是否可以根据条件重试一次？

DennisWelu 发表于 Dev

丹尼斯·韦鲁

我有一个有效的方法，看起来基本上是这样的：

public IObservable<List<Stuff>> GetGoodStuff()
{
    return Observable.FromAsync(GetAccessTokenAsync)
        .SelectMany(accessToken =>
        {
            return httpClient.SendAsync(request);
        })
        .SelectMany(response => 
        { 
            response.EnsureSuccessStatusCode(); 
            return response.Content.ReadAsStringAsync(); 
        })
        .Select(json => 
        {
            return JsonConvert.DeserializeObject<List<Stuff>>(json);
        });
}

“ GetAccessTokenAsync”返回api的缓存访问令牌，否则，第一次将获取令牌。就httpclient和Rx而言，其余的都是非常标准的。

事情是这样的：我想捕获401错误，更新访问令牌，然后重试整个过程。但是只有一次-之后，它将异常抛出给调用者。

在该中间块中，我可以这样做：

            if (response.StatusCode == HttpStatusCode.Unauthorized)
            {
                InvalidateAccessToken();
                // what now???
            }

但是那又怎样呢？没有看到递归调用将如何工作。不知何故包裹了整个东西？还没看到...

编辑1-7Nov2015

到目前为止，两个答案都不错。更具声明性的方法似乎没有多大变化，并且能够隐藏很多“管道”，但是我无法完全使它在所有情况下都能正常工作。

因此，根据@Timothy Shields的建议，我想到了此代码，它看起来不错，并且很好地隐藏了管道（是的，它可以工作:-)

/// <summary>
/// Makes an httpclient request using the access token. If Unauthorized is received the access
/// token will be reacquired and the request will be retried once.
/// </summary>
/// <returns>The json result from a successful request.</returns>
async Task<string> MakeRequestWithAccessToken(string requestUri, CancellationToken cancellationToken)
{
    const int RetryCount = 1;

    HttpResponseMessage response = null;
    for (int i = 0; i <= RetryCount; i++)
    {
        var accessToken = await GetAccessTokenAsync();

        var request = new HttpRequestMessage(HttpMethod.Get, requestUri);
        request.Headers.Add("Authorization", "Bearer " + accessToken);

        var client = new RemoteService(ApiUrl).NewClient();

        response = await client.SendAsync(request, cancellationToken);
        if (i < RetryCount && response.StatusCode == HttpStatusCode.Unauthorized)
        {
            InvalidateAccessToken();
            continue;
        }

        response.EnsureSuccessStatusCode();
    }

    return await response.Content.ReadAsStringAsync();
}

public IObservable<List<Stuff>> GetGoodStuff(int maxCount)
{
    return Observable.FromAsync(async cancellationToken =>
    {
        var requestUri = string.Format("mypath.json?count={0}", maxCount);
        var json = await MakeRequestWithAccessToken(requestUri, cancellationToken);
        return JsonConvert.DeserializeObject<List<Stuff>>(json);
    });
}

蒂莫西·希尔兹

您应该使用async-await来做到这一点：

public IObservable<List<Stuff>> GetGoodStuff()
{
    return Observable.FromAsync(async cancellationToken =>
    {
        const int RetryCount = 1;
        for (int i = 0; i <= RetryCount; i++)
        {
            var accessToken = await GetAccessTokenAsync();
            var request = MakeRequest(accessToken);
            var response = await httpClient.SendAsync(request, cancellationToken);
            if (i < RetryCount && response.StatusCode == HttpStatusCode.Unauthorized)
            {
                InvalidateAccessToken();
                continue;
            }
            response.EnsureSuccessStatusCode();
            var json = await response.Content.ReadAsStringAsync(cancellationToken);
            return JsonConvert.DeserializeObject<List<Stuff>>(json);
        }
    });
}