相关疑难解决方法(0)

我已经创建了这样的AppBar布局

<android.support.design.widget.AppBarLayout
    xmlns:app="http://schemas.android.com/apk/res-auto"
    xmlns:android="http://schemas.android.com/apk/res/android"
    android:id="@+id/appbar_layout"
    android:layout_height="@dimen/app_bar_height"
    android:layout_width="match_parent"
    android:fitsSystemWindows="true"
    android:theme="@style/ThemeOverlay.AppCompat.Dark.ActionBar"
    app:elevation="20dp">

    <android.support.design.widget.CollapsingToolbarLayout...>
</android.support.design.widget.AppBarLayout>

它工作并在LinearLayout中投射阴影:

<LinearLayout
    xmlns:android="http://schemas.android.com/apk/res/android"
    android:layout_width="match_parent"
    android:layout_height="match_parent">

    <include layout="@layout/app_bar_large" />
</LinearLayout>

但是,当我把它放入CoordinatorLayout时,阴影消失了:

<android.support.design.widget.CoordinatorLayout
    xmlns:android="http://schemas.android.com/apk/res/android" 
    android:layout_width="match_parent"
    android:layout_height="match_parent">

    <include layout="@layout/app_bar_large" />
</android.support.design.widget.CoordinatorLayout>

如何让appbar再次展示它的影子？