这种Floating Action Button 的位置感觉在两个 ViewGroup之间，怎么实现？谢谢~
<android.support.design.widget.FloatingActionButton
android:id="@+id/fab"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:layout_margin="@dimen/fab_margin"
android:src="@drawable/ic_add_black_24dp"
custom:layout_anchor="@id/app_bar"
custom:layout_anchorGravity="bottom|left"/>
where custom refers to xmlns http://schemas.android.com/apk/res-auto, and app_bar is a android.support.design.widget.AppBarLayout.