MVVM Binding to InkCanvas

I seem to have ran into a road block. We are using MVVM with Prism and have a View that requires an Ink Canvas. I've create a StrokeCollection that is being bound from my ViewModel to the View. I am able to set the collection from my viewmodel but changes are not coming up to the ViewModel while the user draws. Is there a way to make this work?

My Property in my ViewModel is as follows:

private StrokeCollection _strokes;
public StrokeCollection Signature
{
     get
     {
         return _strokes;
     }
     set
     {
         _strokes = value;
         OnPropertyChanged("Signature");
     }
}

Here is my XAML binding line:

<InkCanvas x:Name="MyCanvas" Strokes="{Binding Signature, Mode=TwoWay}" />

For some reason apparently the InkCanvas never notifies the ViewModel of any change.

From stackoverflow

• The problem with your approach is that you assume the InkCanvas creates the StrokeCollection. It does not - it merely adds and removes items from it. And if the collection isn't available (ie. is null), the binding will fail and the InkCanvas won't do anything with it. So:

  1. You need to create a single StrokeCollection
  2. You need to assume the contents of the collection will change, not the collection itself

  Example code:

  public class ViewModel : INotifyPropertyChanged
  {
      private readonly StrokeCollection _strokes;
  
      public ViewModel()
      {
       _strokes = new StrokeCollection();
       (_strokes as INotifyCollectionChanged).CollectionChanged += delegate
       {
        //the strokes have changed
       };
      }
  
      public event PropertyChangedEventHandler PropertyChanged;
  
      public StrokeCollection Signature
      {
       get
       {
        return _strokes;
       }
      }
  
      private void OnPropertyChanged(string propertyName)
      {
       var handler = PropertyChanged;
  
       if (handler != null)
       {
        handler(this, new PropertyChangedEventArgs(propertyName));
       }
      }
  }
  

  And XAML:

  <InkCanvas Strokes="{Binding Signature}"/>
  

  HTH, Kent

  cjibo : I was missing the INotifyCollectionChanged section. I had everything else including instantiating the StrokeCollection. Thanks Kent.