you can use layout_constraintCircle for center view inside ConstraintLayout.
<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:id="@+id/mparent"
android:layout_width="match_parent"
android:layout_height="match_parent">
<ImageButton
android:id="@+id/btn_settings"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
app:srcCompat="@drawable/ic_home_black_24dp"
app:layout_constraintCircle="@id/mparent"
app:layout_constraintCircleRadius="0dp"
/>
</android.support.constraint.ConstraintLayout>
with constraintCircle to parent and zero radius you can make your view be center of parent.