<Grid>

    <Grid.RowDefinitions>

        <RowDefinition/>

        <RowDefinition/>

        <RowDefinition/>

    </Grid.RowDefinitions>

    <TextBox

        x:Name="tb"

        Grid.Row="0"

        Text="Here is some sample text">

    </TextBox>

    <Button

        x:Name="btnFocusTrue"

        Grid.Row="1"

        Content="Set True">

    </Button>

    <Button

        x:Name="btnFocusFalse"

        Grid.Row="2"

        Content="Set False">

    </Button>

    <Grid.Triggers>

        <EventTrigger RoutedEvent="Button.Click" SourceName="btnFocusTrue">

            <BeginStoryboard Name="FocusTrueStoryboard">

                <Storyboard >

                    <BooleanAnimationUsingKeyFrames

                        Storyboard.TargetName="tb"

                        Storyboard.TargetProperty="(TextBox.Focusable)">

                        <DiscreteBooleanKeyFrame

                            KeyTime="00:00:01"

                            Value="True" />

                    </BooleanAnimationUsingKeyFrames>

                </Storyboard>

            </BeginStoryboard>

        </EventTrigger>

        <EventTrigger RoutedEvent="Button.Click" SourceName="btnFocusFalse">

            <BeginStoryboard Name="FoucsFalseStoryboard">

                <Storyboard >

                    <BooleanAnimationUsingKeyFrames

                        Storyboard.TargetName="tb"

                        Storyboard.TargetProperty="(TextBox.Focusable)">

                        <DiscreteBooleanKeyFrame

                            KeyTime="00:00:01"

                            Value="False" />

                    </BooleanAnimationUsingKeyFrames>

                </Storyboard>

            </BeginStoryboard>

        </EventTrigger>

    </Grid.Triggers>

</Grid>