wpf 通过EventTrigger用一个控件控制另一个控件

<Window x:Class="WpfApp1.MainWindow"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
        xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
        xmlns:local="clr-namespace:WpfApp1"
        mc:Ignorable="d"
        Title="MainWindow" Height="450" Width="800">
    <Grid>
        <Grid.Triggers>
            <EventTrigger RoutedEvent="TextBox.MouseMove" SourceName="txt">
                <BeginStoryboard Name="EnlargeStoryboard">
                    <Storyboard >
                        <DoubleAnimationUsingKeyFrames
                        Storyboard.TargetName="ell"
                        Storyboard.TargetProperty="(Ellipse.Width)">
                            <DiscreteDoubleKeyFrame
                            KeyTime="00:00:0.2"
                            Value="50" />
                        </DoubleAnimationUsingKeyFrames>

                        <DoubleAnimationUsingKeyFrames
                        Storyboard.TargetName="ell"
                        Storyboard.TargetProperty="(Ellipse.Height)">
                            <DiscreteDoubleKeyFrame
                            KeyTime="00:00:0.2"
                            Value="50" />
                        </DoubleAnimationUsingKeyFrames>
                    </Storyboard>
                </BeginStoryboard>
            </EventTrigger>
            <EventTrigger RoutedEvent="TextBox.MouseLeave" SourceName="txt">
                <BeginStoryboard Name="RecoverStoryboard">
                    <Storyboard >
                        <DoubleAnimationUsingKeyFrames
                        Storyboard.TargetName="ell"
                        Storyboard.TargetProperty="(Ellipse.Width)">
                            <DiscreteDoubleKeyFrame
                            KeyTime="00:00:0.2"
                            Value="30" />
                        </DoubleAnimationUsingKeyFrames>

                        <DoubleAnimationUsingKeyFrames
                        Storyboard.TargetName="ell"
                        Storyboard.TargetProperty="(Ellipse.Height)">
                            <DiscreteDoubleKeyFrame
                            KeyTime="00:00:0.2"
                            Value="30" />
                        </DoubleAnimationUsingKeyFrames>
                    </Storyboard>
                </BeginStoryboard>
            </EventTrigger>
        </Grid.Triggers>
        <StackPanel>
            <TextBox x:Name="txt"></TextBox>
            <Ellipse x:Name="ell" Fill="Green" Width="30" Height="30"></Ellipse>
           
            
        </StackPanel>
    </Grid>
</Window>