Check out example codes for "constraint barrier example". It will help you in understanding the concepts better.

Code Example 1

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.

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