When is a reference to the viewmodel available

[drei97]

I am working on a MVVM Avalonia application and got stuck.

My Mainwindow has to set an event handler

public partial class MainWindow : Window
{
    private ViewModels.MainWindowViewModel _vm => (ViewModels.MainWindowViewModel)DataContext!;

    public MainWindow()
    {
        InitializeComponent();
        _vm.Eventhandler += (s, e) => { ... }
    }

}

the Problem I have is, that DataContext and therefore _vm is null. The DataContext is set in the file App.axml.cs and is available not before the constructor of MainWindow has been finished.

Where to put _vm.Eventhandler += (t, e)=>{ ... }?

Joachim

[tobyfirth]

Override OnPropertyChanged. That would also allow you to unsubscribe the event handler if you changed the viewmodel.

protected override void OnPropertyChanged(AvaloniaPropertyChangedEventArgs change)
{
    base.OnPropertyChanged(change);

    if (change.Property == DataContextProperty)
    {
        // your code here
    }
}

There is also a DataContextChanged event but it does not give you the old value.

[drei97]

It is working

thnx

[dmitry-gr]

I believe that DataContextChanged event is designed for this purpose

  public partial class MainView : UserControl
  {
      public MainView()
      {
          InitializeComponent();
          this.DataContextChanged += (t, e)=>{
              if (DataContext is MainViewModel model)
              {
                 //TODO: Initialize the model or perform any setup needed
              }
          };
      }
      
  }

[stevemonaco]

When it's available depends on the pattern(s) you're using. You're responsible for wiring the VM to DataContext in most cases (besides a few like ContentControl or DataTemplate projections).

I basically use the same as private ViewModels.MainWindowViewModel _vm => (ViewModels.MainWindowViewModel)DataContext!;

I found the extra code overhead of DataContextChanged and null-checking to provide zero value. I have a pattern that the VM is available soon after construction and before the control is attached. So I only access it from event handlers that happen afterwards. eg. AttachedToLogicalTree, AttachedToVisualTree, Loaded, etc. If it fails, then I want it to fail hard because my design has been broken.