WPFボタンのテキストが変更されないUIがスレッドコードからロックされる

Question

基本的なペイントプログラムで作業していて、現在のペイントウィンドウをpngファイルにエクスポートするセーブコードを書きました。私はそのコードを新しいスレッドで実行するように変更したので、私は考えました。しかし、保存ボタンのテキストを変更するコードを実行すると、画像が保存されるまで更新されません。

私の最初のことは奇妙です。SaveBtnClickの行にブレークポイントを置くと、テキストが「保存」に変わり、ボタンには影響しません。私はSaveAsPngAsyncで実行されているコードがまだ私のUIをロックしていることを認識しています。私はそれがschedulerのためだと思うが、そのコードブロックがなければスレッドSTAについてのエラーが出る。

コード：

private void SaveBtn_Click(object sender, RoutedEventArgs e) 
     { 
      ChangeSaveBtnText("Saving"); 
      if (pictureSaveTask == null) 
      { 
       pictureSaveTask = SaveAsPngAsync(); 
       pictureSaveTask.ContinueWith((t) => 
       { 
        pictureSaveTask = null; 
        ChangeSaveBtnText("Save"); 

       }); 
      } 
      else 
      { 
       cts.Cancel(); 
       ChangeSaveBtnText("Save"); 
      } 

     } 

     private void ChangeSaveBtnText(string text) 
     { 
      Application.Current.Dispatcher.BeginInvoke(DispatcherPriority.Normal, new Action(() => { 
       SaveBtn.Content = text; 
      })); 
     } 

     /// <summary> 
     /// Saves the current state of the DrawingCanvas as Xaml so that it can re parsed on a different thread then exported as a png. 
     /// </summary> 
     /// <returns>Task of executing code</returns> 
     private async Task SaveAsPngAsync() 
     { 
      //Stores current DrawingCanvas at time of save as a string to be parsed later. 
      var canvasAtSaveState = XamlWriter.Save(DrawingCanvas); 
      var ActualWidth = DrawingCanvas.ActualWidth; 
      var ActualHeight = DrawingCanvas.ActualHeight; 

      //Not really sure why this is required but without, throws Error: Thread must be STA. 
      var scheduler = TaskScheduler.FromCurrentSynchronizationContext(); 

      //This is the short hand way of creating a task and running it instantly 
      await Task.Factory.StartNew(() => { 
       try 
       { 
        // 'as' object syntax is a short hand conversion 
        InkCanvas cc = XamlReader.Parse(canvasAtSaveState) as InkCanvas; 

        //Pretty clear that this gets the users path to their desktop 
        var desktopPath = Environment.GetFolderPath(Environment.SpecialFolder.DesktopDirectory); 

        //File stream is the easiest way to export a stream of bytes into a file 
        //Important usage of a C# syntax "using" statements are very important to prevent memory leaks especially with streams 
        using (var fs = new FileStream(System.IO.Path.Combine(desktopPath, "picture.png"), FileMode.Create, FileAccess.ReadWrite)) 
        { 
         //Must use Actual(Value) because Simply using Width or Height doesn't change the window size changes or scales 
         RenderTargetBitmap rtb = new RenderTargetBitmap((int)ActualWidth, (int)ActualHeight, 96d, 96d, PixelFormats.Default); 

         rtb.Render(cc); 

         //PngBitmapEncoder because my InkCanvas has a transparent background 
         BitmapEncoder pngEncoder = new PngBitmapEncoder(); 

         //Adds a single frame to the image because this isn't a Gif 
         pngEncoder.Frames.Add(BitmapFrame.Create(rtb)); 

         //Begins saving the pngBytes into a stream that will then store the bytes into a file 
         pngEncoder.Save(fs); 
        } 
       } 
       catch (Exception ex) 
       { 
        //Catches anything just in case something explodes 
       } 
      }, cts.Token, TaskCreationOptions.PreferFairness, scheduler); //PreferFairness tells the thread pool that this new worker thread isn't super important 
     } 

Answer 1

あなたの非同期コードは次のようになります。あなたはボタンが更新され表示されていない、なぜあなたの仕事は、UIスレッドで同期実行されている

class YourClass 
{ 
    private CancellationTokenSource cts; 

    private async void SaveBtn_Click(object sender, RoutedEventArgs e) 
    { 
     // if cts isn't null, then a saving task is already running -> cancel it 
     if (cts != null) 
     { 
      cts.Cancel(); 
     } 
     else 
     { 
      // otherwise, create a new token source and await the saving task 
      // this runs on the UI thread 
      cts = new CancellationTokenSource(); 
      SaveBtn.Content = "Saving"; 
      try 
      { 
       // the task itself will probably run on a thread pool's thread, 
       // but after it finishes, this method will continue on the UI thread 
       await Task.Run(() => SaveAsPng(cts.Token), cts.Token); 
      } 
      catch (OperationCanceledException) 
      { 
       // insert your cancellation handling if needed 
       // this runs on the UI thread 
      } 
      finally 
      { 
       // This runs on the UI thread 
       SaveBtn.Content = "Save"; 
       cts.Dispose(); 
       cts = null; 
      } 
     } 
    } 

    private void SaveAsPng(CancellationToken ct) 
    { 
     // your synchronous implementation 
     // don't forget to check ct.IsCancellationRequested in this method 
     // or call ct.ThrowIfCancellationRequested() 

     // no need to start any tasks here 
     // don't use dispatchers, schedulers etc. 
     // just a plain old synchronous code 
    } 
}