[移动开发]ConstraintLayout完全解析

前言：

ConstraintLayout在 2016 年 Google I/O 中面世，是为了解决开发中过于复杂的页面层级嵌套过多的问题，因为层级过深会增加绘制界面需要的时间，影响用户体验。

ConstraintLayout以灵活的方式定位和调整控件，从 Android Studio 2.3起，创建layout文件就已经是默认ConstraintLayout了。

ConstraintLayout是升级版的相对布局，推荐使用约束布局和线性布局进行界面布局开发

为什么要用约束布局？

1.功能强大，支持角度定位，百分比布局。

2.布局扁平化，减少布局层级，界面性能优化。

如何在约束布局中定位一个布局？

约束布局中的布局有四个方向：上下左右，我们需要在水平方向上至少有一个约束，在竖直方向也至少要有一个约束，也就是说，在不同方向上至少有两个约束，才能在界面上定位一个布局。

XML中的基础约束：

<?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">

    <TextView
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="Hello World!"
        app:layout_constraintBottom_toBottomOf="parent"
        app:layout_constraintLeft_toLeftOf="parent"
        app:layout_constraintRight_toRightOf="parent"
        app:layout_constraintTop_toTopOf="parent" />
</androidx.constraintlayout.widget.ConstraintLayout>

layout_constraintBottom_toBottomOf="parent"：我的底部在父元素的底部 layout_constraintLeft_toLeftOf="parent"：我的左边在父元素的左边 layout_constraintRight_toRightOf="parent"：我的右边在父元素的右边 ayout_constraintTop_toTopOf="parent" ：我的顶部在父元素的顶部

example：让一个按钮竖直居中在一张图片的中间

match?constraint(0dp)：充满约束

match_parent：匹配父元素

wrap_content：包裹内容

<?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">
    
    <ImageView
        android:id="@+id/imageView"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_marginStart="20dp"
        app:layout_constraintBottom_toBottomOf="parent"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toTopOf="parent"
        app:srcCompat="@drawable/ic_launcher_background" />

    <Button
        android:id="@+id/button"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="Button"
        android:layout_marginEnd="20dp"
        app:layout_constraintBottom_toBottomOf="@+id/imageView"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintTop_toTopOf="@+id/imageView" />
</androidx.constraintlayout.widget.ConstraintLayout>

如果把Button的高设置为0dp，也就是match?constraint，Button的高度就和图片的高度一致

<?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">

    <ImageView
        android:id="@+id/imageView"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_marginStart="20dp"
        app:layout_constraintBottom_toBottomOf="parent"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toTopOf="parent"
        app:srcCompat="@drawable/ic_launcher_background" />

    <Button
        android:id="@+id/button"
        android:layout_width="wrap_content"
        android:layout_height="0dp"
        android:layout_marginEnd="20dp"
        android:text="Button"
        app:layout_constraintBottom_toBottomOf="@+id/imageView"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintTop_toTopOf="@+id/imageView" />
</androidx.constraintlayout.widget.ConstraintLayout>

权重：

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

    <View
        android:id="@+id/view1"
        android:layout_width="0dp"
        android:layout_height="80dp"
        android:background="@color/purple_200"
        app:layout_constraintEnd_toStartOf="@+id/view2"
        app:layout_constraintHorizontal_weight="1"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toTopOf="parent" />

    <View
        android:id="@+id/view2"
        android:layout_width="0dp"
        android:layout_height="80dp"
        android:background="@color/purple_700"
        android:text="TextView"
        app:layout_constraintEnd_toStartOf="@id/view3"
        app:layout_constraintHorizontal_weight="1"
        app:layout_constraintStart_toEndOf="@id/view1"
        app:layout_constraintTop_toTopOf="parent" />

    <View
        android:id="@+id/view3"
        android:layout_width="0dp"
        android:layout_height="80dp"
        android:background="@color/purple_500"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintHorizontal_weight="1"
        app:layout_constraintStart_toEndOf="@id/view2"
        app:layout_constraintTop_toTopOf="parent" />
</androidx.constraintlayout.widget.ConstraintLayout>

Baseline：基准线对齐

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

    <TextView
        android:id="@+id/textView1"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_marginLeft="50dp"
        android:text="90"
        android:textColor="@color/black"
        android:textSize="60sp"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toTopOf="parent" />

    <TextView
        android:id="@+id/textView2"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="%"
        android:textColor="@color/black"
        android:textSize="20sp"
        app:layout_constraintBaseline_toBaselineOf="@+id/textView1"
        app:layout_constraintStart_toEndOf="@+id/textView1" />

    <TextView
        android:id="@+id/textView3"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_marginStart="50dp"
        android:layout_marginLeft="50dp"
        android:text="200"
        android:textColor="@color/black"
        android:textSize="60sp"
        app:layout_constraintStart_toEndOf="@id/textView2"
        app:layout_constraintTop_toTopOf="parent" />
    
    <TextView
        android:id="@+id/textView4"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="￥"
        android:textColor="@color/black"
        android:textSize="20sp"
        app:layout_constraintBaseline_toBaselineOf="@+id/textView3"
        app:layout_constraintStart_toEndOf="@+id/textView3" />
</androidx.constraintlayout.widget.ConstraintLayout>

角度定位：

<?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:id="@+id/root"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    tools:ignore="MissingConstraints">
    
    <androidx.appcompat.widget.AppCompatImageView
        android:id="@+id/sun"
        android:layout_width="80dp"
        android:layout_height="80dp"
        android:src="@mipmap/sun"
        app:layout_constraintBottom_toBottomOf="parent"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toTopOf="parent" />
    
    <androidx.appcompat.widget.AppCompatImageView
        android:id="@+id/earth"
        android:layout_width="40dp"
        android:layout_height="40dp"
        android:src="@mipmap/earth"
        app:layout_constraintCircle="@id/sun"
        app:layout_constraintCircleAngle="45"
        app:layout_constraintCircleRadius="150dp" />

    <androidx.appcompat.widget.AppCompatImageView
        android:id="@+id/moon"
        android:layout_width="30dp"
        android:layout_height="30dp"
        android:src="@mipmap/moon"
        app:layout_constraintCircle="@id/earth"
        app:layout_constraintCircleAngle="270"
        app:layout_constraintCircleRadius="40dp" />
</androidx.constraintlayout.widget.ConstraintLayout>

package com.zhoujian.constraintlayoutdemo;

import android.animation.ValueAnimator;
import android.os.Bundle;
import android.view.View;
import android.view.animation.LinearInterpolator;
import android.widget.ImageView;

import androidx.appcompat.app.AppCompatActivity;
import androidx.constraintlayout.widget.ConstraintLayout;

public class CircularActivity extends AppCompatActivity {

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_example_circular);

        ImageView imageViewMoon = findViewById(R.id.moon);
        ImageView imageViewEarth = findViewById(R.id.earth);
        ImageView imageViewSun = findViewById(R.id.sun);

        ValueAnimator earthAnimator = ValueAnimator.ofFloat(0f, 1f);
        earthAnimator.setDuration(10000L);
        earthAnimator.setRepeatCount(ValueAnimator.INFINITE);
        earthAnimator.setInterpolator(new LinearInterpolator());
        earthAnimator.addUpdateListener(new ValueAnimator.AnimatorUpdateListener() {
            @Override
            public void onAnimationUpdate(ValueAnimator valueAnimator) {
                ConstraintLayout.LayoutParams layoutParams = (ConstraintLayout.LayoutParams) imageViewEarth.getLayoutParams();
                layoutParams.circleAngle = 45 + earthAnimator.getAnimatedFraction() * 360;
                imageViewMoon.requestLayout();
            }
        });

        ValueAnimator moonAnimator = ValueAnimator.ofFloat(0f, 1f);
        moonAnimator.setDuration(10000L);
        moonAnimator.setRepeatCount(ValueAnimator.INFINITE);
        moonAnimator.setInterpolator(new LinearInterpolator());
        moonAnimator.addUpdateListener(new ValueAnimator.AnimatorUpdateListener() {
            @Override
            public void onAnimationUpdate(ValueAnimator valueAnimator) {
                ConstraintLayout.LayoutParams layoutParams = (ConstraintLayout.LayoutParams) imageViewMoon.getLayoutParams();
                layoutParams.circleAngle = 270 + moonAnimator.getAnimatedFraction() * 360;
                imageViewMoon.requestLayout();
            }
        });


        imageViewSun.setOnClickListener(view -> {
            earthAnimator.start();
            moonAnimator.start();
        });
    }
}

constrainedWidth：约束宽度

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

    <ImageView
        android:id="@+id/avatar"
        android:layout_width="200dp"
        android:layout_height="200dp"
        android:layout_marginTop="40dp"
        android:src="@mipmap/ic_launcher_round"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toTopOf="parent" />

    <TextView
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_marginTop="12dp"
        android:text="测试数据测试数据测试数据测试数据测试数据"
        app:layout_constrainedWidth="true"
        android:gravity="center"
        app:layout_constraintEnd_toEndOf="@+id/avatar"
        app:layout_constraintStart_toStartOf="@+id/avatar"
        app:layout_constraintTop_toBottomOf="@+id/avatar" />

</androidx.constraintlayout.widget.ConstraintLayout>

goneMargin：

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

    <TextView
        android:id="@+id/textview"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="测试数据"
        android:textSize="28sp"
        android:visibility="visible"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toTopOf="parent" />


    <ImageView
        android:id="@+id/avatar"
        android:layout_width="40dp"
        android:layout_height="40dp"
        android:src="@mipmap/ic_launcher_round"
        app:layout_constraintStart_toEndOf="@id/textview"
        app:layout_constraintTop_toTopOf="@id/textview"
        app:layout_goneMarginLeft="20dp" />

</androidx.constraintlayout.widget.ConstraintLayout>

chainStyle：约束链风格

<?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"
    android:layout_width="match_parent"
    android:layout_height="match_parent">
    
    <ImageView
        android:id="@+id/view1"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:src="@mipmap/ic_launcher"
        app:layout_constraintBottom_toBottomOf="parent"
        app:layout_constraintEnd_toStartOf="@+id/view2"
        app:layout_constraintHorizontal_chainStyle="spread_inside"
        app:layout_constraintStart_toStartOf="parent" />

    <ImageView
        android:id="@+id/view2"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:src="@mipmap/ic_launcher"
        app:layout_constraintBottom_toBottomOf="parent"
        app:layout_constraintEnd_toStartOf="@+id/view3"
        app:layout_constraintStart_toEndOf="@+id/view1" />

    <ImageView
        android:id="@+id/view3"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:src="@mipmap/ic_launcher"
        app:layout_constraintBottom_toBottomOf="parent"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintStart_toEndOf="@+id/view2" />

</androidx.constraintlayout.widget.ConstraintLayout>

constraintDimensionRatio：

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

    <ImageView
        android:layout_width="0dp"
        android:layout_height="100dp"
        android:scaleType="fitXY"
        android:src="@mipmap/ic_launcher"
        app:layout_constraintBottom_toBottomOf="parent"
        app:layout_constraintDimensionRatio="2:1"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toTopOf="parent" />


</androidx.constraintlayout.widget.ConstraintLayout>

constraintWidth_percent：

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

    <ImageView
        android:layout_width="0dp"
        android:layout_height="300dp"
        android:scaleType="fitXY"
        android:src="@mipmap/ic_launcher"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toTopOf="parent"
        app:layout_constraintWidth_percent="0.5" />

</androidx.constraintlayout.widget.ConstraintLayout>

GuideLine：

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

    <androidx.constraintlayout.widget.Guideline
        android:id="@+id/guideline"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:orientation="vertical"
        app:layout_constraintGuide_begin="100dp"
        app:layout_constraintGuide_percent="0.2" />

    <TextView
        android:id="@+id/tv_username"
        android:layout_width="wrap_content"
        android:layout_height="60dp"
        android:layout_marginTop="72dp"
        android:gravity="center_vertical"
        android:text="用户名"
        app:layout_constraintEnd_toStartOf="@+id/guideline"
        app:layout_constraintTop_toTopOf="parent" />

    <TextView
        android:id="@+id/tv_password"
        android:layout_width="wrap_content"
        android:layout_height="60dp"
        android:gravity="center_vertical"
        android:text="密码"
        app:layout_constraintEnd_toStartOf="@+id/guideline"
        app:layout_constraintTop_toBottomOf="@+id/tv_username" />

    <EditText
        android:id="@+id/et_username"
        android:layout_width="200dp"
        android:layout_height="wrap_content"
        app:layout_constraintBottom_toBottomOf="@+id/tv_username"
        app:layout_constraintStart_toStartOf="@+id/guideline" />

    <EditText
        android:layout_width="200dp"
        android:layout_height="wrap_content"
        app:layout_constraintBottom_toBottomOf="@+id/tv_password"
        app:layout_constraintStart_toEndOf="@+id/tv_password" />
</androidx.constraintlayout.widget.ConstraintLayout>

Group：