Question

I’m using Caliburn and the MVVM pattern in a WPF application, and am trying to use as many conventions as possible. One issue I’m running into is when I have to wire-up some property-changed notifications on an imported class.

Say I have a ViewModel like so:

class ViewModelA
{
    private readonly IViewModelB _b;

    public ViewModelA(IViewModelB b)
    {
        _b = b;
    }

    public bool CanGo
    {
        get { return _b.MyBoolProperty; }
    }

    public void Go()
    {
        //Does something here
    }
}

What is the recommended (correct) way to call NotifyOfPropertyChange(() => CanGo) when the PropertyChanged event for MyBoolProperty is fired off ViewModelB?

In the past I’ve used a PropertyObserver type class to manage this.

Or am I designing this scenario completely wrong?

Answer 1

If your “sub”-model is exposed with a public property, you could use DependenciesAttribute to track changes:

 
class ViewModelA
{
    public IViewModelB B {get; private set;}

    public ViewModelA(IViewModelB b)
    {
        B = b;
    }

    public bool CanGo
    {
        get { return B.MyBoolProperty; }
    }

    [Dependencies("B.MyBoolProperty")]
    public void Go()
    {
        //Does something here
    }
}

To work properly, the whole property path should be composed of notifying objects.
You can also put a final “*”

[Dependencies("B.*")]

to indicate that all properties of B should cause the precondition re-evaluation; note that “*” only acts on the end of the proprerty path and just for one level of depth (it doesn’t track changes on sub-models of B).