Binding to an initially NULL property in the ViewModel doesn't rebind - wpf

Question

I have a UserControl that contains a Telerik RadDataForm. The form's ItemsSource is bound to a property on the UserControl's ViewModel:
<telerik:RadDataForm
ItemsSource="{Binding Path=viewModel.items, RelativeSource={RelativeSource AncesterType=local:MyUserControl}}"
/>
Where viewModel is:
public partial class MyUserControl: UserControl
{
public MyUserControlVM viewModel
{ get { return this.DataContext as MyUserControlVM; } }
}
Within the viewmodel, items is a fairly ordinary collection:
public class MyUserControlVM : MyViewModelBase
{
private ObservableCollection<AnItem> items_;
public ObservableCollection<AnItem> items
{
get { return this.items_; }
set
{
this.items_ = value;
notifyPropertyChanged("items");
}
}
...
}
And where, of course, MyViewModelBase implements INotifyPropertyChanged.
The user control has an items dependency property, and when it is set, it sets the matching property on the view model:
public partial class MyUserControl : UserControl
{
public ObservableCollection<AnItem> items
{
get { return GetValue itemsProperty as ObservableCollection<AnItem>; }
set { SetValue(itemsProperty, value); }
}
public static readonly DependencyProperty itemsProperty =
DependencyProperty.Register("items",
typeof(ObservableCollection<AnItem>),
typeof(MyUserControl), new PropertyMetadata(
new PropertyChangedCallback(itemsPropertyChanged)));
private static void itemsPropertyChanged(DependencyObject d,
DependencyPropertyChangedEventArgs e)
{
MyUserControl myUserControl = d as MyUserControl;
ObservableCollection<AnItem> items =
e.NewValue as ObservableCollection<AnItem>;
if (myUserControl != null && myUserControl.viewModel != null)
myUserControl.viewModel.items = items;
}
}
All of which seems pretty straightforward, if a bit tedious.
The problem is that the items dependency property on MyUserControl is bound to a property of the current item of another collection, and that the current item is initially null, and so when MyUserControl is initially loaded, its items property is null. And hence, so is the items property on MyUserControlVM that the RadDataForm is binding to.
Later, when an item in that outer collection is made current, the items dependency property on MyUserControl is set, and that sets the items property on MyUserControlVM. And MyUserControlVM calls notifyPropertyChanged so that listeners will be informed of the change. But this last is not working.
Afterwards, if I examine RadDataForm, its ItemsSource property is still null.
It's like the RadDataForm isn't listening for the propertychanged event, because what it was bound to was initially null. In similar circumstances where the bound property is not null at the start, this pattern works fine as the current item changes from one item to another, but it doesn't seem to work from having no current item to having one.
So, any ideas as to how to make this work? I can't, given the circumstances, make it so that items always has a value when the form loads - it is always going to be null, at the start. How do I get the RadDataForm to notice when the property becomes non-null?

Answer 1

When I want to reference something at the root of my UserControl (a custom property, for instance, or as in your case, the DataContext), I usually give my UserControl a Name. Then I use this name together with the ElementName property on the Binding to set it up.
<UserControl
...
Name="TheControl">
<Grid>
<TextBlock Text={Binding Path=DataContext.items, ElementName=TheControl}" />
</Grid>
</UserControl>
Due to the viewModel property, you can use that and DataContext interchangeably.
However, in your case it might actually be simpler. First, there's a typo in your code. It should be AncestorType (with an 'o'). Second, you might want to try setting up the binding using only {Binding Path=items} since I believe that your control already inherits the correct DataContext. (Not sure about that last one, though.)
If the problem persists, and you suspect that it does in fact has something to do with the items property returning null initially, you could always initialize the items_ with an empty collection to avoid the null.
private ObservableCollection<AnItem> items_ = new ObservableCollection<AnItem>();