How to use attached behaviour for textbox lost focus set back default value

Same case as this previous post link but different implementation.

Because I have around 10 textboxes, and I think is it not good to create a lots of lost focus command in view model, plus this reset to default value when lost focus is one of the requirements. If I need to implement property changed with icon notification then my view model will be very messy and complex.

Anyone else have better suggestions ?

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How to implement the textbox will get back to the default value when textbox in lost focus by using attached behavior ?

E.g

Any changes that has been done by user on textbox A, if he/she does not press the enter key and lost focus to another text box B. The textbox A value will change back to the default value.

If the user has press the enter key, the default value will be replaced by changes that has been done by user.

<Window x:Class="TestValidationRule.MainWindow"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        xmlns:vr="clr-namespace:TestValidationRule"
        xmlns:i="http://schemas.microsoft.com/expression/2010/interactivity"
        xmlns:WPFBehaviorsSample="clr-namespace:TestValidationRule"
        Title="MainWindow" Height="350" Width="525"><Window.Resources><Style x:Key="TextBoxErrorStyle" TargetType="TextBox"><Style.Triggers><Trigger Property="Validation.HasError" Value="True"><Setter Property="Foreground" Value="Red" /></Trigger></Style.Triggers></Style></Window.Resources><StackPanel Orientation="Horizontal"><TextBox x:Name="txtDiagPeriod" Width="150" Height="30" Validation.ErrorTemplate="{x:Null}" Style="{StaticResource TextBoxErrorStyle}"><TextBox.Text><Binding Path="DiagPeriod" Mode="TwoWay" UpdateSourceTrigger="PropertyChanged"><Binding.ValidationRules><vr:DiagPeriodValidationRule x:Name="DiagPeriodValidation" /></Binding.ValidationRules></Binding></TextBox.Text><TextBox.InputBindings><KeyBinding Key="Enter" Command="{Binding WriteDiagPeriodCommand}" CommandParameter="{Binding ElementName=txtDiagPeriod, Path=(Validation.HasError)}"></KeyBinding></TextBox.InputBindings><i:Interaction.Behaviors><WPFBehaviorsSample:ShowMessageOnLostFocusBehavior /></i:Interaction.Behaviors></TextBox><TextBox Width="150" Height="30" /></StackPanel></Window>

using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Linq;
using System.Text;
using System.Windows;
using System.Windows.Input;

namespace TestValidationRule
{
    public class MainWindowViewModel : INotifyPropertyChanged
    {
        private uint diagPeriod;

        public uint DiagPeriod
        {
            get { return diagPeriod; }
            set { diagPeriod = value; OnPropertyChanged("DiagPeriod"); }
        }

        private ICommand writeDiagPeriodCommand;

        public ICommand WriteDiagPeriodCommand
        {
            get { return writeDiagPeriodCommand; }
            set { writeDiagPeriodCommand = value; }
        }

        private uint defaultDiagPeriod = 100;

        public MainWindowViewModel()
        {
            writeDiagPeriodCommand = new RelayCommand(ExecuteWriteDiagPeriodCommand);
            //initial when first launch
            diagPeriod = defaultDiagPeriod;
        }

        public void ExecuteWriteDiagPeriodCommand(object parameter)
        {
            var IsDiagPeriodHasError = (bool)parameter;

            if (!IsDiagPeriodHasError && defaultDiagPeriod != diagPeriod)
            {
                defaultDiagPeriod = diagPeriod;

                MessageBox.Show("Hello: " + defaultDiagPeriod);
            }
            else
            {

            }
        }

        public event PropertyChangedEventHandler PropertyChanged;

        public void OnPropertyChanged(string name)
        {
            if (PropertyChanged != null)
            {
                PropertyChanged(this, new PropertyChangedEventArgs(name));
            }
        }
    }
}

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Windows;
using System.Windows.Controls;
using System.Windows.Interactivity;

namespace TestValidationRule
{
    public class ShowMessageOnLostFocusBehavior : Behavior<TextBox>
    {
        protected override void OnAttached()
        {
            AssociatedObject.LostFocus += AssociatedObjectOnLostFocus;
            base.OnAttached();
        }

        protected override void OnDetaching()
        {
            AssociatedObject.LostFocus -= AssociatedObjectOnLostFocus;
            base.OnDetaching();
        }

        private void AssociatedObjectOnLostFocus(object sender, RoutedEventArgs routedEventArgs)
        {
            MessageBox.Show(AssociatedObject.Text);
        }
    }
}