Question

I have a custom UserControl who’s position is bound to parameters in a ViewModel.

<Canvas Name="CursorCanvas">
    <ctrl:HandCursor x:Name="RightHandCursor" State="{Binding RightHandState}" Visibility="{Binding IsHandTracking, Converter={StaticResource BoolToVisibilityConverter}}" Canvas.Top="{Binding RightHandY}" Canvas.Left="{Binding RightHandX}" />
</Canvas>

Inside of this control I am performing a HitTest, to determine what is below it. Currently this HitTest is hooked up to a DispatcherTimer ta 1-second intervals. What I’d rather do tell the control to perform the HitTest when it moves on the Canvas, but I am unable to find how to capture the Canvas position updates in the UserControl that is being moved.

How can I capture the Canvas.Top/Left movement updates in the code-behind for the “HandCursor” UserControl defined above?

Many thanks!

Answer 1

You could add a PropertyChangedCallback by overriding the property metadata of the Canvas.Left and Canvas.Top attached properties:

static HandCursor()
{
    Canvas.LeftProperty.OverrideMetadata(
        typeof(HandCursor),
        new FrameworkPropertyMetadata(
            (o, e) => ((HandCursor)o).LeftPropertyChanged((double)e.NewValue)));

    Canvas.TopProperty.OverrideMetadata(
        typeof(HandCursor),
        new FrameworkPropertyMetadata(
            (o, e) => ((HandCursor)o).TopPropertyChanged((double)e.NewValue)));
}

private void LeftPropertyChanged(double left)
{
    ...
}

private void TopPropertyChanged(double top)
{
    ...
}