Question

使用WPF在C＃中异步加载BitmapImage的最佳方法是什么？

Answer 1

我只是在考虑这个并且不得不投入我的两分钱，虽然在原始帖子后几年（以防万一其他人来寻找我正在研究的同样的事情）。

我有一个图像控件，需要使用 Stream 在后台加载图像，然后显示。

我一直遇到的问题是 BitmapSource ，它是 Stream 来源和图片控件都必须在同一个线程。

在这种情况下，使用Binding并设置它的IsAsynch = true将引发跨线程异常。

BackgroundWorker非常适合WinForms，你可以在WPF中使用它，但我更喜欢避免在WPF中使用WinForm程序集（不建议对项目进行膨胀，这也是一个很好的经验法则）。在这种情况下，这也应该抛出无效的交叉引用异常，但我没有测试它。

事实证明，一行代码可以完成其中任何一项工作：

//Create the image control
Image img = new Image {HorizontalAlignment = System.Windows.HorizontalAlignment.Stretch, VerticalAlignment = System.Windows.VerticalAlignment.Stretch};

//Create a seperate thread to load the image
ThreadStart thread = delegate
     {
         //Load the image in a seperate thread
         BitmapImage bmpImage = new BitmapImage();
         MemoryStream ms = new MemoryStream();

         //A custom class that reads the bytes of off the HD and shoves them into the MemoryStream. You could just replace the MemoryStream with something like this: FileStream fs = File.Open(@"C:\ImageFileName.jpg", FileMode.Open);
         MediaCoder.MediaDecoder.DecodeMediaWithStream(ImageItem, true, ms);

         bmpImage.BeginInit();
         bmpImage.StreamSource = ms;
         bmpImage.EndInit();

         //**THIS LINE locks the BitmapImage so that it can be transported across threads!! 
         bmpImage.Freeze();

         //Call the UI thread using the Dispatcher to update the Image control
         Dispatcher.BeginInvoke(new ThreadStart(delegate
                 {
                         img.Source = bmpImage;
                         img.Unloaded += delegate 
                                 {
                                         ms.Close();
                                         ms.Dispose();
                                 };

                          grdImageContainer.Children.Add(img);
                  }));

     };

//Start previously mentioned thread...
new Thread(thread).Start();

Answer 2

假设您正在使用数据绑定，将Binding.IsAsync属性设置为True似乎是实现此目标的标准方法。如果你使用后台线程在代码隐藏文件中加载位图+ Dispatcher对象是更新UI异步的常用方法

Answer 3

这将允许您通过使用HttpClient进行异步下载在UI线程上创建BitmapImage：

private async Task<BitmapImage> LoadImage(string url)
{
    HttpClient client = new HttpClient();

    try
    {
        BitmapImage img = new BitmapImage();
        img.CacheOption = BitmapCacheOption.OnLoad;
        img.BeginInit();
        img.StreamSource = await client.GetStreamAsync(url);
        img.EndInit();
        return img;
    }
    catch (HttpRequestException)
    {
        // the download failed, log error
        return null;
    }
}

Answer 4

 BitmapCacheOption.OnLoad

var bmp = await System.Threading.Tasks.Task.Run(() => 
{ 
BitmapImage img = new BitmapImage(); 
img.BeginInit(); 
img.CacheOption = BitmapCacheOption.OnLoad; 
img.UriSource = new Uri(path); 
img.EndInit(); 
ImageBrush brush = new ImageBrush(img); 

}

Answer 5

详细说明aku的答案，这是一个关于在哪里设置IsAsync的小例子：

ItemsSource="{Binding IsAsync=True,Source={StaticResource ACollection},Path=AnObjectInCollection}"

这就是你在XAML中所做的。

Answer 6

使用或扩展System.ComponentModel.BackgroundWorker：
http://msdn.microsoft.com/en-us/library/system.componentmodel.backgroundworker.aspx

就个人而言，我发现这是在客户端应用程序中执行异步操作的最简单方法。（我在WinForms中使用过它，但不是WPF。我假设这也适用于WPF。）

我通常扩展Backgroundworker，但你不必这样做。

public class ResizeFolderBackgroundWorker : BackgroundWorker
{

    public ResizeFolderBackgroundWorker(string sourceFolder, int resizeTo)
    {
        this.sourceFolder = sourceFolder;
        this.destinationFolder = destinationFolder;
        this.resizeTo = resizeTo;

        this.WorkerReportsProgress = true;
        this.DoWork += new DoWorkEventHandler(ResizeFolderBackgroundWorker_DoWork);
    }

    void ResizeFolderBackgroundWorker_DoWork(object sender, DoWorkEventArgs e)
    {
        DirectoryInfo dirInfo = new DirectoryInfo(sourceFolder);
        FileInfo[] files = dirInfo.GetFiles("*.jpg");


        foreach (FileInfo fileInfo in files)
        {
            /* iterate over each file and resizing it */
        }
    }
}

您可以在表单中使用它：

    //handle a button click to start lengthy operation
    private void resizeImageButtonClick(object sender, EventArgs e)
    {
        string sourceFolder = getSourceFolderSomehow();
        resizer = new ResizeFolderBackgroundWorker(sourceFolder,290);
        resizer.ProgressChanged += new progressChangedEventHandler(genericProgressChanged);
        resizer.RunWorkerCompleted += new RunWorkerCompletedEventHandler(genericRunWorkerCompleted);

        progressBar1.Value = 0;
        progressBar1.Visible = true;

        resizer.RunWorkerAsync();
    }

    void genericRunWorkerCompleted(object sender, RunWorkerCompletedEventArgs e)
    {
        progressBar1.Visible = false;
        //signal to user that operation has completed
    }

    void genericProgressChanged(object sender, ProgressChangedEventArgs e)
    {
        progressBar1.Value = e.ProgressPercentage;
        //I just update a progress bar
    }

使用WPF在C＃中异步加载BitmapImage

6 个答案: