我们要完成如此一个UI

<?xml version="1.0" encoding="utf-8"?>
<androidx.constraintlayout.widget.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=".MainActivity">


    <LinearLayout
        android:id="@+id/target"
        android:layout_width="match_parent"
        android:layout_height="100dp"
        android:layout_margin="20dp"
        android:background="@android:color/holo_green_dark"
        android:orientation="horizontal"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toTopOf="parent" />

    <LinearLayout
        android:layout_width="40dp"
        android:layout_height="40dp"
        android:background="@android:color/holo_green_light"
        android:orientation="horizontal"
        app:layout_constraintEnd_toEndOf="@id/target"
        app:layout_constraintStart_toStartOf="@id/target"
        app:layout_constraintTop_toBottomOf="@id/target"
        app:layout_constraintBottom_toBottomOf="@id/target"

        />

</androidx.constraintlayout.widget.ConstraintLayout>

 效果

下面这两句很难理解

        app:layout_constraintTop_toBottomOf="@id/target"
        app:layout_constraintBottom_toBottomOf="@id/target"

那我们删除一个不就明白了吗

 在删除一个

 所以是顶部对着底部，底部对着顶部。他俩一约束。自然而然的跑到中间去。非常的时尚