如何等待 UploadStringAsync 方法完成

Question

我 trying/wanting 等待异步任务完成这就是我正在尝试做的事情

public class savetodbsingle 
{
    public static  string  insert(string id, string type, string cat)
    {
        Uri uri = new Uri("http://yxz.com");

        WebClient wc = new WebClient();
        wc.Headers[HttpRequestHeader.ContentType] = "application/json";
        wc.UploadStringAsync(uri, "POST", json);

        wc.UploadStringCompleted += wc_UploadComplete6;
    }

    private static   void wc_UploadComplete6(object sender, UploadStringCompletedEventArgs e)
    {
        //Saving to database logic here 
    }
}

并从其他页面调用该方法

//calling method
savetodbsingle.insert(id,type,cat)); 

// i want to show messegebox from here when above task completes 
MessegeBox.Show("Completed");

那么如何实现呢？我曾尝试使方法异步然后等待它，但这没有帮助

Answer 1

不知道WebClient.UploadStringTaskAsync comes with Microsoft.Bcl.Async for Windows Phone 8, but in case someone is wondering how it is implemented and how to use TaskCompletionSource, here it is its implementation, taken from the source:

[HostProtection(ExternalThreading = true)]
[ComVisible(false)]
public Task<string> UploadStringTaskAsync(Uri address, string method, string data)
{
    // Create the task to be returned
    var tcs = new TaskCompletionSource<string>(address);

    // Setup the callback event handler
    UploadStringCompletedEventHandler handler = null;
    handler = (sender, e) => HandleCompletion(tcs, e, (args) => args.Result, handler, (webClient, completion) => webClient.UploadStringCompleted -= completion);
    this.UploadStringCompleted += handler;

    // Start the async operation.
    try { this.UploadStringAsync(address, method, data, tcs); }
    catch
    {
        this.UploadStringCompleted -= handler;
        throw;
    }

    // Return the task that represents the async operation
    return tcs.Task;
}

Answer 2

您可以使用回调来完成。如何定义回调取决于您。作为示例，我将其定义为一个带有两个参数的 Action：一个对象，可用于将某些东西返回给调用者，以及一个异常，以防在保存过程中出现问题。

public class savetodbsingle 
{
    private Action<object, Exception> _callback;

    public static  string  insert(string id, string type, string cat, Action<object,Exception> callback)
    {
        Uri uri = new Uri("http://yxz.com");

        WebClient wc = new WebClient();
        wc.Headers[HttpRequestHeader.ContentType] = "application/json";
        wc.UploadStringCompleted += wc_UploadComplete6;

        _callback = callback;

        wc.UploadStringAsync(uri, "POST", json);

    }

    private static void wc_UploadComplete6(object sender, UploadStringCompletedEventArgs e)
    {
        //Saving to database logic here 

        // When done, you can return something if needed
        // or return an exception when something bad happened.
        _callback(someObject, someException)
    }
}

并从其他页面调用该方法

//calling method
savetodbsingle.insert(id,type,cat, handleSaved)); 

private void handleSaved(object o, Exception e)
{
    MessegeBox.Show("Completed");
}

Answer 3

改用 UploadStringTaskAsync 方法

这将 return 一个您可以等待的任务。

https://msdn.microsoft.com/en-us/library/hh159423%28v=vs.110%29.aspx

 public static  async string  insert(string id, string type, string cat, Action<object,Exception> callback)
    {
         ----
        string result  = await wc.UploadStringTaskAsync(uri, "POST", json);

    }

Answer 4

这是我如何实现它的解决方案

 public static async  Task  insert(string id, string type, string cat)
            {
       WebClient wc = new WebClient();
                wc.Headers[HttpRequestHeader.ContentType] = "application/json";


           var responce=     await wc.UploadStringTaskAsync(uri, "POST", json);
             jsons = responce.ToString().Trim();

             saving();//save to database logic here
    }

和这样的调用方法

 await savetodbsingle.insert(id, type, cat);
 Messegebox.Show("Completed");

如何等待 UploadStringAsync 方法完成

how to await till UploadStringAsync method completes

c#

async-await

windows-phone-8