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.