Communication between two viewmodels

Question

I\'m newbie in MVVM design pattern, and I have these viewmodels :

ClassAViewModel

public class ClassAViewModel : INotifyPro         


        

Answer 1

I agree with other commenters that the mediator/pub-sub/event aggregator/messenger is a good way to go. If you're not using an MVVM framework with a built-in solution, then I recommend this simple approach that takes advantage of the Reactive extensions:

public class EventPublisher : IEventPublisher
{
    private readonly ConcurrentDictionary<Type, object> subjects
        = new ConcurrentDictionary<Type, object>();

    public IObservable<TEvent> GetEvent<TEvent>()
    {
        var subject =
            (ISubject<TEvent>) subjects.GetOrAdd(typeof (TEvent),
                        t => new Subject<TEvent>());
        return subject.AsObservable();
    }

    public void Publish<TEvent>(TEvent sampleEvent)
    {
        object subject;
        if (subjects.TryGetValue(typeof(TEvent), out subject))
        {
            ((ISubject<TEvent>)subject)
                .OnNext(sampleEvent);
        }
    }
}

That's your whole event aggregator. Pass an instance of it into each view model, and store it as a reference. Then create a class to store your event details, let's say "ValueChangedEvent":

public class ValueChangedEvent
{
    public int Value
    {
        get { return _value; }
    }
    private readonly int _value;

    public ValueChangedEvent(int value)
    {
        _value = value;
    }
}

Publish like this from the first view model:

set
{
    _nbre = value;
    PropertyChanged(this, new PropertyChangedEventArgs("Nbre"));
    _eventPublisher.Publish(new ValueChangedEvent(value));
}

Subscribe in the other class using GetEvent:

public class ClassBViewModel: INotifyPropertyChanged, IDisposable
{
    private readonly IDisposable _subscriber;

    public ClassBViewModel(IEventPublisher eventPublisher)
    {
        _subscriber = eventPublisher.Subscribe<ValueChangedEvent>(next => 
        {
            IsBiggerthanFive = next.Value > 5;
        });
    }

    public void Dispose()
    {
        _subscriber.Dispose();
    }
}

Answer 2

A messenger service is a solution. MVVM Light Toolkit has an implementation of this. What you can do with it, is listen to a specific type of message in your viewmodel and handle it through the messenger. http://www.mvvmlight.net/