7

我正在尝试在我的 wpf 应用程序中从网络加载图像。

这个想法如下：当我点击一个按钮时，会弹出一个带有附加信息的弹出窗口。在这个弹出窗口中，我使用了一些来自网络的图像。

问题：加载弹出窗口时，系统在等待图像时挂起。我正在从后面的代码中绑定图像。图像存储在 ObservableCollection 中。我尝试使用线程来加载图像，但每次遇到异常时都说线程不是对象的所有者。

我尝试使用 Invoke 将下载的图像获取到 UserinterfaceThread 但我无法访问它。我的代码如下：

        IList<Image> imagesFromWeb = downloadImagesFromWeb(url);


        DispatcherHelper.UIDispatcher.Invoke(DispatcherPriority.Normal, (ThreadStart)delegate()
        {
            foreach (Image img in imagesFromWeb 
            {
                this.ObservableCollection_Images.Add(img);
            }
    }

下载图像并尝试将图像添加到（已经打开的）弹出窗口后，我得到异常说线程不是对象的所有者

有人可以指出我正确的方向吗？

4

3 回答 3

15

如果您在公共 Web 服务器上有可用的图像，可以使用普通的 HTTP URI 进行寻址，那么您可以直接将源设置为：

<Image Source="http://www.someserver.com/myimage.png" />

WPF 将负责下载它 - 我认为它甚至会异步执行，尽管我不是 100% 确定。

您当然也可以使用数据绑定来执行此操作：

<Image Source="{Binding TheImage}" />

在视图模型中

public string TheImage 
{ 
    get { return "http://www.someserver.com/myimage.png"; } 
}
于 2010-12-07T10:19:13.683 回答
3

您可能会遇到集合、WPF、绑定和线程的各种问题

最好的事情（在我看来）是使用调度员安全的可观察集合

这是一个实现，还包括线程安全：

public class SafeObservable<T> : IList<T>, INotifyCollectionChanged, INotifyPropertyChanged
{
    private readonly IList<T> collection = new List<T>();
    private readonly Dispatcher dispatcher;
    public event NotifyCollectionChangedEventHandler CollectionChanged;
    public event PropertyChangedEventHandler PropertyChanged;
    private readonly ReaderWriterLock sync = new ReaderWriterLock();

    public SafeObservable()
    {
        dispatcher = Dispatcher.CurrentDispatcher;
    }

    public void Add(T item)
    {
        if (Thread.CurrentThread == dispatcher.Thread)
            DoAdd(item);
        else
            dispatcher.BeginInvoke((Action)(() => DoAdd(item)));
        if (PropertyChanged != null)
            PropertyChanged(this, new PropertyChangedEventArgs("Count"));
    }

    private void DoAdd(T item)
    {
        sync.AcquireWriterLock(Timeout.Infinite);
        collection.Add(item);
        if (CollectionChanged != null)
            CollectionChanged(this,
                new NotifyCollectionChangedEventArgs(NotifyCollectionChangedAction.Add, item));
        sync.ReleaseWriterLock();
    }

    public void Clear()
    {
        if (Thread.CurrentThread == dispatcher.Thread)
            DoClear();
        else
            dispatcher.BeginInvoke((Action)(DoClear));
        if (PropertyChanged != null)
            PropertyChanged(this, new PropertyChangedEventArgs("Count"));
    }

    private void DoClear()
    {
        sync.AcquireWriterLock(Timeout.Infinite);
        collection.Clear();
        if (CollectionChanged != null)
            CollectionChanged(this,
                new NotifyCollectionChangedEventArgs(NotifyCollectionChangedAction.Reset));
        sync.ReleaseWriterLock();
    }

    public bool Contains(T item)
    {
        sync.AcquireReaderLock(Timeout.Infinite);
        var result = collection.Contains(item);
        sync.ReleaseReaderLock();
        return result;
    }

    public void CopyTo(T[] array, int arrayIndex)
    {
        sync.AcquireWriterLock(Timeout.Infinite);
        collection.CopyTo(array, arrayIndex);
        sync.ReleaseWriterLock();
    }

    public int Count
    {
        get
        {
            sync.AcquireReaderLock(Timeout.Infinite);
            var result = collection.Count;
            sync.ReleaseReaderLock();
            return result;
        }
    }

    public bool IsReadOnly
    {
        get { return collection.IsReadOnly; }
    }

    public bool Remove(T item)
    {
        if (Thread.CurrentThread == dispatcher.Thread)
            return DoRemove(item);
        var op = dispatcher.BeginInvoke(new Func<T, bool>(DoRemove), item);
        if (op == null || op.Result == null)
            return false;
        if (PropertyChanged != null)
            PropertyChanged(this, new PropertyChangedEventArgs("Count"));
        return (bool)op.Result;
    }

    private bool DoRemove(T item)
    {
        sync.AcquireWriterLock(Timeout.Infinite);
        var index = collection.IndexOf(item);
        if (index == -1)
        {
            sync.ReleaseWriterLock();
            return false;
        }

        var result = collection.Remove(item);
        if (result && CollectionChanged != null)
            CollectionChanged(this, new
                NotifyCollectionChangedEventArgs(NotifyCollectionChangedAction.Reset));

        sync.ReleaseWriterLock();
        return result;
    }

    public IEnumerator<T> GetEnumerator()
    {
        return collection.GetEnumerator();
    }

    System.Collections.IEnumerator System.Collections.IEnumerable.GetEnumerator()
    {
        return collection.GetEnumerator();
    }

    public int IndexOf(T item)
    {
        sync.AcquireReaderLock(Timeout.Infinite);
        var result = collection.IndexOf(item);
        sync.ReleaseReaderLock();
        return result;
    }

    public void Insert(int index, T item)
    {
        if (Thread.CurrentThread == dispatcher.Thread)
            DoInsert(index, item);
        else
            dispatcher.BeginInvoke((Action)(() => DoInsert(index, item)));
        if (PropertyChanged != null)
            PropertyChanged(this, new PropertyChangedEventArgs("Count"));
    }

    private void DoInsert(int index, T item)
    {
        sync.AcquireWriterLock(Timeout.Infinite);
        collection.Insert(index, item);
        if (CollectionChanged != null)
            CollectionChanged(this,
                new NotifyCollectionChangedEventArgs(NotifyCollectionChangedAction.Add, item, index));
        sync.ReleaseWriterLock();
    }

    public void RemoveAt(int index)
    {
        if (Thread.CurrentThread == dispatcher.Thread)
            DoRemoveAt(index);
        else
            dispatcher.BeginInvoke((Action)(() => DoRemoveAt(index)));
        if (PropertyChanged != null)
            PropertyChanged(this, new PropertyChangedEventArgs("Count"));
    }

    private void DoRemoveAt(int index)
    {
        sync.AcquireWriterLock(Timeout.Infinite);
        if (collection.Count == 0 || collection.Count <= index)
        {
            sync.ReleaseWriterLock();
            return;
        }
        collection.RemoveAt(index);
        if (CollectionChanged != null)
            CollectionChanged(this,
                new NotifyCollectionChangedEventArgs(NotifyCollectionChangedAction.Reset));
        sync.ReleaseWriterLock();
    }

    public T this[int index]
    {
        get
        {
            sync.AcquireReaderLock(Timeout.Infinite);
            var result = collection[index];
            sync.ReleaseReaderLock();
            return result;
        }

        set
        {
            sync.AcquireWriterLock(Timeout.Infinite);
            if (collection.Count == 0 || collection.Count <= index)
            {
                sync.ReleaseWriterLock();
                return;
            }
            collection[index] = value;
            sync.ReleaseWriterLock();
        }
    }
}
于 2010-11-19T09:20:10.997 回答
1

我想有一个更好的方法来加载图像。

与其绑定到后面代码中的图像，不如绑定到包含图像位置的字符串。之后，我在 xaml 代码中使用转换器将字符串转换为图像。（图像下载器现在在转换器类中）

xaml 中的代码：

<Image Source="{Binding imageUrl, Converter={StaticResource url}}" Height="200" Width="200"></Image>

转换器的代码：


    class ImageDownloader : IValueConverter
    {
        public object Convert(object value, Type targetType, object parameter, System.Globalization.CultureInfo culture)
        {
            string url =(string)value;
            return getImage(url);



    }

    private object getImage(string imagefile)
    {
       /// IMPLEMENT FUNCTION TO DOWNLOAD IMAGE FROM SERVER HERE
    }

    public object ConvertBack(object value, Type targetType, object parameter, System.Globalization.CultureInfo culture)
    {
        return null;
    }
}

当然不要忘记在 app.xaml 中设置资源：

<Application.Resources>
   <ResourceDictionary>
       <namespace:ImageDownloader x:Key="ImageDownloader" />
   </ResourceDictionary>
</Application.Resources>
于 2010-11-19T12:47:32.973 回答