Why do I get the previous selected item from the SelectedItem Listbox?

Question

WPF

I have a listbox:

<ListBox ItemsSource="{Binding Notes}" 
                         HorizontalContentAlignment="Stretch" 
                         VerticalContentAlignment="Stretch"
                         Name="NoteList"
                         SelectedItem="{Binding SelectedNote}"
                         >
                    <ListBox.ItemTemplate>
                        <DataTemplate>
                            <StackPanel Orientation="Horizontal" Background="#FFD7B4F3" >
                                <TextBlock Text="{Binding Title}" FontSize="15" Margin="2"/>
                            </StackPanel>
                        </DataTemplate>
                    </ListBox.ItemTemplate>
                    <i:Interaction.Triggers>
                        <i:EventTrigger EventName="GotFocus">
                            <i:InvokeCommandAction Command="{Binding ActivatePanelCommand}" CommandParameter="{Binding SelectedNote}"/>
                        </i:EventTrigger>
                    </i:Interaction.Triggers>
                </ListBox>

ViewModel code:

class MainVM : INotifyPropertyChanged
    {
        static public ApplicationContext DB { get; private set; }
        IEnumerable<Note> notes;
        Note selectedNote;

        public IEnumerable<Note> Notes
        {
            get { return notes; }
            set
            {
                notes = value;
                OnPropertyChanged();
            }
        }

        public Note SelectedNote
        {
            get { return selectedNote; }
            set
            {
                selectedNote = value;
                OnPropertyChanged();
            }
        }
        
        public RelayCommand ActivatePanelCommand
        {
            get
            {
                return activatePanelCommand ??
                    (activatePanelCommand = new RelayCommand((selectedItem) =>
                    {
                        Note note = selectedItem as Note;
                        if(note != null)
                        {
                            MessageBox.Show(note.Title);
                        }
                    }));
            }

        public MainVM()
        {
            CurrentContentVM = new WriteNoteVM();
            DB = new ApplicationContext();
            DB.Notes.Load();
            Notes = DB.Notes.Local.ToBindingList();
        }

        public event PropertyChangedEventHandler PropertyChanged;
        public void OnPropertyChanged([CallerMemberName]string prop = "")
        {
            if (PropertyChanged != null)
                PropertyChanged(this, new PropertyChangedEventArgs(prop));

        }
    }

The problem is that from SelectedNote I get the previous selected item (and nothing is displayed on the first click, since SelectedNote is initially null). What to do?

Answer 1

I'm not an expert, but it's possible that the GotFocus event fires before the SelectedItem is assigned a value.
You can decide like this.
In addition to the interactivity library, include the interactions library:

xmlns:i="http://schemas.microsoft.com/expression/2010/interactivity"
xmlns:ei ="http://schemas.microsoft.com/expression/2010/interactions"

The event connection will look like this:

<i:Interaction.Triggers>
    <i:EventTrigger EventName="GotFocus">
        <ei:CallMethodAction MethodName="NoteList_GotFocus"
                             TargetObject="{Binding }"/>
    </i:EventTrigger>
</i:Interaction.Triggers>

viewmodel:

public void NoteList_GotFocus(object sender, RoutedEventArgs e)
{
    if (e.OriginalSource is ListBoxItem lvi)
    {
        Note note = lvi.DataContext as Note;
        MessageBox.Show(note.Title);
    }
}

Proof: