有没有一种方法可以在创建时启动异步任务，但不能在创建时启动异步任务 Xamarin

Question

I want the asynchronous task to be performed by the application, but the content is to be displayed first, then start the async task. When I use protected async override void OnCreate(Bundle bundle) , I don't get to see the contents like button or the content on the screen before the async task is performed. This completely makes it useless to use async task.

I could achieve it with Button.Click . But then again, it is not what I want. I want to start the async task immediately after OnCreate sets all the views. Maybe the problem is with protected async override void OnCreate(Bundle bundle) .

Is there any other way around to start the task onCreate ?

Here is my code.

[Activity(Label = "NewsDetails")]
public class NewsDetails : Activity {
  protected async override void OnCreate(Bundle savedInstanceState) {
    base.OnCreate(savedInstanceState);
    SetContentView(Resource.Layout.NewsDetails);
    TextView Title = FindViewById<TextView>(Resource.Id.textTitle);
    TextView Source = FindViewById<TextView>(Resource.Id.textSource);
    WebView webDisplay = FindViewById<WebView>(Resource.Id.webDisplay);
    string thisid=Intent.GetStringExtra ("id");
    string url = "http://MyApiUrl/";
    url = url + thisid;
    var result = await GetNewsAsync(url);
    Title.Text = result.GetString("Title");
    Source.Text = result.GetString("Source");
    string ExternalReference = result.GetString("ExternalReference");
    webDisplay.Settings.JavaScriptEnabled = true;
    webDisplay.LoadUrl(ExternalReference);
  }
  private async Task<JSONObject> GetNewsAsync(string url) {
    // Create an HTTP web request using the URL:
    // Send the request to the server and wait for the response:
    // Return some JSONObject after async task here
    // Create an HTTP web request using the URL:
    HttpWebRequest request = (HttpWebRequest)HttpWebRequest.Create(new Uri(url));
    request.ContentType = "application/json";
    request.Method = "GET";
    // Send the request to the server and wait for the response:
    using (WebResponse response = await request.GetResponseAsync()) {
      // Get a stream representation of the HTTP web response:
      using (Stream stream = response.GetResponseStream()) {
        // Use this stream to build a JSON document object:
        JSONObject jsonResponse;
        Stream jsonDoc = stream;
        Console.Out.WriteLine("Response: {0}", jsonDoc.ToString());
        StreamReader reader = new StreamReader(stream, Encoding.UTF8);
        String responseString = reader.ReadToEnd();
        jsonResponse = new JSONObject(responseString);
        JSONObject jResult = jsonResponse.GetJSONObject("Result");
        return jResult;
      }
    }
  }
}

I would like to know if there is something I am doing wrong or if there is a completely different method to achieve what I am trying to do.

Any suggestions in the code are welcome.

Edit: code for async Task<JSONObject> GetNewsAsync .

Answer 1

I would not recommend to mark the OnCreate method as async.

Try something like that:

protected override void OnCreate(Bundle savedInstanceState)
{
    base.OnCreate(savedInstanceState);

    SetContentView(Resource.Layout.NewsDetails);

    TextView Title = FindViewById<TextView>(Resource.Id.textTitle);
    TextView Source = FindViewById<TextView>(Resource.Id.textSource);
    WebView webDisplay = FindViewById<WebView>(Resource.Id.webDisplay);
    webDisplay.Settings.JavaScriptEnabled = true;

    string thisid=Intent.GetStringExtra ("id");
    string url = "http://MyApiUrl/";
    url = url + thisid;

    GetNewsAsync(url).ContinueWith(t=>
    {
        var result = t.Result;

        Title.Text = result.GetString("Title");
        Source.Text = result.GetString("Source");
        string ExternalReference = result.GetString("ExternalReference");

        webDisplay.LoadUrl(ExternalReference);
    });
  }