Unsubscribe EventAggregator events in ViewModels

Question

I start using WPF with PRISM and MVVM. One problem I am facing is that I can’t find a good place / best practice to unsubscribe EventAggregator events formerly subscribed in the ViewModel. The following solution - calling Unsubscribe in the destructor – is much too late. It is just running with the next garbage collection.

public class ViewModel : ViewModelBase
{
    public ViewModel()
    {
        var eventAggregator = ServiceLocator.Current.GetInstance<IEventAggregator>();
        eventAggregator.GetEvent<SeriesSelectionChangedEvent>().Subscribe(OnSeriesSelectionChanged);
    }

    ~ViewModel()
    {
        var eventAggregator = ServiceLocator.Current.GetInstance<IEventAggregator>();
        eventAggregator.GetEvent<SeriesSelectionChangedEvent>().Unsubscribe(OnSeriesSelectionChanged);
    }

    void OnSeriesSelectionChanged(SeriesSelectionChangedEventArgs e)
    {
    }
}

Answer 1

It's up to you! If your application can notify ViewModel when it is no longer needed, so you should unsubscribe there.

For example, in our project we have IViewDisposeService. If view (or its model) needs deterministic finalization, it registers itself in IViewDisposeService when showing. Then the Core use the same service to notify registered views when they've been removed from regions.

Another way is to use commands. Your model expose command which must be invoked by a view when it is closing. ViewModel may use command handler to unsubscribe.

By the way, if you worry that EventAggregator will hold your ViewModel, it's not a problem, because Prism's EventAggregator use weak references.