使用TAP的WebClient进度报告

Question

I'm wondering if there is a way to report WebClient progress without using EAP( Event-based Asynchronous Pattern ). Old way(using EAP) would be:

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

With async/await this can be written as:

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

But in the second example i'm using both EAP and TAP( Task-based Asynchronous Pattern ). Isn't mixing two patterns of asynchrony considered as a bad practice?

Is there a way to achieve the same without using EAP? I have read about IProgress interface, but I think there is no way to use it to report WebClient progress.

Answer 1

The bad news is that the answer is NO!

The good news is that any EAP API can be converted into a TAP API.

Try this:

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;
        }
    }
}

And just use it like this:

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();
}