フォーカスについて

<Grid  FocusManager.FocusedElement="{Binding ElementName=*****}" >

    public static class FocusExtension
    {
         public static bool GetIsFocused(DependencyObject obj)
         {
             return (bool) obj.GetValue(IsFocusedProperty);
         }


         public static void SetIsFocused(DependencyObject obj, bool value)
        {
            obj.SetValue(IsFocusedProperty, value);
        }


        public static readonly DependencyProperty IsFocusedProperty =
                DependencyProperty.RegisterAttached(
                   "IsFocused", typeof(bool), typeof(FocusExtension),
                    new UIPropertyMetadata(false, OnIsFocusedPropertyChanged));


        private static void OnIsFocusedPropertyChanged(DependencyObject d,
                DependencyPropertyChangedEventArgs e)
        {
            var uie = (UIElement)d;
            if ((bool) e.NewValue)
            {
                uie.Focus();
            }
        }
    }

        public bool IsTextBoxFocused
        {
            get { return _isFocused; }
            set
            {
                this.SetProperty(ref this._isFocused, value);
            }
        }

<TextBox local:FocusExtension.IsFocused="{Binding IsTextBoxFocused}" /> 

{
    public class FocusBehavior : Behavior<FrameworkElement>
    {
        /// <summary>
        /// フォーカス設定
        /// </summary>
        public void Focus()
        {
            this.AssociatedObject.Focus();
        }
    }

        <CheckBox Content="flg1" HorizontalAlignment="Left" IsChecked="{Binding Flg1}">
            <i:Interaction.Behaviors>
                <local:FocusBehavior x:Name="forcusTest" />
            </i:Interaction.Behaviors>
        </CheckBox>
        <Button Content="Button" HorizontalAlignment="Left" VerticalAlignment="Top" Width="75" Command="{Binding CalcCommand}" >
            <i:Interaction.Triggers>
                <i:EventTrigger EventName="Click">
                    <ei:CallMethodAction TargetObject="{Binding ElementName=forcusTest}" MethodName="Focus" />
                </i:EventTrigger>
            </i:Interaction.Triggers>
        </Button>