WPF MVVM get UserControl actual position via a DependencyProperty - wpf

Question

I have a quite common design MVVM app: MainWindow has a ContentPresenter defined like this:
<ContentPresenter Grid.Row="1" Grid.Column="1"
Content="{Binding Path=CurrentViewModel}">
</ContentPresenter>
It uses DataTemplate and can switch Views:
<DataTemplate DataType="{x:Type vm:PlateEntireViewModel}">
<v:PlateEntireView/>
</DataTemplate>
PlateEntireView is a UserControl with PlateEntireViewModel as DataContext. Now - I want to have a property in PlateEntireViewModel, that would hold PlateEntireView actual position (Left,Top) inside the MainWindow. Can this be acieved? Is it possible to make some DependencyProperty and use it in PlateEntireView, like:
<Grid ext:CustomProperties.ActualPositionX="{Binding Path=ActualPositionX, Mode=OneWayToSource}">
</Grid>
Can anybody tell me if its the right way to try - and how to use it?

Answer 1

So the shortest answer for this is normally the ViewModel would not care about specific coordinates that are being displayed in the View. That being said it is possible to do this relatively simple.
All you need to do is set up an attached property that will retrieve the point from the left top corner of the screen
public static double GetXCoordinate(DependencyObject obj)
{
var fe = obj as FrameworkElement;
if (fe != null)
{
return (fe.PointToScreen(new Point())).X;
}
return -1;
}
public static void SetXCoordinate(DependencyObject obj, double value)
{
obj.SetValue(XCoordinateProperty, value);
}
// Using a DependencyProperty as the backing store for XCoordinate. This enables animation, styling, binding, etc...
public static readonly DependencyProperty XCoordinateProperty =
DependencyProperty.RegisterAttached("XCoordinate", typeof(double), typeof(CustomProperties), new PropertyMetadata(0.0));
Then you could have the you binding look as such
<local:control Grid.Column="1"
Grid.Row="1"
x:Name="cp"
local:CustomProperties.XCoordinate="{Binding XCoordinate, UpdateSourceTrigger=Explicit}"
/>
You will need to update explicitly though, since this binding will never fire off any change events. You can do that by hooking into a reasonable event in your view. For more information about that look here.