activity_login.xml

<android.support.constraint.ConstraintLayout xmlns:android="http://schemas.android.com/apk/res/android"
                                             xmlns:app="http://schemas.android.com/apk/res-auto"
                                             android:layout_width="match_parent"
                                             android:layout_height="match_parent">

    <EditText
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:inputType="textPersonName"
            android:ems="10"
            android:id="@+id/loginField"
            app:layout_constraintTop_toTopOf="parent"
            android:layout_marginTop="8dp"
            android:layout_marginRight="8dp"
            app:layout_constraintRight_toRightOf="parent"
            android:layout_marginLeft="8dp"
            app:layout_constraintLeft_toLeftOf="parent"
            android:hint="login"
            app:layout_constraintHorizontal_bias="0.503"/>

    <EditText
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:inputType="textPassword"
            android:ems="10"
            android:id="@+id/passwordField"
            android:layout_marginTop="8dp"
            app:layout_constraintTop_toBottomOf="@+id/loginField"
            android:layout_marginRight="8dp"
            app:layout_constraintRight_toRightOf="parent"
            android:layout_marginLeft="8dp"
            app:layout_constraintLeft_toLeftOf="parent"
            android:hint="password"/>

    <Button
            android:text="Login"
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:id="@+id/loginBtn"
            android:layout_marginRight="8dp"
            app:layout_constraintRight_toRightOf="parent"
            android:layout_marginLeft="8dp"
            app:layout_constraintLeft_toLeftOf="parent"
            android:layout_marginTop="8dp"
            app:layout_constraintTop_toBottomOf="@+id/passwordField"/>

    <ProgressBar
            style="?android:attr/progressBarStyle"
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:id="@+id/progressBar"
            android:layout_marginRight="8dp"
            app:layout_constraintRight_toRightOf="parent"
            android:layout_marginLeft="8dp"
            app:layout_constraintLeft_toLeftOf="parent"
            app:layout_constraintTop_toTopOf="@+id/loginBtn"
            android:layout_marginTop="8dp"
            android:visibility="gone"/>

    <Button
            android:text="help"
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:id="@+id/helpBtn"
            android:layout_marginRight="8dp"
            app:layout_constraintRight_toRightOf="parent"
            android:layout_marginLeft="8dp"
            app:layout_constraintLeft_toLeftOf="parent"
            app:layout_constraintBottom_toBottomOf="parent"
            android:layout_marginBottom="8dp"/>
</android.support.constraint.ConstraintLayout>