如何在后台加载图像？ [英] How do I load images in the background?

问题描述

我正在尝试在后台加载图片，然后更新UI。我一整天都在玩，我不知道我失踪了什么。我收到以下错误：

I am trying to load an image in the background and then update the UI. I have been playing with this all day and I don't know what I am missing. I keep getting the following error:

调用线程无法访问此对象，因为不同的线程拥有它。

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

我以下例子中的例子搜到了，但我似乎找不到答案。我还在另一个BeginInvoke中包装了触摸UI的代码。

I've hunted around following example after example, but I cannot seem to find an answer. I also wrapped the code that is touching the UI in another BeginInvoke.

更新3：故事的道德。 ImageSource不是线程安全的访问。

Update 3: The moral of the story. ImageSource is not thread safe for access.

更新2：这必须是一个简单的解决方案:)。我试过克隆，但是并没有取得成功，但是我确实遇到了一个不同的错误：调用目标抛出异常。

Update 2: This has got to be a simple solution :). I tried the cloning, but that didn't result in success, but I did get a different error: "Exception has been thrown by the target of an invocation."

更新1：我尝试了BackgroundWorker，但我仍然收到相同的错误，但它正在刷.ImageSource.Height。我是否正确表示UI？任何建议？

Update 1: I tried the BackgroundWorker, but I am still getting the same error, but it is occurring on the brush.ImageSource.Height. Am I signaling the UI correctly? Any suggestions?

这是我的XAML：

<Window x:Class="Slideshow.Show"
xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml">
    <DockPanel>
        <Canvas Background="Black" Name="canvas">
            <Viewbox Name="background" Stretch="Uniform">
                <Rectangle name="background" />
            </Viewbox>
        </Canvas>
    </DockPanel>
</Window>

以下是一些代码：

namespace Slideshow
{
    public class Show 
    {
        public Show()
        {
            BackgroundWorker bw = new BackgroundWorker();
            bw.DoWork += new DoWorkEventHandler(bw_DoWork);
            bw.RunWorkerCompleted += 
                new RunWorkerCompletedEventHandler(bw_RunWorkerCompleted);
            bw.RunWorkerAsync();
        }

        void bw_RunWorkerCompleted(object sender, RunWorkerCompletedEventArgs e)
        {
            BitmapSource bitmap = e.Result as BitmapSource;

            if (bitmap != null)
            {
                this.Dispatcher.BeginInvoke(DispatcherPriority.Normal 
                    (ThreadStart)delegate()
                {
                    Image image = new Image();
                    image.Source = bitmap;
                    background.Child = image;
                 });
            }
        }

        void bw_DoWork(object sender, DoWorkEventArgs e)
        {
            BitmapSource bitmapSource = CreateBrush(GetRandomFile());
            e.Result = bitmapSource;
         }
    }
}

推荐答案

我倾向于使用 ThreadPool.QueueUserWorkItem 来加载图像，然后当操作完成时，我使用线程安全的 Dispatcher 对象。图像源不是线程安全的，您将不得不使用类似于 JpegBitmapDecoder 的内容，还有一个 PngBitmapDecoder 。

What I tend to use the ThreadPool.QueueUserWorkItem to load the image, then when the operation completes I call back to the UI thread using the thread-safe Dispatcher object. The image source is not thread safe, you will have to use something like the JpegBitmapDecoder, there is also a PngBitmapDecoder.

例如：

public Window()
{
    InitializeComponent();

    ThreadPool.QueueUserWorkItem(LoadImage,
         "http://z.about.com/d/animatedtv/1/0/1/m/simp2006_HomerArmsCrossed_f.jpg");
}

public void LoadImage(object uri)
{
    var decoder = new JpegBitmapDecoder(new Uri(uri.ToString()), BitmapCreateOptions.PreservePixelFormat, BitmapCacheOption.OnLoad);
    decoder.Frames[0].Freeze();
    this.Dispatcher.Invoke(DispatcherPriority.Send, new Action<ImageSource>(SetImage), decoder.Frames[0]);
}

public void SetImage(ImageSource source)
{
    this.BackgroundImage.Source = source;
}

这篇关于如何在后台加载图像？的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！