MVVM - binding to aggregated property

Question

I have the following view modes:

public class ViewModel: INotifyPropertyChanged
{
    public ObservableCollection<Item> Items { get; set; }
    ...
}
public class Item: INotifyPropertyChanged
{
    public SubItem A { get; set; }
    public SubItem B { get; set; }
    ...
}
public class SubItem: INotifyPropertyChanged
{
    public bool Valid { get; set; }
    ...
}

XAML:

<ListBox ItemsSource="{Binding Items}" ..>

If I want to display text "Valid item", if both A.Validand B.Validare equal true, then:

• I can do this with the logic in the view (element data template), for example, using visibility and an additional container:

  <Grid Visibility="{Binding A.Valid}" Converter=...>
      <TextBlock Text="Valid item" Visibility="{Binding B.Valid}" Converter=... \>
  </Grid>
  

• Or I can add a new property in the viewmodel item:

  public class Item: INotifyPropertyChanged
  {
      public bool Valid => A.Valid && B.Valid; // bind to this
      ...
  }
  

  The problem is that notifications of any of these SubItemwill not update the view.

In case (1), the binding will be subscribed to both events PropertyChanged: Itemand matches SubItem. In case (2), the binding only knows about the property Item.Valid, so I need to do something like:

public class Item: INotifyPropertyChanged
{
    SubItem _a;
    public SubItem A
    {
        get { return _a; }
        set
        {
            _a.PropertyChanged -= bla;
            _a = value;
            _a.PropertyChanged += bla;
            OnPropertyChanged(nameof(A));
            OnPropertyChanged(nameof(Valid));
        }
    }
    void bla(object sender, PropertyChangedEventArgs e) =>
        OnPropertyChanged(nameof(Valid));
    ...
}

It's horrible. So I prefer (1) (data triggers are sometimes used, but that doesn't matter).

, viewmodel (2), ?

Answer 1

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