Question

I have usercontrol, and there is a DependencyProperty defined in it.

        #region ImageUri
    public static readonly DependencyProperty ImageUriProperty = DependencyProperty.Register(
       "ImageUri",
       typeof(string),
       typeof(ScrollableCanvas),
       new PropertyMetadata(new PropertyChangedCallback(ImageUriPropertyChangedCallback)));

    private static void ImageUriPropertyChangedCallback(DependencyObject sender, DependencyPropertyChangedEventArgs e)
    {
        ScrollableCanvas main = sender as ScrollableCanvas;
        if (main != null)
        {
            main.ImageUri = (string)e.NewValue;
        }
    }

    public string ImageUri
    {
        get
        {
            return (string)GetValue(ImageUriProperty);
        }
        set
        {
            SetValue(ImageUriProperty, value);
            UpdateImage();
        }
    }
    #endregion

In the Xaml, I bind a value to it like this

<my:ScrollableCanvas  Name="scrollableCanvas1" ImageUri="{Binding Path=LayerImage}" />

when I update the LayerImage in the viewmodel, the ImageUri property does not update.

Can some help on this? Thanks.

BTW: The value is updated when I set the LayerImage in the constructor of the viewmodel.

Answer 1

You shouldn’t include your UpdateImage call in your setter, but rather in the property changed callback.

public static readonly DependencyProperty ImageUriProperty = DependencyProperty.Register(
   "ImageUri",
   typeof(string),
   typeof(ScrollableCanvas),
   new PropertyMetadata(new PropertyChangedCallback(ImageUriPropertyChangedCallback)));

private static void ImageUriPropertyChangedCallback(DependencyObject sender, DependencyPropertyChangedEventArgs e)
{
    ScrollableCanvas main = sender as ScrollableCanvas;
    if (main != null)
    {
        // Since ImageUri has already been called at this point, you can just update your image here...
        main.UpdateImage();
    }
}

public string ImageUri
{
    get
    {
        return (string)GetValue(ImageUriProperty);
    }
    set
    {
        SetValue(ImageUriProperty, value);
    }
}