c# - 如何取消和进度报告 WebClient.DownloadDataTaskAsync 方法返回的任务？

Question

我正在学习任务并行库。我有一些旧代码使用 WebClient 类从 Web 下载数据。我想转换我以前使用 Event-based Asynchronous Pattern(EAP) 的代码至 Task-based Asynchronous Pattern (TAP)

我的旧代码如下:

WebClient client1 = new WebClient();
client1.DownloadDataCompleted += (o, e)=>
{
    if (e.Cancelled)
    {
        //code that update UI report download has been canceled.
    }
    else
    {
        byte[] s = e.Result;
        //code that update UI report downloads has been completed.
    }
};

client1.DownloadProgressChanged += ( o,  e) =>
{
    //code that update UI report downloading progress.
    updateProgress(e.ProgressPercentage);
};

//start download asynchronous
client1.DownloadDataAsync(new Uri("http://stackoverflow.com/"));

//code to cancel download.
client1.CancelAsync();

现在使用任务 API，我有代码:

WebClient client2 = new WebClient();
Task<byte[]> task = client2.DownloadDataTaskAsync("http://stackoverflow.com/");

task.ContinueWith((antecedent) =>
{
    byte[] s = antecedent.Result;
    //code that updateUI report download has been completed.
});

//TODO how to code that can cancel the download and report progress?

所以我的问题是:

当使用任务方法 DownloadDataTaskAsync 时，WebClient 类是否有内置 api，我可以取消下载并报告下载进度？

Answer 1

您可以扩展 WebClient:

public static class WebClientExtensions
{
    public static async Task<byte[]> DownloadDataTaskAsync(this WebClient webClient, string address, CancellationToken cancellationToken)
    {
        cancellationToken.ThrowIfCancellationRequested();

        using (cancellationToken.Register(webClient.CancelAsync))
        {
            return await webClient.DownloadDataTaskAsync(address);
        }
    }
}

然后调用扩展方法:

var data = await webClient.DownloadDataTaskAsync("http://stackoverflow.com/", cancellationToken);