wpf Databinding using XAML not working - wpf

Question

I am developing a small application for learning purpose. I find that when I bind ItemControl's ItemSource to a ViewModel property in XAML, it doesn't work in an expected way. i.e. It loads the underlying collection with values at the loading time, but any changes to it are not reflected.
However, if I set Itemsource in Codebehind, it works.
When the form is loaded, it shows 2 note objects. Clicking on button should show the 3rd one. I don't understand why setting DataContext using XAML doesn't update to changes in collection. I am sharing snippet of the code here. Any help greatly appreciated.
Cut-down version of XAML -
<Window x:Class="NotesApp.MainWindow"
xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
xmlns:local="clr-namespace:NotesApp"
xmlns:vm="clr-namespace:NotesApp.ViewModel"
Title="MainWindow" Height="480" Width="640">
<Window.DataContext >
<vm:MainViewModel/>
</Window.DataContext>
<DockPanel >
<ScrollViewer VerticalScrollBarVisibility="Auto">
<ItemsControl Name="NoteItemControl" ItemsSource="{Binding notes}" Background="Beige" >
<ItemsControl.LayoutTransform>
<ScaleTransform ScaleX="{Binding Value, ElementName=zoomSlider}" ScaleY="{Binding Value, ElementName=zoomSlider}" />
</ItemsControl.LayoutTransform>
<ItemsControl.ItemTemplate>
<DataTemplate>
<Border Name="NoteBorder" Background="Green" CornerRadius="3" Margin="5,3,5,3">
<Grid>
<Grid.ColumnDefinitions>
<ColumnDefinition/>
</Grid.ColumnDefinitions>
<Grid.RowDefinitions>
<RowDefinition/>
<RowDefinition/>
</Grid.RowDefinitions>
<TextBlock Text="{Binding noteText}" Margin="5,3,5,3"/>
<StackPanel Grid.Row="1" Orientation="Vertical" >
<Line X1="0" Y1="0" X2="{Binding ActualWidth,ElementName=NoteBorder}" Y2="0" Stroke="Black" StrokeThickness="1"/>
<TextBlock Text="{Binding Category}" Margin="5,3,5,3"/>
</StackPanel>
</Grid>
</Border>
</DataTemplate>
</ItemsControl.ItemTemplate>
</ItemsControl>
</ScrollViewer>
</DockPanel>
</Window>
View Code behind-
namespace NotesApp
{
public partial class MainWindow : Window
{
MainViewModel ViewModel { get; set; }
public MainWindow()
{
InitializeComponent();
ViewModel = new MainViewModel();
// IT WORKS IF I BRING IN THIS STATEMENT
//NoteItemControl.ItemsSource = ViewModel.notes;
}
private void Button_Click_1(object sender, RoutedEventArgs e)
{
ViewModel.AddNote(new Note("note3", "Category 3"));
}
}
}
ViewModel -
namespace NotesApp.ViewModel
{
public class MainViewModel: INotifyPropertyChanged
{
ObservableCollection<Note> _notes;
public ObservableCollection<Note> notes
{
get
{ return _notes; }
set
{
_notes = value;
OnPropertyChanged("notes");
}
}
public void AddNote(Note note)
{
_notes.Add(note);
OnPropertyChanged("notes");
}
public MainViewModel ()
{
notes = new ObservableCollection<Note>();
notes.Add(new Note("note1", "Category 1"));
notes.Add(new Note("note2", "Category 2"));
}
public event PropertyChangedEventHandler PropertyChanged = delegate { };
protected virtual void OnPropertyChanged(string propertyName = null)
{
PropertyChangedEventHandler handler = PropertyChanged;
if (handler != null)
handler(this, new PropertyChangedEventArgs( propertyName));
}
}
}

Answer 1

You create a MainViewModel instance and assign it to the MainWindow's DataContext in XAML
<Window.DataContext >
<vm:MainViewModel/>
</Window.DataContext>
The bindings in your XAML use this instance as their source object, as long as you do not explicitly specify some other source. So there is no need (and it's an error) to create another instance in code behind.
Change the MainWindow's constructor like this:
public MainWindow()
{
InitializeComponent();
ViewModel = (MainViewModel)DataContext;
}

Answer 2

Try this :
<Window.Resources>
<vm:MainViewModel x:Key="mainVM"/>
</Window.Resources>
Now use this key as a static resource wherever you bind something like :
<ItemsControl Name="NoteItemControl" ItemsSource="{Binding notes,Source={StaticResource mainVM},Mode=TwoWay}" Background="Beige" >
If you do this, you dont need any datacontext