将async/await转换为Task.ContinueWith

Question

对此问题予以评论引发的这一个:

如何在async/await没有的情况下将非线性代码反向移植到.NET 4.0 Microsoft.Bcl.Async？

在链接的问题中,我们有一个WebRequest操作,如果它一直失败,我们想要重试有限次数.该Async/await代码看起来是这样的:

async Task<HttpWebResponse> GetResponseWithRetryAsync(string url, int retries)
{
    if (retries < 0)
        throw new ArgumentOutOfRangeException();

    var request = WebRequest.Create(url);
    while (true)
    {
        WebResponse task = null;
        try
        {
            task = request.GetResponseAsync();
            return (HttpWebResponse)await task;
        }
        catch (Exception ex)
        {
            if (task.IsCanceled)
                throw;

            if (--retries == 0)
                throw; // rethrow last error

            // otherwise, log the error and retry
            Debug.Print("Retrying after error: " + ex.Message);
        }
    }
}

从第一个想法,我会使用TaskCompletionSource,像这样(未经测试):

Task<HttpWebResponse> GetResponseWithRetryAsync(string url, int retries)
{
    if (retries < 0)
        throw new ArgumentOutOfRangeException();

    var request = WebRequest.Create(url);

    var tcs = new TaskCompletionSource<HttpWebResponse>();

    Action<Task<WebResponse>> proceesToNextStep = null;

    Action doStep = () =>
        request.GetResponseAsync().ContinueWith(proceedToNextStep);

    proceedToNextStep = (prevTask) =>
    {
        if (prevTask.IsCanceled)
            tcs.SetCanceled();
        else if (!prevTask.IsFaulted)
            tcs.SetResult((HttpWebResponse)prevTask.Result);
        else if (--retries == 0)
            tcs.SetException(prevTask.Exception);
        else
            doStep();
    };

    doStep();

    return tcs.Task;
}

问题是,如何做到这一点没有TaskCompletionSource？

Answer 1

我已经想出了如何在没有async/await或TaskCompletionSource使用嵌套任务的情况下完成它Task.Unwrap.

首先,要解决@ mikez的评论,这里是GetResponseAsync.NET 4.0 的实现:

static public Task<WebResponse> GetResponseTapAsync(this WebRequest request)
{
    return Task.Factory.FromAsync(
         (asyncCallback, state) =>
             request.BeginGetResponse(asyncCallback, state),
         (asyncResult) =>
             request.EndGetResponse(asyncResult), null);
}

现在,这里的GetResponseWithRetryAsync实现:

static Task<HttpWebResponse> GetResponseWithRetryAsync(string url, int retries)
{
    if (retries < 0)
        throw new ArgumentOutOfRangeException();

    var request = WebRequest.Create(url);

    Func<Task<WebResponse>, Task<HttpWebResponse>> proceedToNextStep = null;

    Func<Task<HttpWebResponse>> doStep = () =>
    {
        return request.GetResponseTapAsync().ContinueWith(proceedToNextStep).Unwrap();
    };

    proceedToNextStep = (prevTask) =>
    {
        if (prevTask.IsCanceled)
            throw new TaskCanceledException();

        if (prevTask.IsFaulted && --retries > 0)
            return doStep();

        // throw if failed or return the result
        return Task.FromResult((HttpWebResponse)prevTask.Result);
    };

    return doStep();
}

这是一项有趣的练习.它有效,但我认为它比async/await版本更难以遵循.