Silverlight-等待异步任务完成

Question

Here is what i have done so far:

All of the below code will get looped through and different URL's will be sent each time. I want to be able to call the address in the loop and then wait for it to complete and then call the completed method.

I have created my URI

Uri address = new Uri("http://dev.virtualearth.net/REST/V1/Imagery/Metadata/OrdnanceSurvey/" + latitude + "," + longitude + "?+zl=" + zoomLevel + "&key=""");

I have then told it where to call when the operation has complete

WebClient webClient = new WebClient();
webClient.DownloadStringCompleted += WebClientDownloadString_Complete;
webClient.DownloadStringAsync(address);

Then i set up what to happen when the operation has completed

    private void WebClientDownloadString_Complete(object sender, DownloadStringCompletedEventArgs e)
    {
        if (e.Error == null)
        {

            string html = e.Result;

            string[] parts = html.Split(',');
            string[] URLs = parts[7].Split('"');
            URL = URLs[3].Replace("{subdomain}", "t0").Replace("{quadkey}", qk.Key).Replace(@"\", string.Empty);


        }
    }

Is there a way so that when the webclient calls the URL I wait till the operation has completed and it calls the completed method?

Answer 1

The easy way to do this is to just use DownloadString instead of DownloadStringAsync . The DownloadString call will block (make sure to run it on a background thread!) and you can loop as you expect.

Your other option is to create a TaskCompletionSource and set it in your event handler.

private TaskCompletionSource<bool> currentTask;

private async Task GetURLs()
{
    while (someCondition)
    {
        currentTask = new TaskCompletionSource<bool>();
        WebClient webClient = new WebClient();
        webClient.DownloadStringCompleted += WebClientDownloadString_Complete;
        webClient.DownloadStringAsync(address);

        await currentTask;
    }
}

private void WebClientDownloadString_Complete(...)
{
    //Process completion

    //C# 6 null check
    curentTask?.TrySetValue(true);
}

"GetURLS" will block asynchronously until the tasks value is set, and then allow the while loop to continue. This has the advantage of not needing an explicit background thread.

Answer 2

The code that you put up already does just that - you created your WebClient, you set an event handler to its completed method, and you start the download. If you want to wait for it to finish, all you need to do is let your method end, and control will pass back to the Silverlight framework which will do your waiting for you.

The other way to do this would be to use the async and await keywords, which your framework appears to support. Simply put, you might be able to put an await keyword before your method call to DownloadStringAsync , which will cause the system to wait for the download (and go on to other tasks), pull out the string once the download completes, and continue as usual:

string html = await webClient.DownloadStringAsync(address);

Answer 3

Wanted to do this myself some time ago. Here is how I did it:

            // create a notifier which tells us when the download is finished
            // in .net 4.5 DownloadFileTaskAsync can be used instead which simplyfies this task
            AutoResetEvent notifier = new AutoResetEvent(false);
            client.DownloadFileCompleted += (object sender, AsyncCompletedEventArgs e) =>
            {
                notifier.Set();
            };

            try
            { 

                client.DownloadFileAsync(downloadUrl, destination);
                // wait for download to finish
                notifier.WaitOne();
             ...