使用 TAP 的 WebClient 进度报告

Question

我想知道是否有一种方法可以在不使用 EAP（基于事件的异步模式）的情况下报告 WebClient 进度。旧方式（使用 EAP）将是：

var client = new WebClient();
client.DownloadProgressChanged += (s,e) => { //progress reporting }
client.DownloadFileCompleted += (s,e) => { Console.Write("download finished" }
client.DownloadFileAsync(file);

使用 async/await 可以写成：

var client = new WebClient();
client.DownloadProgressChanged += (s,e) => { //progress reporting }
await client.DownloadFileTaskAsync(file);
Console.Write("downlaod finished");

但是在第二个示例中，我同时使用了 EAP 和 TAP（基于任务的异步模式）。混合两种异步模式不是被认为是一种不好的做法吗？

有没有办法在不使用 EAP 的情况下实现相同的目标？我已经阅读了IProgress接口，但我认为没有办法使用它来报告 WebClient 进度。

Answer 1

坏消息是答案是否定的！

好消息是任何 EAP API 都可以转换为 TAP API。

尝试这个：

public static class WebClientExtensios
{
    public static async Task DownloadFileTaskAsync(
        this WebClient webClient, 
        Uri address, 
        string fileName, 
        IProgress<Tuple<long, int, long>> progress)
    {
        // Create the task to be returned
        var tcs = new TaskCompletionSource<object>(address);

        // Setup the callback event handler handlers
        AsyncCompletedEventHandler completedHandler = (cs, ce) =>
        {
            if (ce.UserState == tcs)
            {
                if (ce.Error != null) tcs.TrySetException(ce.Error);
                else if (ce.Cancelled) tcs.TrySetCanceled();
                else tcs.TrySetResult(null);
            }
        };

        DownloadProgressChangedEventHandler progressChangedHandler = (ps, pe) =>
        {
            if (pe.UserState == tcs)
            {
                progress.Report(
                    Tuple.Create(
                        pe.BytesReceived, 
                        pe.ProgressPercentage, 
                        pe.TotalBytesToReceive));
            }
        };

        try
        {
            webClient.DownloadFileCompleted += completedHandler;
            webClient.DownloadProgressChanged += progressChangedHandler;

            webClient.DownloadFileAsync(address, fileName, tcs);

            await tcs.Task;
        }
        finally
        {
            webClient.DownloadFileCompleted -= completedHandler;
            webClient.DownloadProgressChanged -= progressChangedHandler;
        }
    }
}

就像这样使用它：

void Main()
{
    var webClient = new WebClient();

    webClient.DownloadFileTaskAsync(
        new Uri("http://feeds.paulomorgado.net/paulomorgado/blogs/en"),
        @"c:\temp\feed.xml",
        new Progress<Tuple<long, int, long>>(t =>
        {
            Console.WriteLine($@"
        Bytes received: {t.Item1,25:#,###}
   Progress percentage: {t.Item2,25:#,###}
Total bytes to receive: {t.Item3,25:#,###}");
        })).Wait();
}