In a ConstraintLayout, we can center a View by aligning its top, bottom, left, and right all to the edges of its parent.

<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">

    <ImageView
        android:id="@+id/imageView"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:scaleType=""
        app:srcCompat="@drawable/grumpy_cat"
        app:layout_constraintTop_toTopOf=""
        app:layout_constraintBottom_toBottomOf=""
        app:layout_constraintLeft_toLeftOf=""
        app:layout_constraintRight_toRightOf="" />

    <EditText
        android:id="@+id/captionField"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:ems="10"
        android:hint="Enter a caption"
        android:layout_marginTop=""
        app:layout_constraintTop_toBottomOf="@+id/imageView"
        app:layout_constraintLeft_toLeftOf="parent"
        app:layout_constraintRight_toRightOf="parent" />

</android.support.constraint.ConstraintLayout>