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 = value;
        OnPropertyChanged("Size");
    }
}

I then create a more specific ViewModel, inheriting from the previous one, which contains the following property:

public Rect Rectangle
{
    get { return new Rect(0, 0, _size.Width, _size.Height); }
}

Now, in some View class I bind to the aforementioned ViewModel's Rectangle property. Everything works fine, until I change the size. When Size changes, Rectangle doesn't know about it and the change doesn't propagate to the View. And since Rectangle is in the child class, I can't simply add an OnPropertyChanged("Rectangle") to the Size setter.

Now imagine that I have many different properties like Rectangle, that all depend on base-class properties, and that none of these changes are being propagated. I need some lightweight and elegant way of chaining change notifications, preferably one that doesn't require a lot of code and doesn't force me into using dependency properties.

Obviously there are a lot of ugly solutions here- what I am looking for is something clean and clever. It seems to me this would be a very common scenario, and it seems to me there might be an MVVM-friendly way of doing this.

Answer 1

I recently blogged about this exact problem. I include a [DependsUpon("Size")] attribute with the Rectangle. I REALLY like this approach, because it keeps the dependency knowledge with the code that creates the dependency, not the other way around.

Take a look: http://houseofbilz.com/archive/2009/11/14/adventures-in-mvvm----dependant-properties-with-inotifypropertychanged.aspx

Answer 2

I use Josh Smith's PropertyObserver, which you can get from his MVVM Foundation library at http://mvvmfoundation.codeplex.com/.

Usage:

_viewmodel_observer = new PropertyObserver<OtherViewModel>(_OtherViewModel)
   .RegisterHandler(m => m.Size, m => RaisePropertyChanged(Rectangle);

Brian's attribute approach is nice too. One thing I like about PropertyObserver is that I can execute arbitrary code; allowing me to check conditions which may make me avoid the raise or perform other actions all together.

Answer 3

You can simply override OnPropertyChanged in the derived ViewModel like so:

protected override void OnPropertyChanged(string propertyName) {
    base.OnPropertyChanged(propertyName);
    if (propertyName == "Size") {
        base.OnPropertyChanged("Rectangle");
    }
}

Another possibility... A while back I put together a pretty nice ViewModel base class that supports attributes on properties like:

[DependsOn("Size")]
public Rect Rectangle {
    get { new Rect(0,0,Size.Width, Size.Height); }
}

Then the ViewModel base class collects these DependsOnAttribute's at runtime and in its OnPropertyChanged method it basically just looks to see what other properties need to be invalidated when a property change occurs.

Answer 4

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 :)