How to bind usercontrol to applications viewmodel - data-binding

Question

I am using the standard pivot template in my WP7 app.
I have the MainViewModel class defined with a few extra properties:
public class MainViewModel : INotifyPropertyChanged
{
...
private MyClass selectedKey_m;
public MyClass SelectedKey
{
get
{
...
}
set
{
if (value != this.selectedKey_m)
{
this.selectedKey_m = value;
NotifyPropertyChanged("SelectedKey");
}
}
}
}
The App class has a view model instance:
private static MainViewModel viewModel = null;
public static MainViewModel ViewModel
{
get
{
// Delay creation of the view model until necessary
if (viewModel == null)
viewModel = new MainViewModel();
return viewModel;
}
}
My MainPage.xaml.cs sets the DataContext:
DataContext = App.ViewModel;
From here, I can set up two way bindings on ListBoxes and I know it works because if I put a breakpoint on the SelecetdKey property in my viewmodel I can see the setter get called.
My problem is that I have my own user control, with a bindable property, bound to the SelectedKey property of the view model, but the property in my user control never gets set when the viewmodel gets updated and I can't figure out why.
Here is my user control:
public partial class MyUserControl : UserControl
{
public static readonly DependencyProperty SelectedKeyProperty = DependencyProperty.Register(
"SelectedKey", typeof(MyClass), typeof(MyUserControl), new PropertyMetadata(null));
public MyClass SelectedKey
{
get { return (MyClass)this.GetValue(SelectedKeyProperty); }
set { this.SetValue(SelectedKeyProperty, value); }
}
}
And here is the xaml in my main page:
<local:MyUserControl x:Name="myUC" SelectedKey="{Binding Path=SelectedKey}">
I would expect that the setter for the SelectedKey property of my user control to get called when the SelectedKey property of the view model gets changed, but it doesn't.
I've also tried setting the datacontext of my user control in the xaml:
DataContext="{Binding Path=App.ViewModel}"

Answer 1

The debugger does not step into the setter, don't know why.
Try adding a callback invoked on property value changes :
public static readonly DependencyProperty SelectedKeyProperty = DependencyProperty.Register(
"SelectedKey", typeof(MyClass), typeof(MyUserControl), new PropertyMetadata(MyPropertyChanged));
private static void MyPropertyChanged( object sender, DependencyPropertyChangedEventArgs args)
{
}

Answer 2

Solved. I had to add the static method as ptauzen suggested, but also remove the DataContext binding statement from my xaml :
DataContext="{Binding Path=App.ViewModel}"
Because the MainPage sets the datacontext in the constructor, so because my user control is a child of the main page, it inherits the data context. All I needed was to ensure the binding of my user controls properties were set up:
SelectedKey="{Binding SelectedKey}"