Update Parent View From Child MVVM

Question

I'm building a WPF MVVM application but I'm having problems updating a parent view from a child view model.

I call the doAction() method on my parent view model which updates a property and raises a PropertyChangeEvent. When this method is called from the MainViewModel everything works great however when I call the same method from my child view model the PropertyChangedEvent get's raised but the view doesn't update.

Example:

ChildViewModel()
{
    private ParentViewModel parent; 

    parent.doAction(); // Raised event but MainView doesn't update
}

ParentViewModel()
{
    public void doAction()
    {
        this.Property = true;
        OnPropertyChange("Property"); 
    }
}

My Views are created using XAML:

<MainView>
   <TabItem>
      <view:ChildView/>
   </TabItem>
</MainView>

Propery Change event is raised like so:

protected void OnPropertyChanged(string name)
{
    LOGGER.Info("Property Changed: " + name);
    PropertyChangedEventHandler handler = PropertyChanged;
    if (handler != null)
    {
        handler(this, new PropertyChangedEventArgs(name));
    }
}

My question is how do I get the parent view to listen and update to a property change event raised by a child view.

Edit:

Base Class:

public abstract class AbstractBaseViewModel : INotifyPropertyChanged
{

        public event PropertyChangedEventHandler PropertyChanged;
        public ICommand CloseCommand { get; set; }

        public AbstractBaseViewModel()
        {
            this.CloseCommand = new CloseCommand(this);
        }

        public void CloseWindow()
        {
            Application.Current.MainWindow.Close();
        }

        protected void OnPropertyChanged(string name)
        {
            LOGGER.Info("Property Changed: " + name);
            PropertyChangedEventHandler handler = PropertyChanged;
            if (handler != null)
            {
                handler(this, new PropertyChangedEventArgs(name));
            }
        }
}

Parent ViewModel:

public class ParentViewModel : AbstractBaseViewModel
{
        private Dispatcher dispatcher;
        private bool visible;

        public bool Visible
        {
            get { return visible; }
            set { visible= value; OnPropertyChanged("Visible"); }
        }

        public MainWindowViewModel()
        {
            this.dispatcher = Dispatcher.CurrentDispatcher;
            this.manager = manager;
        }

        public void ShowTab(){
            this.Visible = true;
        }
 }

Child View Model:

 public class ChildViewModel : AbstractBaseViewModel
 {

    private ParentViewModel parentVm;

    public GeneralViewModel(ParentViewModel vm)
    {
        this.parentVm= vm;
    }

    public void Command(){
        vm.ShowTab();
    }
 }

ParentView Xaml:

<TabItem Header="ViewWeWantToHide/Show" 
     Visibility="{Binding Visible,Converter={converter:BooleanToVisibilityConverter}}">
     <views:SomeOtherView/>
</TabItem>

<TabItem Header="ChildView Tab"> 
     <views:ChildView/>
</TabItem>

Answer 1

Without seeing all of your code, it'd be hard to guess what is causing your problem. However, I'd have to say that it is much more common in WPF to display view model instances in the UI and have WPF automatically display the relevant views, rather than displaying the views directly as you have. If you use view models rather than views then you'll have access to the child view model(s) from the parent view models.

If you did, or do have access to the child view model from the parent view model then I would advise that you use a simple delegate to 'pass your message' from the child view model to the parent view model instead. You can even define a parameter-less delegate and use it to just send a signal, rather than any property value.

You can declare a delegate, add a property of that type to your child UserControl and attach a handler in the main view model. Then you can call the delegate from your child UserControl and in the handler in the parent view model, you can call your method. For more information on this technique, please see my answer to the Passing parameters between viewmodels question (which answers a similar, but not exactly the same problem).

Answer 2

Thank you everyone for your assistance.

I found the issue which was unrelated to WPF and was actually a product of how I was setting the datacontext on my child views. In my parent window I was creating and registering a singleton instance of my ParentViewModel with the Unity container. This instance would then be injected in to all child views, the problem was the InitializeComponent method was being called before my parent view model was created and registered with the unity container meaning all child views were receiving completely a different instance.

Not working:

public MainWindow()
{

    InitializeComponent();

    if (DesignerProperties.GetIsInDesignMode(this))
    {
        this.DataContext = new ParentViewModel();
    }
    else
    {      
         IUnityContainer container = UnityFactory.Retrieve();

         ParentViewModel parentVM = container.Resolve<ParentViewModel>();
         container.RegisterInstance<ParentViewModel>(parentVM);

         this.DataContext = parentVM;

     }
}

Working:

public MainWindow()
{

    if (DesignerProperties.GetIsInDesignMode(this))
    {
        this.DataContext = new ParentViewModel();
    }
    else
    {     
         IUnityContainer container = UnityFactory.Retrieve();

         ParentViewModel parentVM = container.Resolve<ParentViewModel>();
         container.RegisterInstance<ParentViewModel>(parentVM );

         this.DataContext = parentVM;
     }

     /**
       * Initialize after registering parent VM and setting the datacontext
       */
     InitializeComponent();

  }