How can I tell when the ItemsSource of my Custom control has changed

Question

I have a custom control in the early stages of development as I endeavour to learn about wpf custom control development. The custom control inherits from ItemsControls which gives me access to an ItemsSource property to which I am binding an enumerable collection.

Currently I have a simple two project solution comprising my custom control in one and a test project in the other to test the former. In my test project I have a simple mainwindow onto which I have put my custom control and bound its ItemsSource.

    <WpfControls:VtlDataNavigator x:Name="MyDataNavigator"
                                      ItemsSource="{Binding ElementName=MainWindow, Path=Orders}" />

In the loaded event of the main window (which implements INotifyPropertyChanged) I instantiate the Orders collection. The customcontrols gets initialised before the main window loads but I can see from examining the Live Visual Tree in visual studio that once the main form loads the custom controls Items Source property is indeed set to Orders. Now of course I'd actually like to count the orders and have my custom control display that (it's a simple data navigator so what I'm after is the record count). I know how to get the count but how do I know when the itemsSource has changed so that I can react to it and get the count. There's no itemsSourceChanged event that I can see.

I've seen this blog article, but I'm wondering if there is a more straightforward approach to this as it seems such an obvious thing to want to know about.

Answer 1

You can do that using OverrideMetaData.

Try this:

public class Class1 : ItemsControl
{
    static Class1()
    {
        ItemsSourceProperty.OverrideMetadata(typeof(Class1), 
        new FrameworkPropertyMetadata(null, OnItemSourceChanged));
    }

    private static void OnItemSourceChanged(DependencyObject d, 
                                            DependencyPropertyChangedEventArgs e)
    {
        Debug.WriteLine("Why you haz Changed me!");
    }
}