constraint barrier example

Let's have two buttons, @id/button1 and @id/button2. 
The constraint_referenced_ids field will reference them 
by simply having them as comma-separated list:
     
<androidx.constraintlayout.widget.Barrier
     android:id="@+id/barrier"
     android:layout_width="wrap_content"
     android:layout_height="wrap_content"
     app:barrierDirection="bottom"
     app:constraint_referenced_ids="button1,button2" />
     
This way the barrier will allways be under button1 and button2.