創建和更新用戶界面元素異步Wpf

Question

我想加載幾個圖像異步到用戶界面。

加載圖像時（位圖是從路徑創建的），應該將圖像設置爲窗口上矩形的填充。

當我將Bitmapimage的創建也放在Dispatcher.Invoke方法中時，代碼就可以工作。但顯然，我想要花費大量工作（創建位圖），而不是調用。

我已經嘗試了幾個解決方案，其中包括backgroundworker，但我無法讓它工作。現在，我有以下代碼：

private void Window_ContentRendered(object sender, EventArgs e) 
{ 
    Thread loadThread = new Thread(new ThreadStart(LoadImagesAsync)); 
    loadThread.Start(); 
} 

private void LoadImagesAsync() 
{ 
    IEnumerable<string> images = System.IO.Directory.GetFiles(IMAGE_FOLDER, "*.jpg").Skip(_PageNumber * NUMBER_OF_IMAGES).Take(NUMBER_OF_IMAGES); 

    for (int i = 0; i < NUMBER_OF_IMAGES; i++) 
    { 
     var bitm = new BitmapImage(new Uri(images.ElementAt(i)));     

     this.Dispatcher.Invoke(() => 
     { 
      Grid grid = (Grid)grd_photoBox.Children[i]; 

      var rectangle = (from e in grid.Children.OfType<Rectangle>() 
           where e is Rectangle 
           select e).First(); 

      ImageBrush brush = new ImageBrush(bitm); 

      rectangle.Fill = brush; 
     }); 
    } 
} 

我得到以下異常：

The calling thread cannot access this object because a different thread owns it. 

任何線索？

Answer 1

在大圖像的情況下，使用下面的代碼，它會直接重新調整圖像和加載其他線程，則主線程這可以確保圖像的即時加載。編輯下面的值來改變負載格式：

bitm.DecodePixelWidth = 200; 
bitm.DecodePixelHeight = 100; 

---

private void LoadImagesAsync() 
     { 
      IEnumerable<string> images = System.IO.Directory.GetFiles(IMAGE_FOLDER, "*.jpg").Skip(_PageNumber * NUMBER_OF_IMAGES).Take(NUMBER_OF_IMAGES); 

      for (int i = 0; i < NUMBER_OF_IMAGES; i++) 
      { 
       int j = i; 
       var bitm = new BitmapImage(); 

       bitm.BeginInit(); 
       bitm.CacheOption = BitmapCacheOption.OnLoad; 
       bitm.UriSource = new Uri(images.ElementAt(i)); 
       bitm.DecodePixelWidth = 200; 
       bitm.DecodePixelHeight = 100; 
       bitm.EndInit(); 

       ImageBrush brush = new ImageBrush(bitm); 
       brush.Freeze(); 

       this.Dispatcher.BeginInvoke(new Action(() => 
       { 
        Grid grid = (Grid)grd_photoBox.Children[j]; 

        var rectangle = (from e in grid.Children.OfType<Rectangle>() 
             where e is Rectangle 
             select e).First(); 

        rectangle.Fill = brush; 
       })); 
      } 
     } 

Answer 2

我試過下面的代碼，它是爲我工作。

Task<IEnumerable<string>>.Factory.StartNew(() => System.IO.Directory.GetFiles(
      imagePath, 
      "*.jpg")). 
      ContinueWith(task => 
         { 
          foreach (var item in task.Result) 
          { 
           this.Dispatcher.BeginInvoke((Action)(() => 
           { 
            var img = new Image 
                { 
                 Source = 
                  new BitmapImage(
                  new Uri(item)) 
                }; 
            LayoutRoot.Children.Add(img); 

           })); 

          } 
         }); 

LayoutRoot是我在xaml中的網格。

希望它會有所幫助。

Answer 3

訣竅是freeze位圖允許從另一個線程訪問。因此，您還必須確保在創建時立即加載位圖，因爲默認行爲是延遲加載。

var bitm = new BitmapImage(); 
bitm.BeginInit(); 
bitm.CacheOption = BitmapCacheOption.OnLoad; // load immediately 
bitm.UriSource = new Uri(images.ElementAt(i)); 
bitm.EndInit(); 
bitm.Freeze();