Cant Keep track of Adding/Removing an Item to/from a List in CustomControl WPF

Question

I have a ObservationCollection in my CustomControl's class.

 public static readonly DependencyProperty ItemsProperty = DependencyProperty.Register("Items", typeof(ObservableCollection<Draggable>), typeof(Draggable), new PropertyMetadata(new ObservableCollection<Draggable>(), OnItemsChanged));

    private static void OnItemsChanged(DependencyObject obj, DependencyPropertyChangedEventArgs e)
    {
        Draggable draggable = (Draggable)obj;

        if (e.NewValue != null)
        {
            if (((ObservableCollection<Draggable>)e.NewValue).Count > 0)
                draggable.HasItem = true;
            else
                draggable.HasItem = false;
        }
        else
        {
            draggable.Items = new ObservableCollection<Draggable>();
            draggable.HasItem = false;
        }
    }

and By default it has assigned by a new ObservableCollection<Draggable>.

I cant keep track of adding or removing the Items to/from this collection.

OnItemsChanged will fires whenever i assign a new ObservableCollection to this property. so Adding or removing the Items will not raise anything for me.

I have another property called HasItem. In my ControlTemplate i will show a Grid Whenever this property is True but it will never be True.

How can i solve this problem ?

EDIT:

My Main Class is Draggable and the ObservableCollection is the same in type. I have tried two solution:

1. I have written a ValueConverter:

   [ValueConversion(typeof(ObservableCollection), typeof(Visibility))] public class IntToVisibilityConverter : IValueConverter { public static IntToVisibilityConverter Instance = new IntToVisibilityConverter();

   public object Convert(object value, Type targetType, object parameter, CultureInfo culture)
   {
       ObservableCollection<Draggable> Source = (ObservableCollection<Draggable>)value;
       if (Source.Count > 0)
           return Visibility.Visible;
       else
           return Visibility.Collapsed;
   }
   
   public object ConvertBack(object value, Type targetType, object parameter, CultureInfo culture)
   {
       throw new NotImplementedException();
   }
   

   }

and i used it in this way:

 <Border x:Name="ItemsContainer" Visibility="{Binding Path=Items,Converter={x:Static MyConverters:IntToVisibilityConverter.Instance}}" ... >

2. And I have written a constructor for my class and I added CollectionChanged event:

   public Draggable()
   {
       Items.CollectionChanged += Items_CollectionChanged;
   }
   
   private void Items_CollectionChanged(object sender, System.Collections.Specialized.NotifyCollectionChangedEventArgs e)
   {
       if (Items.Count > 0)
           HasItem = true;
       else
           HasItem = false;
   }
   

both of them will throw in an Infinite Loop As VisualStudio Says. I can't Understand this because I cant see the loop it says.

Answer 1

The only time you are setting HasItem is when you are setting the dependency property. I'm guessing you want it to recalculate that value when the collection's contents changes (e.g. by adding or removing items).

If you only need this from Xaml, then I wouldn't bother with the HasItem property: just bind the Visibility of the Grid to the Count property of the DP, using a ValueIsNotZeroConverter, which converts an int to a bool.

Answer 2

OnItemsChanged will fires whenever property value changes(in case of ObservableCollection,if new value assign to the ObservableCollection property). For getting notification about the Adding or removing of the Items on ObservableCollection, we have to register CollectionChanged event of the ObservableCollection class. For example

private static void OnItemsChanged(DependencyObject obj, DependencyPropertyChangedEventArgs e)
        {
            Draggable draggable = (Draggable)obj;

            if (e.NewValue != null)
            {
                if (((ObservableCollection<Draggable>)e.NewValue).Count > 0)
                {
                    draggable.HasItem = true;
                    (ObservableCollection<Draggable>)e.NewValue).CollectionChanged += CollectionChanged;
                }
                else
                    draggable.HasItem = false;
            }
            else
            {
                draggable.Items = new ObservableCollection<Draggable>();
                draggable.HasItem = false;
            }
        }
private void CollectionChanged(object sender, System.Collections.Specialized.NotifyCollectionChangedEventArgs e)
        {

        }