A behavior making a TextBox update the source on every key press.

TextBoxImmediateUpdateBehavior.cs

using System;
using System.Windows;
using System.Windows.Controls;
using System.Windows.Interactivity;

namespace MonkeyBusters
{
    // This behavior allows for TextBoxes which can't be tabbed away from - and would thus
    // never update their source. They do update on key release now, which is not
    // right away as it would be if the TextChanged event was used.
    public class TextBoxImmediateUpdateBehavior : Behavior<TextBox>
    {
        protected override void OnAttached()
        {
            AssociatedObject.TextChanged += HandleAssociatedObjectTextChanged;
            AssociatedObject.DataContextChanged += HandleAssociatedObjectDataContextChanged;
            
        }

        protected override void OnDetaching()
        {
            AssociatedObject.TextChanged -= HandleAssociatedObjectTextChanged;
            AssociatedObject.DataContextChanged -= HandleAssociatedObjectDataContextChanged;
        }

        void HandleAssociatedObjectDataContextChanged(object sender, DependencyPropertyChangedEventArgs e)
        {
            hadNonemptyText = false;
        }

        void HandleAssociatedObjectTextChanged(object sender, TextChangedEventArgs e)
        {
            if (hadNonemptyText || AssociatedObject.Text != "")
            {
                var binding = AssociatedObject.GetBindingExpression(TextBox.TextProperty);
                if (binding != null) binding.UpdateSource();
                hadNonemptyText = true;
            }
        }

        Boolean hadNonemptyText = false;
    }
}