[WPF 学习] 6 求助：Ellipse的DataTrigger 失效

    <stackpanel width="200">
        <textbox x:name="tbs" text="a" />
        <textblock >
            <textblock.style>
                <style targettype="textblock">
                    <style.triggers>
                        <datatrigger binding="{binding elementname=tbs,path=text}" value="a">
                            <setter property="text" value="data trigger is raised"></setter>
                        </datatrigger>
                    </style.triggers>
                </style>
            </textblock.style>
        </textblock>
        <ellipse width="100" height="100" strokethickness="5" stroke="red">
            <ellipse.style>
                <style targettype="ellipse">
                    <style.triggers>
                        <datatrigger binding="{binding elementname=tbs,path=text}" value="a">
                            <setter property="stroke" value="black"/>
                        </datatrigger>
                    </style.triggers>
                </style>
            </ellipse.style>
        </ellipse>
    </stackpanel>

结果：

textblock的datatrigger触发了有效果，ellipse的datatrigger也应该触发了，但是无效，有谁知道原因？