Question

Say, for instance, that I have a combobox whose itemsSource is bound to a dependency property. The bound list values can change depending on many factors. Calculating those values requires a non-trivial amount of time therefore I do not want the application to update the collection unless necessary. What is the correct way in MVVM to ‘lazily’ update a bound property only when the data is required in the view (i.e. when the combobox is dropping down)?

Answer 1

I would add a method to the ViewModel:

class ViewModel {
    public void Refresh() {
        // ...
    }

    // ...
}

… and call it when necessary in your control.

class Control : UserControl {
    private ViewModel ViewModel = new ViewModel();

    void OnDropDown() {
        ViewModel.Refresh();
    }

    // ...
}