如何把导航默认在最底下显示？

<?xml version="1.0" encoding="utf-8"?>
<DirectionalLayout
    xmlns:ohos="http://schemas.huawei.com/res/ohos"
    ohos:height="match_parent"
    ohos:width="match_parent"
    ohos:orientation="vertical"
    ohos:background_element="#FFE0E3E3">

    <Text
        ohos:id="$+id:text_helloworld"
        ohos:height="match_content"
        ohos:width="match_content"
        ohos:layout_alignment="horizontal_center"
        ohos:text="$string:mainability_HelloWorld"
        ohos:text_size="40vp"

        />
    <DirectionalLayout
        xmlns:ohos="http://schemas.huawei.com/res/ohos"
        ohos:height="match_content"
        ohos:width="match_parent"
        ohos:orientation="horizontal"
        ohos:background_element="#FFFFFF"
        >
        <Button
            ohos:id="$+id:dynamic"
            ohos:height="match_content"
            ohos:width="match_parent"
            ohos:text="动态"
            ohos:text_size="10vp"
            ohos:element_top="$media:dt"
            ohos:weight="1"/>
        <Button
            ohos:id="$+id:friend"
            ohos:height="match_content"
            ohos:width="match_parent"
            ohos:text="好友"
            ohos:text_size="10vp"
            ohos:element_top="$media:hy"
            ohos:weight="1"/>
        <Button
            ohos:id="$+id:function"
            ohos:height="match_content"
            ohos:width="match_parent"
            ohos:text="功能"
            ohos:text_size="10vp"
            ohos:element_top="$media:gn"
            ohos:weight="1"/>
        <Button
            ohos:id="$+id:set"
            ohos:height="match_content"
            ohos:width="match_parent"
            ohos:text="我的"
            ohos:text_size="10vp"
            ohos:element_top="$media:yh"
            ohos:weight="1"/>
    </DirectionalLayout>
</DirectionalLayout>
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