Question

My event below (OnSourceUpdated) is not getting handled.

XAML:

    <StackPanel x:Name="MyStackPanel" 
                Orientation="Horizontal" 
                DockPanel.Dock="Top">
        <TextBox Text="{Binding Side, Mode=TwoWay}" 
                 Width="100"/>
        <TextBlock Background="Yellow" 
                   Text="{Binding Side, Mode=OneWay, 
                                  NotifyOnSourceUpdated=True}"
                   Width="100" 
                   SourceUpdated="OnSourceUpdated"
                   Binding.SourceUpdated="OnSourceUpdated"/>
    </StackPanel>

C#:

....
MyStackPanel.DataContext = new MyItemClass() { Side = "Test" };
....

private void OnSourceUpdated(Object sender, DataTransferEventArgs args)
{
    var i = args.Property;
}

public class MyItemClass : INotifyPropertyChanged
{
    private string _side;
    public string Side
    {
        get { return _side; }

        set 
        {
            _side = value;
            OnPropertyChanged("Side");
        }
    }

    #region INotifyPropertyChanged Members

    public event PropertyChangedEventHandler PropertyChanged;

    private void OnPropertyChanged(string propertyName)
    {
        if (this.PropertyChanged != null)
        {
            this.PropertyChanged(this, new PropertyChangedEventArgs(propertyName));
        }
    }

    #endregion
}

I have all the relevant settings done like NotifyOnSourceUpdated & SourceUpdated & Binding.SourceUpdated etc.

Answer 1

From msdn: Binding.SourceUpdated attached event occurs when a value is transferred from the binding target to the binding source, but only for bindings with the NotifyOnSourceUpdated value set to true

In the Binding of TextBlock, there is no value transfer from the binding target (TextBlock.Text) to the binding source (Side). Thus SourceUpdated cannot be fired.

Instead SourceUpdated can be fired on the first binding. Indeed here the target binding TextBox.Text can change the binding source (Side).