activity_main.xml

<?xml version="1.0" encoding="utf-8"?>
<RelativeLayout
    xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:paddingBottom="@dimen/activity_vertical_margin"
    android:paddingLeft="@dimen/activity_horizontal_margin"
    android:paddingRight="@dimen/activity_horizontal_margin"
    android:paddingTop="@dimen/activity_vertical_margin"
    tools:context="com.google.firebase.udacity.friendlychat.MainActivity">

    <ListView
        android:id="@+id/messageListView"
        android:layout_width="match_parent"
        android:layout_height="match_parent"
        android:layout_above="@+id/linearLayout"
        android:stackFromBottom="true"
        android:divider="@android:color/transparent"
        android:transcriptMode="alwaysScroll"
        tools:listitem="@layout/item_message"/>

    <LinearLayout
        android:id="@+id/linearLayout"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:layout_alignParentBottom="true"
        android:layout_alignParentLeft="true"
        android:layout_alignParentStart="true"
        android:orientation="horizontal">

        <ImageButton
            android:id="@+id/photoPickerButton"
            android:layout_width="36dp"
            android:layout_height="36dp"
            android:background="@android:drawable/ic_menu_gallery" />

        <EditText
            android:id="@+id/messageEditText"
            android:layout_width="0dp"
            android:layout_height="wrap_content"
            android:layout_gravity="center_vertical"
            android:layout_weight="1" />

        <Button
            android:id="@+id/sendButton"
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:layout_gravity="bottom"
            android:enabled="false"
            android:text="@string/send_button_label"/>

    </LinearLayout>

    <ProgressBar
        android:id="@+id/progressBar"
        style="?android:attr/progressBarStyleLarge"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_centerHorizontal="true"
        android:layout_centerVertical="true"/>
</RelativeLayout>