WPF通过EventTrigger改变其他控件的值

<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>