Updates a source binding for a TextBox when the text changes and not just when the box loses focus.

UpdateSourceOnTextChanged.cs

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

public class UpdateSourceOnTextChanged : Behavior<TextBox>
{
    BindingExpression textBinding;

    protected override void OnAttached()
    {
        if (this.AssociatedObject == null)
            return;
            
        // Get the binding
        textBinding = this.AssociatedObject.GetBindingExpression(TextBox.TextProperty);

        // Subscribe to text changed events.
        this.AssociatedObject.TextChanged += AssociatedObject_TextChanged;
            
        base.OnAttached();
    }

    void AssociatedObject_TextChanged(object sender, TextChangedEventArgs e)
    {   
        // Update the binding source.
        if(textBinding != null)
            textBinding.UpdateSource();
    }

    protected override void OnDetaching()
    {
        if (this.AssociatedObject == null)
            return;

        // Clean up...
        this.textBinding = null;
        this.AssociatedObject.TextChanged -= AssociatedObject_TextChanged;
            
        base.OnDetaching();
    }
}

UpdateSourceOnTextChangedExample.xaml

<TextBox
    Text="{Binding SomeTextProperty, Mode=TwoWay}">
    <i:Interaction.Behaviors>
        <con:UpdateSourceOnTextChanged />
    </i:Interaction.Behaviors>
</TextBox>