Question

I have an Objectmodel with some nested Collections:

public class MainViewModel :  BaseViewModel
{
    private Dictionary<String, Diagram> _diagrams;

    public Dictionary<String, Diagram> Diagrams
    {
        get { return _diagrams; }
        set
        {
            _diagrams = value;
            OnPropertyChanged("Diagrams");
        }
    }

}

Base Viewmodel implements INotifyPropertyChanged. Inside Diagram there is a Collection of Curves:

public class Diagram
{
    private  ObservableCollection<DiagramCurve> curves;
    [JsonIgnoreAttribute]
    public ObservableCollection<DiagramCurve> Curves
    {
        get { return curves; }
    }

    public Diagram()
    {
        curves = new ObservableCollection<DiagramCurve>();
    }
}

I bind to an instance of DiagramCanvas in my Xaml like this:

    <wd:DiagramCanvas x:Name="diagramCanvas" 
                          Diagram ="{Binding Diagrams[Diagram1]}"  
                          Grid.Row="2" Grid.Column="1" 
                          Height="auto" Width="Auto"  
                          HorizontalAlignment="Stretch"  
                          VerticalAlignment="Stretch" 
                          Margin="10"   
                          Background="{StaticResource DiagrambackgroundBrush }" />

which works fine if I assign the the Diagrams property of MainView a completely new Diagrams collection. But what I need is that the DiagramCanvas Control get’s updated when the Curves Collection changes. But this does not happen.

Answer 1

I tried to inherit Diagram from an ‘ObservableCollection’, but even this didn’t help as in ObservableCollection dependency property does not update when item in collection is deleted explained.

As the DependencyProperty of my CustomControl DiagramCanvas binds to a value item of the Dictionary MainViewModel.Diagrams not to a property of a Diagram instance, no OnPropertyChanged event of the DependencyProperty of DiagramCanvasis fired, when properties of the Diagram instance change.

Only when a new value is assigned to the value item inside the Dictionary MainViewModel.Diagrams, this event gets fired, beacause this assigns a new value to the DependencyProperty of DiagramCanvas.

Therefore I have to subscribe to the events of the collection manually:.

public class DiagramCanvas : Canvas
{
  // Is called only when new Items get inserted info the Dictionary
  static void OnDiagramPropertyChanged(DependencyObject obj, 
                                     DependencyPropertyChangedEventArgs args)
  {
      DiagramCanvas THIS = (DiagramCanvas)obj;

      Diagram new_diagram = (Diagram)args.NewValue;
      Diagram old_diagram = (Diagram)args.NewValue;

      if (old_diagram != null)
      {
        // Unsubscribe from CollectionChanged on the old collection
        old_diagram.Curves.CollectionChanged -= THIS.OnCollectionChanged;
      }

      if (new_diagram != null)
      {
        // Subscribe to CollectionChanged on the new collection
        new_diagram.Curves.CollectionChanged += THIS.OnCollectionChanged;
      }
  }

  //Is called everytime the Curves collecion changes
  void OnCollectionChanged(object sender, NotifyCollectionChangedEventArgs e)
  {

  }
}