Change Notification in MVVM Hierarchies

Question

Let\'s say in some abstract ViewModel base-class I have a plain-old property as follows:

public Size Size
{
    get { return _size; }
    set
    {
        _size         


        

Answer 1

A clean MVVM way would be to use a Messenger subscribe/notify mechanism (like in Josh Smith's MvvmFoundation)

Create a singleton Messenger object somewhere - the main App class is always a good place for this

public partial class App : Application
{
    private static Messenger _messenger;
    public static Messenger Messenger
    {
        get
        {
            if (_messenger == null)
            {
                _messenger = new Messenger();
            }
            return _messenger;
        }
    }
}

In the Size setter from the base class, notify changes:

public Size Size
{
    get { return _size; }
    set
    {
        _size = value;
        OnPropertyChanged("Size");

        App.Messenger.NotifyColleagues("SIZE_CHANGED");
    }
}

Now you can let your inherited ViewModel's listen for these changes, and raise PropertyChanged events as appropriate...

public MyViewModel : MyViewModelBase
{
    public MyViewModel()
    {
        App.Messenger.Register("SIZE_CHANGED", () => OnPropertyChanged("Rectangle"));
    }
}

Of course - you can add as many subscriptions to this message as you need - one for each property that needs changes to be notified back to the View...

Hope this helps :)