Question

I have a ViewModel class that looks like this.

class MyViewModel : Screen
{
  public BindableCollection<MyObject> MyObjects { get; set; }

  private MyObject selectedObject;
  public MyObject SelectedMyObject
  {
    get { return selectedObject; }
    set
    {
      selectedObject = value:
      //some additional unrelated logic
    }
  }

  public void SaveObject()
  {
    //some logic
  }
  public bool CanSaveObject{
    get{
      //logic to determine if the selectedObject is valid
    }
  }

That is the relevant code. Now the problem.

MyObject is a class with three properties. In the View I have a ListView that is bound to the MyObjects collection, and three TextBoxes that are bound to the SelectedItem in the ListView.

When I fill in the textboxes, the related object gets changed in the Model, but I want to make sure that the object is in a valid state before you can save it. CanSaveObject has the necessary logic, but the problem is that is never gets called since I don’t have any oppurtunity to call NotifyOfPropertyChanged when the textboxes are filled since only the properties of selectedObject are called, and no properties on MyViewModel.

So the question is: Are there any good way to do this without making properties on the ViewModel that encapsulate the properties inside MyObject.

I have got it working if I make properties like these, and then bind to these instead of the SelectedItem directly in the view, but the viewmodel gets cluttered up in the hurry if hacks like this is the only way to do it. I hope it’s not 🙂

public string SelectedObjectPropertyOne{
  get{ return selectedObject.PropertyOne; }
  set{
    selectedObject.PropertyOne = value;
    NotifyOfPropertyChange(() => SelectedObjectPropertyOne);
    NotifyOfPropertyChange(() => CanSaveObject);
  }
}

Answer 1

ActionMessage.EnforceGuardsDuringInvocation is a static boolean field that can be set to enforce a guard check when an action is about to be invoked. This will guard the actual Save action from being invoked, however it will not help with the issue of the UI appearance based on the guard state immediately after an update to the selected model.

Without doing that, the only other modification I could suggest would be to create a VM type for MyObject model and move the validation and save logic there. This would also allow you to simplify your Views…