Sometimes it is annoying that the TextBox in WPF needs to loose focus in order to updating the underlying value in the model. Having the application moving the focus in order to force the binding update is a bad trick, unacceptable if you are using MVVM. In such a case worked for me deriving a custom class from the textbox ( actually I did for the Fluent Ribbon textbox, but the concept is the same ) and programmatically updating the source, as below:

public class UpdatingBindingTextBox:TextBox
    {
        public UpdatingBindingTextBox()
        {
            this.TextChanged += new System.Windows.Controls.TextChangedEventHandler(UpdatingBindingTextBox_TextChanged);
        }

        void UpdatingBindingTextBox_TextChanged(object sender, System.Windows.Controls.TextChangedEventArgs e)
        {
            BindingOperations.GetBindingExpression(this, TextBox.TextProperty).UpdateSource();
        }
    }

ADDITION:

By investigating a little more, I found this alternative on stack overflow. It is basically a behavior attached to the textbox that call a command in the ViewModel. Don’t know which one is better, I personally like the behavior way, but in any case I do need to update the binding manually into the view model…