Android ConstraintLayout约束布局的居中

xml代码：

<?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"
    android:layout_width="match_parent"
    android:layout_height="match_parent">

    <TextView
        android:id="@+id/text1"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:background="@android:color/holo_red_light"
        android:text="text1：顶部水平居中"
        android:textColor="@android:color/white"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toTopOf="parent" />

    <TextView
        android:id="@+id/text2"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:background="@android:color/holo_red_light"
        android:text="text2：位于text1下且水平居中"
        android:textColor="@android:color/white"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toBottomOf="@id/text1" />

    <TextView
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:background="@android:color/holo_green_light"
        android:text="水平居中，且居于text2下"
        android:textColor="@android:color/white"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toBottomOf="@id/text2" />


    <TextView
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:background="@android:color/holo_red_light"
        android:text="居中位于底部"
        android:textColor="@android:color/white"
        app:layout_constraintBottom_toBottomOf="parent"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintStart_toStartOf="parent" />


    <TextView
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:background="@android:color/holo_blue_bright"
        android:text="全局居中"
        android:textColor="@android:color/white"
        app:layout_constraintBottom_toBottomOf="parent"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toTopOf="parent" />
</android.support.constraint.ConstraintLayout>

输出结果如图：