app:layout_marginBottom is not working well with android constraint layout

Question

Is there any reason why the following layout_marginBottom is not working? However, if I use layout_marginTop on the second view it does work well

<android.support.constraint.ConstraintLayout
    xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:app="http://schemas.android.com/apk/res-auto"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:background="#ade4ad">
    <TextView
        android:id="@+id/first"
        android:layout_width="90dp"
        android:layout_height="40dp"
        app:layout_marginBottom="10dp"
        android:background="#000"/>
    <TextView
        android:id="@+id/second"
        android:layout_width="90dp"
        android:layout_height="40dp"
        android:background="#fff"
        app:layout_constraintTop_toBottomOf="@+id/first"/>
</android.support.constraint.ConstraintLayout>

Answer 1

In order to

android:layout_marginBottom="20dp" 

work well, you should use

app:layout_constraintBottom_toBottomOf="parent"