Android – Constraint layout – How to align a view centered over edge of other view?

Try this:

<?xml version="1.0" encoding="utf-8"?>
<android.support.constraint.ConstraintLayout
    xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:app="http://schemas.android.com/apk/res-auto"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:orientation="vertical"
    android:padding="20dp">

    <android.support.v7.widget.CardView
        android:id="@+id/card_1"
        android:layout_width="0dp"
        android:layout_height="200dp"
        android:layout_marginStart="8dp"
        android:layout_marginEnd="8dp"
        android:layout_marginTop="8dp"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintTop_toTopOf="parent" />

    <android.support.v7.widget.CardView
        android:id="@+id/card_2"
        android:layout_width="100dp"
        android:layout_height="100dp"
        android:layout_marginBottom="8dp"
        android:layout_marginStart="8dp"
        android:layout_marginEnd="8dp"
        android:layout_marginTop="0dp"
        app:cardBackgroundColor="#69F"
        app:layout_constraintBottom_toBottomOf="@+id/card_1"
        app:layout_constraintStart_toStartOf="@+id/card_1"
        app:layout_constraintEnd_toEndOf="@+id/card_1"
        app:layout_constraintTop_toBottomOf="@+id/card_1" />

</android.support.constraint.ConstraintLayout>

Explanation :- This works because of these four lines

Following lines sets the blue CardView centered on the bottom edge of White CardView.

<!-- top constraint is set to bottom of White CardView -->
app:layout_constraintTop_toBottomOf="@+id/card_1"

<!-- bottom constraint is set to bottom of White CardView -->
app:layout_constraintBottom_toBottomOf="@+id/card_1"

Following lines set the blue CardView centered horizontally

<!-- left/start constraint is set to left/start of White CardView -->
app:layout_constraintStart_toStartOf="@+id/card_1"

<!-- right/end constraint is set to right/end of White CardView -->
app:layout_constraintEnd_toEndOf="@+id/card_1"