Question

I have a MVVM application and somewhere in the application our company use a Third-Party that cannot use {Binding}. It’s a component that draw shapes, etc. What I want it, when the ViewModel load from the persisted storage all shapes to notify the View to draw them. In a perfect world I would just have the take the Third-party and bind it to the ViewModel Shapes collection but I cannot.

From there, my idea was that I could get from the View the ViewModel (via the DataContext) and to hook the PropertyChanged event. The problem is that the DataContext is not yet initialized in the constructor, so it’s NULL, and I cannot hook the event. Here is a sample of the code:

        public CanvasView()
        {
            InitializeComponent();
            ((CanvasViewModel)this.DataContext).PropertyChanged += new PropertyChangedEventHandler(CanvasView_PropertyChanged); //Exception Throw here because DataContext is null
        }

        void CanvasView_PropertyChanged(object sender, PropertyChangedEventArgs e)
        {
            if (e.PropertyName == "Shapes")
            {
                DrawShapes(); 
            }
        }

How can I get information from my ViewModel to my View in that case?

Answer 1

All of the answers so far breaks the MVVM pattern with having code-behind on the view. Personally I would wrap the 3rd party control in a UserControl and then wire up a few dependency properties with property change events.

C#

public partial class MyWrappedControl : UserControl
{
  public static readonly DependencyProperty ShapesProperty = DependencyProperty.Register("Shapes", typeof(ObservableCollection<IShape>), typeof(MyWrappedControl),
      new PropertyMetadata(null, MyWrappedControl.OnShapesPropertyChanged);

  public ObservableCollection<IShape> Shapes
  {
    get { return (ObservableCollection<IShape>)GetValue(ShapesProperty); }
    set { SetValue(ShapesProperty, value); }
  }

  private static void OnShapesPropertyChanged(DependencyObject o, DependencyPropertyChangedEventArgs e)
  {
    ((MyWrappedControl)o).OnShapesPropertyChanged(e);
  }

  private void OnShapesPropertyChanged(DependencyPropertyChangedEventArgs e)
  {
    // Do stuff, e.g. shapeDrawer.DrawShapes(); 
  }
}

XAML

<UserControl 
    Name="MyWrappedControl"
    x:Class="MyWrappedControl"
    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation">
    <!-- your control -->
    <shapeDrawerControl x:Name="shapeDrawer" />
</UserControl>