MVVM, Multidimensional property, reflect change of element - mvvm

Question

I have an application in MVVM style. There is a custom control in there with a dependency property holding an ObservableCollection of A. A has a ObservableCollection of B. A and B implement INotifyPropertyChanged.
When I add an object of B to A in my ViewModel, the changes will not be propagated to the control. The binding in xaml is correct, because the Bs are displayed and Mode is TwoWay.

Answer 1

This is normal behaviour, as the control only listens on INotifyPropertyChanged and not on ICollectionChanged. The collection property itself does not change and, therefore, the control does not know that it has to refresh.
In order to pass the collection changes to the control you will have to subscribe to the CollectionChanged event of you ObservableCollection, and then raise the the property changed event for collection property when the collection is changed (items added, removed, moved, or the collection is cleared).
#region [BViewModelCollection]
/// <summary>
/// The <see cref="BViewModelCollection" /> property's name.
/// </summary>
public const string BViewModelCollectionPropertyName = "BViewModelCollection";
private ObservableCollection<BViewModel> _bViewModelCollection = new ObservableCollection<BViewModel>();
/// <summary>
/// Gets the BViewModelCollection property.
/// TODO Update documentation:
/// Changes to that property's value raise the PropertyChanged event.
/// This property's value is broadcasted by the Messenger's default instance when it changes.
/// </summary>
public ObservableCollection<BViewModel> BViewModelCollection {
get {
return _bViewModelCollection;
}
set {
if (_bViewModelCollection != value) {
SetBViewModelCollection(value);
RaisePropertyChanged(BViewModelCollectionPropertyName);
}
}
}
private void SetBViewModelCollection(ObservableCollection<BViewModel> value) {
if (_bViewModelCollection != null)
_bViewModelCollection.CollectionChanged -= this.BViewModelCollection_CollectionChanged;
_bViewModelCollection = value;
if (_bViewModelCollection != null)
_bViewModelCollection.CollectionChanged += this.BViewModelCollection_CollectionChanged;
}
private void BViewModelCollection_CollectionChanged(object sender, System.Collections.Specialized.NotifyCollectionChangedEventArgs e) {
RaisePropertyChanged(BViewModelCollectionPropertyName);
}
#endregion
You can now use the SetBViewModelCollection to set the ObservableCollection and automatically register the events correctly, without raising a PropertyChanged event for the collection - e.g. for usage in the constructor or a load data method.
When an item is now added to or removed from the collection your control should get notified that the collection propterty has been changed.
Note: Adjust the method names to your own framework.