mvvm update calculated fields - wpf

Question

Do you know best practices in wpf+mvvm to update Calculated fields?
What I can do instead OnPropertyChanged(nameof(Summary))?
Also calculated field can be in another viewmodel and this viewmodel should not know about all dependences.
This is my code :
public class Model
{
public int Prop1 { get; set; }
public int Prop2 { get; set; }
public int Summary => Prop1 + Prop2;
}
public class ViewModel : BaseViewModel
{
public Model Model { get; }
public int Prop1
{
get
{
return Model.Prop1;
}
set
{
Model.Prop1 = value;
OnPropertyChanged();
OnPropertyChanged(nameof(Summary));
}
}
public int Prop2
{
get
{
return Model.Prop2;
}
set
{
Model.Prop2 = value;
OnPropertyChanged();
OnPropertyChanged(nameof(Summary));
}
}
public int Summary => Model.Summary;
}

Answer 1

Calling OnPropertyChanged on the calculated property is perfectly acceptable. If you have a relatively simple model like the one you wrote that'll be enough.
If you have multiple calculated properties on the model, you might consider creating a method to call all of them from a single place, instead of calling each one from every property.
Something like this:
public int Prop1
{
get
{
return _prop1;
}
set
{
_prop1 = value;
OnPropertyChanged();
NotifyCalculatedProperties();
}
}
public int Calc1 { get { /* ... */ } }
public int Calc2 { get { /* ... */ } }
public int Calc3 { get { /* ... */ } }
public void NotifyCalculatedProperties()
{
OnPropertyChanged(nameof(Calc1));
OnPropertyChanged(nameof(Calc2));
OnPropertyChanged(nameof(Calc3));
}
In case the calculated properties exist in a different model, you can register in that Model\VM to the source's PropertyChanged event, and then invoke the change notification there.
Like that:
void ModelPropertyChanged(object sender, PropertyChangedEventArgs e)
{
if(e.PropertyName == "Prop1")
OnPropertyChanged(nameof(Calc1));
OnPropertyChanged(nameof(Calc2));
OnPropertyChanged(nameof(Calc3));
}
Just don't forget to unsubscribe when the Model\VM changes, or you'll have a memory leak on your hands.
Lastly, you can always use the Messenger to pass messages between unrelated VMs, though you should use caution since it's a very powerful tool, and can easily be misused.
I don't know what MVVM framework you're using, but each has it's own implementation. You can find more general details on the Messenger pattern here: https://msdn.microsoft.com/en-us/magazine/jj694937.aspx