如何在Windows Phone 8 Silverlight中创建datacontextchanged事件

Question

I want to extent my UserControl class so that it could have datacontextchanged event in it, as it is in a Windows phone xaml application.

But I am unable to find that event.

This DataContextChangedEventArgs is in xaml based application in windows phone but when project is created in silverlight not able to find that event

Answer 1

[Disclaimer: I'm not a WindowsPhone developer, so everything I say about mobile Silverlight is based on the stuff I read on msdn (but never tried out myself) and the stuff I know from working with the ordinary fullfledged-browser Silverlight]

The DataContextChanged event is only supported in Silverlight 5. The mobile version of Silverlight (as far as I know) is based on Silverlight 4. That means you have to implement the event yourself. Just define a dependency property of type object in your UserControl and directly bind against the DataContext. As soon as the DataContext changes your bound object will reflect this change and therefore you can easily detect it and fire your own event.

public class MyControl : UserControl
{
    public MyControl()
    {
        BindingOperations.SetBinding( this, MyControl.ObservedDataContextProperty, new Binding() );
    }

    public object ObservedDataContext
    {
        get { return (object) GetValue( ObservedDataContextProperty ); }
        set { SetValue( ObservedDataContextProperty, value ); }
    }

    public static readonly DependencyProperty ObservedDataContextProperty =
        DependencyProperty.Register( "ObservedDataContext", typeof( object ), typeof( MyControl ), new PropertyMetadata( null, OnObservedDataContextChanged ) );

    private static void OnObservedDataContextChanged( DependencyObject d, DependencyPropertyChangedEventArgs e )
    {
        ((MyControl) d).OnObservedDataContextChanged();
    }

    private void OnObservedDataContextChanged()
    {
        RaiseDataContextChanged();
    }

    private void RaiseDataContextChanged()
    {
        var h = DataContextChanged;
        if (h != null) h( this, EventArgs.Empty );
    }

    public event EventHandler DataContextChanged;
}