WPF accessing code-behind from view models

Question

Using the NotifyIcon project to show balloons in my c# app.

I want a trigger from the program to be displayed as a popup/balloon when it occurs. The problem is that I can only see how to show a balloon using the .ShowCustomBalloon method in code-behind of the .xaml.cs file, which doesn't have context in my ViewModel. The examples in the project work because they use code-behind to show the balloon contents.

What I need is a way to have an event listener on the .xaml file that can trigger this show balloon, that is bound to a property of the viewModel. Problem is my experience in c# app dev. is not great and I am wondering how to go about this.

Answer 1

In code-behind (the View) you should subscribe for property changed event (or some other event, it really depends on your ViewModel implementation).

Here is sample ViewModel:

public class SomeViewModel : INotifyPropertyChanged
{
  public event PropertyChangedEventHandler PropertyChanged;

  private string someProperty;
  public string SomeProperty
  {
      get { return someProperty; }
      set
      {
        someProperty = value;
        OnPropertyChanged();
      }
  }

  protected virtual void OnPropertyChanged([CallerMemberName] string propertyName = null)
  {
    PropertyChanged?.Invoke(this, new PropertyChangedEventArgs(propertyName));
  }
}

Here is sample View:

public partial class SomeView : Window
{
    public DiscImageView()
    {
        //  ...

        viewModel.PropertyChanged += ViewModel_PropertyChanged;
    }

    private void ViewModel_PropertyChanged(object sender, PropertyChangedEventArgs e)
    {
        if (e.PropertyName == nameof(SomeViewModel.SomeProperty))
        {
            //  Logic for changed property event
        }
    }
}

Also consider using of some MVVM library or framework, they simplify life a lot. I suggest MVVMLight, besides other it has messaging capabilities.