Question

I currently write an image viewer control that encapsulates a WPF Image Control and further stuff (Controls for applying filters and changing views). Here’s the relevant portion of control’s source code:

public partial class ImageViewPort : UserControl, INotifyPropertyChanged
{
    private BitmapSource _source;

    public static readonly DependencyProperty ImageDescriptorSourceProperty =
        DependencyProperty.Register("ImageDescriptorSource",
                                 typeof(ImageDescriptor),
                                 typeof(ImageViewPort),
                                 new UIPropertyMetadata(ImageDescriptorSourceChanged));

    public ImageDescriptor ImageDescriptorSource
    {
        get { return (ImageDescriptor)GetValue(ImageDescriptorSourceProperty); }
        set { SetValue(ImageDescriptorSourceProperty, value); }
    }

    public BitmapSource Source //the image control binds to this beauty!
    {
        get { return _source; }
        set { _source = value; OnPropertyChanged("Source"); }
    }

   public ImageViewPort() { InitializeComponent(); }

    private static void ImageDescriptorSourceChanged(DependencyObject d, DependencyPropertyChangedEventArgs e)
    {
        ImageViewPort viewPort = (ImageViewPort)d;
        if (viewPort != null)
        {
            viewPort.TransformImage();
        }
    }

    private BitmapSource TransformImage()
    {
        //do something that sets the "Source" property to a BitmapSource
    }
}

The XAML Code (only relevant parts) of the user control:

<UserControl x:Name="viewPort">
<Image Source="{Binding ElementName=viewPort,Path=Source}"/>
</UserControl>

And finally the usage:

<WPF:ImageViewPort ImageDescriptorSource="{Binding Path=CurrentImage}"/>

In my window, I basically iterate a Collection and, as I do so, throw PropertyChanged notification for the CurrentImage property. That works, the getter is called each time, so the binding seems to work.

What I would expect to happen now is that the PropertyChanged-Callback of my UserControl is fired, but no such thing happens (It never steps in there, I’ve tried using breakpoints). I’ve tried the same thing binding a primitive type (int), and that worked.

Do you see any flaw in my implementation? Why isn’t the user control updating?
Thanks a lot in advance for any help!

Cheers

Sebi

Answer 1

Check the output … do you get any binding warnings? Also are you setting a new value? WPF knows when you try to set a value that is already set and ignores it. I would suggest thange the Metadata type to FrameworkPropertyMetadata and supply a proper default value.

To give this “comment” more value: Adding “PresentationTraceSources.TraceLevel=High” on the binding gives alot more information about how the binding tries to get its value, which also helps alot, finding problems that are not errors for WPF.

<TextBox Text="{Binding MyText, PresentationTraceSources.TraceLevel=High}"/>