【转载】约束布局ConstraintLayout_layout_constrainttop_totopof

参考文章1：约束布局ConstraintLayout看这一篇就够了
参考文章2：学习Placeholder的使用

补充点

1. 文章1中的“3.3 角度定位”

使用角定位时，app:layout_constraintCircleRadius指的是两个控件的中心点的距离

2. 位置偏移说明

margin配置，如android:layout_marginLeft，优先级高于app:layout_constraintLeft_toLeftOf="parent"、app:layout_constraintRight_toRightOf="parent"配置，高于layout_constraintHorizontal_bias配置

3. 文章1中的“3.7 链”

配置layout_constraintHorizontal_weight之后，layout_constraintHorizontal_chainStyle无效

4. 文章1中的“4.3 Group”

Group中app:constraint_referenced_ids引用其他Group的id无效

补充学习代码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=".MainActivity">

    <TextView
        android:layout_width="wrap_content"
        android:layout_height="50dp"
        android:text="TextView1"
        android:gravity="center"
        android:id="@+id/tv1"
        android:layout_marginLeft="10dp"
        android:layout_marginTop="10dp"
        app:layout_constraintTop_toTopOf="parent"
        app:layout_constraintLeft_toLeftOf="parent"
        app:layout_constraintRight_toRightOf="parent"
        android:background="@android:color/holo_red_dark"/>

    <TextView
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="TextView2"
        android:id="@+id/tv2"
        app:layout_constraintCircle="@id/tv1"
        app:layout_constraintCircleAngle="120"
        app:layout_constraintCircleRadius="80dp"
        android:background="@android:color/holo_blue_dark"/>

    <TextView
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="TextView3"
        android:id="@+id/tv3"
        app:layout_goneMarginLeft="100dp"
        app:layout_constraintLeft_toRightOf="@id/tv1"
        app:layout_constraintTop_toTopOf="@id/tv1"
        app:layout_constraintBaseline_toBaselineOf="@id/tv1"
        android:background="@android:color/holo_green_dark"/>

    <TextView
        android:layout_width="wrap_content"
        android:layout_height="0dp"
        android:text="TextView4"
        android:id="@+id/tv4"
        android:layout_marginTop="50dp"
        app:layout_constraintDimensionRatio="4:2"
        app:layout_constraintTop_toBottomOf="@+id/tv3"
        app:layout_constraintLeft_toLeftOf="parent"
        app:layout_constraintRight_toRightOf="parent"
        app:layout_constraintHorizontal_bias="0.1"
        android:layout_marginLeft="100dp"
        android:background="@android:color/holo_orange_dark"/>

    <TextView
        android:layout_marginTop="20dp"
        app:layout_constraintTop_toBottomOf="@+id/tv4"
        android:background="@android:color/holo_blue_light"
        android:id="@+id/tv5"
        android:text="TextView5"
        android:layout_width="0dp"
        app:layout_constraintHorizontal_weight="1"
        android:layout_height="wrap_content"
        app:layout_constraintHorizontal_chainStyle="spread"
        app:layout_constraintLeft_toLeftOf="parent"
        app:layout_constraintRight_toLeftOf="@+id/tv6"/>

    <TextView
        app:layout_constraintTop_toTopOf="@+id/tv5"
        android:background="@android:color/holo_blue_dark"
        android:id="@+id/tv6"
        android:text="TextView6"
        android:layout_width="0dp"
        app:layout_constraintHorizontal_weight="2"
        android:layout_height="50dp"
        app:layout_constraintLeft_toRightOf="@+id/tv5"
        app:layout_constraintRight_toLeftOf="@+id/tv7"/>

    <TextView
        app:layout_constraintTop_toTopOf="@+id/tv5"
        android:background="@android:color/holo_blue_bright"
        android:id="@+id/tv7"
        android:text="TextView7"
        android:layout_width="0dp"
        app:layout_constraintHorizontal_weight="3"
        android:layout_height="60dp"
        app:layout_constraintLeft_toRightOf="@+id/tv6"
        app:layout_constraintRight_toLeftOf="@id/tv8" />

    <TextView
        app:layout_constraintTop_toTopOf="@+id/tv5"
        android:background="@android:color/holo_blue_light"
        android:id="@+id/tv8"
        android:text="TextView8"
        android:layout_width="0dp"
        app:layout_constraintHorizontal_weight="4"
        android:layout_height="70dp"
        app:layout_constraintLeft_toRightOf="@+id/tv7"
        app:layout_constraintRight_toRightOf="parent"/>

    <android.support.constraint.Barrier
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:id="@+id/barrier"
        app:barrierDirection="bottom"
        app:constraint_referenced_ids="tv5,tv6,tv7,tv8"/>

    <TextView
        app:layout_constraintTop_toBottomOf="@id/barrier"
        android:id="@+id/tv9"
        android:text="TextView9"
        android:layout_marginTop="20dp"
        android:background="@android:color/holo_orange_dark"
        android:layout_width="0dp"
        android:layout_height="wrap_content"
        app:layout_constraintLeft_toLeftOf="parent"
        app:layout_constraintRight_toRightOf="parent"/>

    <android.support.constraint.Group
        android:id="@+id/group1"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:visibility="visible"
        app:constraint_referenced_ids="tv5,tv6,tv7,tv8"/>

    <android.support.constraint.Group
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:id="@+id/group2"
        android:visibility="visible"
        app:constraint_referenced_ids="group1,tv9"/>

    <android.support.constraint.Placeholder
        android:layout_marginTop="20dp"
        android:layout_width="100dp"
        android:layout_height="100dp"
        android:id="@+id/placeholder"
        app:layout_constraintTop_toBottomOf="@+id/tv9"
        app:layout_constraintLeft_toLeftOf="parent"
        app:layout_constraintRight_toRightOf="parent"/>

    <android.support.constraint.Guideline
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:orientation="horizontal"
        android:id="@+id/guideline1"
        app:layout_constraintGuide_percent="0.5"/>

    <android.support.constraint.Guideline
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:orientation="vertical"
        android:id="@+id/guideline2"
        app:layout_constraintGuide_percent="0.5"/>

    <TextView
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="TextView10"
        app:layout_constraintTop_toTopOf="@id/guideline1"
        app:layout_constraintBottom_toBottomOf="@id/guideline1"
        app:layout_constraintLeft_toLeftOf="@id/guideline2"
        app:layout_constraintRight_toRightOf="@id/guideline2"
        android:background="@android:color/holo_green_light"/>

</android.support.constraint.ConstraintLayout>
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运行截图