使用PRISM的WPF级联组合框

为了简单起见，我的视图中有两个组合框。我希望在遵循MVVM设计模式和PRISM 6框架的同时将它们级联起来。在Brian Lagunas MVVM/Prism网络研讨会之后，他提到了DelegateCommand的ObservesProperty。我有一种预感，我应该用它，但不确定。我提供了一个非常简单的视图和视图模型，我不能得到工作。

XAML(视图):

<ComboBox ItemsSource="{Binding PackageNames}" SelectedItem="{Binding PackageNameSelected}" />
<ComboBox ItemsSource="{Binding PackageOptions}"/>

ViewModel:

public MainWindowViewModel() // Constructor
{
    PackageNamesCommand = new DelegateCommand(PackageNamesCommandExecute);
    PackageOptionsCommand = new DelegateCommand(PackageOptionsCommandExecute).ObservesProperty(() => PackageNameSelected);
}
private string _packageNameSelected;
public string PackageNameSelected
{
    get { return _packageNameSelected; }
    set { SetProperty(ref _packageNameSelected, value); }
}
private ObservableCollection<string> _packeNames;
public ObservableCollection<string> PackageNames
{
    get { return _packeNames; }
    set { SetProperty(ref _packeNames, value); }
}
private ObservableCollection<string> _packageOptions;
public ObservableCollection<string> PackageOptions
{
    get { return _packageOptions; }
    set { SetProperty(ref _packageOptions, value); }
}
public DelegateCommand PackageNamesCommand { get; set; }
private async void PackageNamesCommandExecute()
{
    await Task.Factory.StartNew(() => GetPackageNames()).ContinueWith(t => PackageNames = ToObservableCollection(t.Result));
}
private IQueryable<string> GetPackageNames()
{
    return //My LINQ Query Here
}
public DelegateCommand PackageOptionsCommand { get; set; }
private void PackageOptionsCommandExecute()
{
    Task.Factory.StartNew(() => GetPackageOptions()).ContinueWith(t => PackageOptions = ToObservableCollection(t.Result));
}
private IQueryable<string> GetPackageOptions()
{
    return // My LINQ query here
}