        dispatch_async(dispatch_get_main_queue(), ^{
            if(callBack) callBack(YES,@"00000");
        });
    });
}

//接口3依赖接口2的结果
- (void)fetchData3:(NSString *)userid callBack:(void(^)(BOOL ret))callBack{
    
    dispatch_async(dispatch_get_global_queue(0, 0), ^{
        [NSThread sleepForTimeInterval:2];
        dispatch_async(dispatch_get_main_queue(), ^{
            if(callBack) callBack(NO);
        });
    });
}

2、把三个接口转换成signal

- (RACSignal *)creatFetchData1Signal{
    NSLog(@"start fetchData1");
    return [RACSignal createSignal:^RACDisposable * _Nullable(id<RACSubscriber>  _Nonnull subscriber) {
        [self fetchData1:^(BOOL ret, NSString *userid) {
            if(ret){
                [subscriber sendNext:userid];
            }else{
                [subscriber sendError:nil];
            }
            [subscriber sendCompleted];
        }];
        return nil;
    }];
}

- (RACSignal *)creatFetchData2Signal:(NSString *)userid{
    NSLog(@"start fetchData2 param>>>>%@",userid);
    return [RACSignal createSignal:^RACDisposable * _Nullable(id<RACSubscriber>  _Nonnull subscriber) {
        [self fetchData2:userid callBack:^(BOOL ret, NSString *userid) {
            if(ret){
                [subscriber sendNext:userid];
            }else{
                [subscriber sendError:nil];
            }
            [subscriber sendCompleted];
        }];
        return nil;
    }];
}

- (RACSignal *)creatFetchData3Signal:(NSString *)userid{
    NSLog(@"start fetchData3 param>>>>%@",userid);
    return [RACSignal createSignal:^RACDisposable * _Nullable(id<RACSubscriber>  _Nonnull subscriber) {
        [self fetchData3:userid callBack:^(BOOL ret) {
            if(ret){
                [subscriber sendNext:@(ret)];
            }else{
                [subscriber sendError:nil];
            }
            [subscriber sendCompleted];
        }];
        return nil;
    }];
}

3、使用signal

    [[[[self creatFetchData1Signal] flattenMap:^__kindof RACSignal * _Nullable(id  _Nullable value) {
        return [self creatFetchData2Signal:value];
    }]flattenMap:^__kindof RACSignal * _Nullable(id  _Nullable value) {
        return [self creatFetchData3Signal:value];
    }]subscribeNext:^(id  _Nullable x) {
        NSLog(@"ret==%@",x);
    } error:^(NSError * _Nullable error) {
        NSLog(@"error");
    } completed:^{
        NSLog(@"completed");
    }];