objective-c - ReactiveCocoa 信号，用于在观察身份验证状态时获取数据

Question

ReactiveCocoa 相当新，我正在尝试构建一个信号，该信号从客户端必须首先验证的远程 API 异步获取一些资源。身份验证的处理方式是首先从 API 获取 token ，然后通过一些自定义 HTTP header 将其传递给每个后续请求。但是，自定义 header 可能会在订阅 fetchResource 信号后设置，这在当前情况下会导致未经身份验证的请求。我想我实际上可以在 self.authenticationStatus 的 subscribeNext block 中构建请求，从而确保设置 token ，但我该如何处理信号的处理呢？

- (RACSignal *)fetchResource
{
    return [RACSignal createSignal:^RACDisposable *(id<RACSubscriber> subscriber) {
        NSURLRequest *request = [self.requestSerializer
                                 requestWithMethod:@"GET"
                                 URLString:[[NSURL URLWithString:@"resource" relativeToURL:self.baseURL] absoluteString]
                                 parameters:nil error:nil];
        NSURLSessionDataTask *task = [self dataTaskWithRequest:request
                                      completionHandler:^(NSURLResponse *response, id responseObject, NSError *error) {
            if (error) {
                [subscriber sendError:error];
            } else {
                [subscriber sendNext:responseObject];
                [subscriber sendCompleted];
            }
        }];

        // Actually trigger the request only once the authentication token has been fetched.
        [[self.authenticationStatus ignore:@NO] subscribeNext:^(id _) {
            [task resume];
        }];

        return [RACDisposable disposableWithBlock:^{
            [task cancel];
        }];
    }];
}

Answer 1

- (RACSignal *)fetchTokenWithCredentials:(Credentials *)credentials
{
    return [RACSignal createSignal:^RACDisposable *(id<RACSubscriber> subscriber) {

        // Fetch the token and send it to `subscriber`.
        Token *t = ... ;
        [subscriber sendNext:t];

        return nil;

    }];
}

- (RACSignal *)fetchResourceWithToken:(Token *)token
{
    return [RACSignal createSignal:^RACDisposable *(id<RACSubscriber> subscriber) {

        // Use `token` to set the request header. Then fetch
        // the resource and send it to `subscriber`. Basically
        // this part is what you already have.
        Resource *r = ... ;
        [subscriber sendNext:r];

        return nil;

    }];
}

如果您没有有效的 token ，请在您的 View Controller 中显示模态身份验证对话框。当用户点击“提交”按钮时，执行如下操作:

- (IBAction)handleAuthenticationSubmit:(id)sender
{
    Credentials *c = ... ;
    RACSignal *resourceSignal = [[[self fetchTokenWithCredentials:c]
            flattenMap:^(Token *t) {

                return [self fetchResourceWithToken:t];

            }]
            deliverOn:RACScheduler.mainThreadScheduler];

    [self rac_liftSelector:@selector(receiveResource:) withSignals:resourceSignal, nil];
}

- (void)receiveResource:(Resource *)resource
{
    [self.delegate authenticationController:self didReceiveResource:resource];
}