activity_message_list.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"
    tools:context="com.sendbird.chattutorial.MessageListActivity">
    
<android.support.v7.widget.RecyclerView
    android:id="@+id/reyclerview_message_list"
    android:layout_width="0dp"
    android:layout_height="0dp"
    app:layout_constraintTop_toTopOf="parent"
    app:layout_constraintLeft_toLeftOf="parent"
    app:layout_constraintRight_toRightOf="parent">
</android.support.v7.widget.RecyclerView>

<!-- A horizontal line between the chatbox and RecyclerView -->
<View
    android:layout_width="0dp"
    android:layout_height="2dp"
    android:background="#dfdfdf"
    android:layout_marginBottom="0dp"
    app:layout_constraintBottom_toTopOf="@+id/layout_chatbox"
    app:layout_constraintLeft_toLeftOf="parent"
    app:layout_constraintRight_toRightOf="parent"/>

<LinearLayout
    android:id="@+id/layout_chatbox"
    android:layout_width="0dp"
    android:layout_height="wrap_content"
    android:orientation="horizontal"
    android:minHeight="48dp"
    android:background="#ffffff"
    app:layout_constraintBottom_toBottomOf="parent"
    app:layout_constraintRight_toRightOf="parent"
    app:layout_constraintLeft_toLeftOf="parent">

<EditText
    android:id="@+id/edittext_chatbox"
    android:hint="Enter message"
    android:background="@android:color/transparent"
    android:layout_gravity="center"
    android:layout_marginLeft="16dp"
    android:layout_marginRight="16dp"
    android:layout_width="0dp"
    android:layout_weight="1"
    android:layout_height="wrap_content"
    android:maxLines="6"/>

<Button
    android:id="@+id/button_chatbox_send"
    android:text="SEND"
    android:textSize="14dp"
    android:background="?attr/selectableItemBackground"
    android:clickable="true"
    android:layout_width="64dp"
    android:layout_height="48dp"
    android:gravity="center"
    android:layout_gravity="bottom" />
  
</LinearLayout>

</android.support.constraint.ConstraintLayout>