main_activity.xml

<?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">

    <ImageView
        android:id="@+id/imageRecon"
        android:layout_width="match_parent"
        android:layout_height="match_parent"
        android:contentDescription="Select image for text recognition"
        android:scaleType="fitStart"
        app:layout_constraintBottom_toTopOf="parent"
        app:layout_constraintLeft_toLeftOf="parent"
        app:layout_constraintRight_toRightOf="parent"
        app:layout_constraintTop_toBottomOf="parent" />

    <bferrari.com.mlkitdemo.GraphicOverlay
        android:id="@+id/graphicOverlay"
        android:layout_width="match_parent"
        android:layout_height="match_parent"
        android:layout_alignParentStart="true"
        android:layout_alignParentTop="true"
        android:layout_alignParentBottom="true"
        android:layout_alignParentLeft="true" />

    <Spinner
        android:id="@+id/spinner"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:layout_marginBottom="8dp"
        android:background="@android:drawable/btn_dropdown"
        android:spinnerMode="dropdown"
        app:layout_constraintBottom_toTopOf="@+id/textRecognitionBtn"
        app:layout_constraintTop_toBottomOf="@+id/imageRecon"/>

    <Button
        android:id="@+id/textRecognitionBtn"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_marginBottom="16dp"
        android:layout_marginStart="8dp"
        android:text="@string/text_recognition"
        app:layout_constraintBottom_toTopOf="@id/spinner"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintStart_toStartOf="parent" />

    <android.support.design.widget.FloatingActionButton
        android:id="@+id/fab"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_margin="16dp"
        android:layout_marginBottom="8dp"
        android:layout_marginEnd="8dp"
        android:src="@drawable/ic_photo_camera_black_24dp"
        app:fabSize="normal"
        app:layout_constraintBottom_toBottomOf="parent"
        app:layout_constraintEnd_toEndOf="parent" />

</android.support.constraint.ConstraintLayout>