MVVM setting default values

Question

Hi,

I'm sure this is a pretty common scenario, and I would like to know how MVVM developers tackle this.

I have a ViewModel that is instantiated on demand, and persists until it is explicitly removed by the user. It's corresponding View is loaded onto the UI on demand. The View is able to unload and it's ViewModel may still exists in the application.

In my scenario, I have a ListBox of preset colors in the View(by setting it's ItemsSource to a Xaml-defined ObservableCollection of SolidColorBrush).

I bound the ListBox's SelectedItem property to a property in the ViewModel so that when the View is to be loaded again, the SelectedItem correctly shows the last selected item in the ListBox and also when the user selects a different color, the VM will handle the change.

My question is, how would u set the default value, say the third item in the ObservableCollection of SolidColorBrush to the ViewModel when the View is first loaded?

Answer 1

typically In MVVM your ObservableCollection of SolidBrushColor should be defined as a property in ViewModal. and this property should be bound to your ListBox. Now, to set default value you can have it in your ViewModal's constructor

Answer 2

If the property bound to SelectedItem is declared as dependency property then you can set the default value of the dependency propery to whatever you want.

 public static readonly DependencyProperty SelectedColorProperty
   = DependencyProperty.Register("SelectedColor", typeof(SolidBrushColor ), 
                                                typeof(<yourviewmodel>),
                                                new UIPropertyMetadata(GetDefaultColor()));

  Where GetDefaultColor() is a static method in your ViewModel, 
  which will return the required color from your ObservableCollection .

Answer 3

I believe you error is in your implementation. The reason to have MVVM is to have a "separation of concerns". That makes your view just an implementation, that can get switched or updated out if/when the need arises. Once you start putting stuff in your view that is part of the application logic, you are traveling down a path of a maintenance headache, and then spaghetti code can quickly ensue.

Some people say, "Don't put any code in your view", I agree 99% of the time. I say "Don't put any domain/application/business logic in your view."

Whenever you're trying to put some code into your view ask yourself "If I switched from WPF to another framework would my app still work?" If the answer is no, then modify your ViewModel to incorporate what you were trying to put in your view.