ConstraintLayout, 即约束布局, 在2016年由Google I/O推出. 从支持力度而言, 将成为主流布局样式, 完全代替其他布局, 减少布局的层级, 优化渲染性能. 在新版Android Studio中, ConstraintLayout已替代RelativeLayout, 成为HelloWorld项目的默认布局. ConstraintLayout作为非绑定(Unbundled)的支持库, 命名空间是app:, 即来源于本地的包命名空间. 最新版本是1.0.1(2017.4.21), 在项目的build.gradle中声明.

dependencies {
    compile 'com.android.support.constraint:constraint-layout:1.0.1'
}

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Constraint Layout

概念

ConstraintLayout约束布局的含义: 根据布局中的其他元素或视图, 确定View在屏幕中的位置, 受到三类约束, 即其他视图, 父容器(parent), 基准线(Guideline).

layout_constraint[本源位置]_[目标位置]="[目标ID]"

例如: 

app:layout_constraintBottom_toBottomOf="@+id/constraintLayout"

约束当前View的底部至目标View的底部, 目标View是constraintLayout. 即, 把当前View的底部对齐到constraintLayout的底部.

演示

本例复用的Activity页面, 根据不同的参数设置对应的标题和布局ID.

public class LayoutDisplayActivity extends AppCompatActivity {
    private static final String TAG = LayoutDisplayActivity.class.getSimpleName();
    static final String EXTRA_LAYOUT_ID = TAG + ".layoutId"; // 布局ID

    @Override protected void onCreate(@Nullable Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setTitle(getIntent().getStringExtra(Intent.EXTRA_TITLE));
        final int layoutId = getIntent().getIntExtra(EXTRA_LAYOUT_ID, 0);
        setContentView(layoutId); // 设置页面布局, 复用布局
    }
}

主页面使用ListView展示多项, 每项对应不同的布局页面.

public class MainActivity extends AppCompatActivity {
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);

        ListView list = (ListView) findViewById(R.id.activity_main);
        ArrayAdapter<String> adapter = new ArrayAdapter<>(this,
                android.R.layout.simple_list_item_1, LIST_ITEMS);
        list.setAdapter(adapter);
        list.setOnItemClickListener(new AdapterView.OnItemClickListener() {
            @Override
            public void onItemClick(AdapterView<?> adapterView, View view, int i, long l) {
                // 复用显示布局
                Intent intent = new Intent(MainActivity.this, LayoutDisplayActivity.class);
                intent.putExtra(Intent.EXTRA_TITLE, LIST_ITEMS[i]); // 标题
                intent.putExtra(LayoutDisplayActivity.EXTRA_LAYOUT_ID, LAYOUT_IDS[i]); // 布局Id
                startActivity(intent);
            }
        });
    }
}

基础

ConstraintLayout最基本的使用方式, 就是指定约束. 如, 取消按钮的底部对齐constraintLayout(父容器)的底部, 左侧对齐父容器的左侧. 下一步按钮的底部对齐父容器的底部, 而左侧对齐取消按钮的右侧, 每个按钮添加Margin空隙.

父容器可以直接指定ID, 也可以使用parent代指.

<?xml version="1.0" encoding="utf-8"?>
<android.support.constraint.ConstraintLayout
    android:id="@+id/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">

    <Button
        android:id="@+id/cancel_button"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_marginBottom="16dp"
        android:layout_marginStart="16dp"
        android:text="取消"
        app:layout_constraintBottom_toBottomOf="@id/constraintLayout"
        app:layout_constraintStart_toStartOf="@id/constraintLayout"/>

    <Button
        android:id="@+id/next_button"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_marginBottom="16dp"
        android:layout_marginStart="16dp"
        android:text="下一步"
        app:layout_constraintBottom_toBottomOf="@id/constraintLayout"
        app:layout_constraintStart_toEndOf="@id/cancel_button"/>

</android.support.constraint.ConstraintLayout>

ConstraintLayout的属性设置, 最重要的就是理解属性的表示含义.

Base

比例

ConstraintLayout除了指定约束, 还支持设置比例. Center按钮的全部边界与constraintLayout(父容器)边界对齐, 则为居中. 同时还可以设置水平与竖直的比例, 如Bias按钮, 在对齐父容器后, 设置水平与竖直的比例均为0.25, 表示左侧与右侧比例是1:4, 上部与下部的比例是1:4.

constraintHorizontal_bias设置水平比例, constraintVertical_bias设置竖直比例. 

<?xml version="1.0" encoding="utf-8"?>
<android.support.constraint.ConstraintLayout
    xmlns:tools="http://schemas.android.com/tools"
    android:id="@+id/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">

    <Button
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="Center"
        app:layout_constraintEnd_toEndOf="@id/constraintLayout"
        app:layout_constraintStart_toStartOf="@id/constraintLayout"
        app:layout_constraintTop_toTopOf="@+id/constraintLayout"
        app:layout_constraintBottom_toBottomOf="@+id/constraintLayout"/>

    <Button
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="Bias"
        app:layout_constraintEnd_toEndOf="@id/constraintLayout"
        app:layout_constraintStart_toStartOf="@id/constraintLayout"
        app:layout_constraintTop_toTopOf="@+id/constraintLayout"
        app:layout_constraintBottom_toBottomOf="@+id/constraintLayout"
        app:layout_constraintHorizontal_bias="0.25"
        app:layout_constraintVertical_bias="0.25"/>

</android.support.constraint.ConstraintLayout>

Bias

引导线

ConstraintLayout除了与视图约束以外, 还支持与引导线(Guideline)约束. 如, 设置竖直引导线(Guideline)距离左侧72dp. 两个按钮的左侧都与引导线约束, 上下使用比例方式排列, 一个是0.25, 一个是0.75.

<?xml version="1.0" encoding="utf-8"?>
<android.support.constraint.ConstraintLayout
    xmlns:tools="http://schemas.android.com/tools"
    android:id="@+id/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">

    <android.support.constraint.Guideline
        android:id="@+id/guideLine"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:orientation="vertical"
        app:layout_constraintGuide_begin="72dp"/>

    <Button
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="Guide Up"
        app:layout_constraintStart_toStartOf="@id/guideLine"
        app:layout_constraintTop_toTopOf="@+id/constraintLayout"
        app:layout_constraintBottom_toBottomOf="@+id/constraintLayout"
        app:layout_constraintVertical_bias="0.25"/>

    <Button
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="Guide Down"
        app:layout_constraintStart_toStartOf="@id/guideLine"
        app:layout_constraintTop_toTopOf="@+id/constraintLayout"
        app:layout_constraintBottom_toBottomOf="@+id/constraintLayout"
        app:layout_constraintVertical_bias="0.75"/>

</android.support.constraint.ConstraintLayout>

Guide Line

视图尺寸

ConstraintLayout也支持自动填充宽高, 把宽高设置为0dp会根据位置自动填充. 如, Large按钮, 左侧与Small按钮的左侧对齐, 右侧与constraintLayout(父控件)的右侧对齐, 宽度设置为0dp, 则会填充全部空位.

<?xml version="1.0" encoding="utf-8"?>
<android.support.constraint.ConstraintLayout
    android:id="@+id/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">

    <Button
        android:id="@+id/small"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="Small"
        app:layout_constraintStart_toStartOf="@id/constraintLayout"
        app:layout_constraintTop_toTopOf="@id/constraintLayout"/>

    <Button
        android:layout_width="0dp"
        android:layout_height="wrap_content"
        android:text="Large"
        app:layout_constraintBottom_toBottomOf="@id/constraintLayout"
        app:layout_constraintEnd_toEndOf="@id/constraintLayout"
        app:layout_constraintStart_toEndOf="@id/small"
        app:layout_constraintTop_toTopOf="@id/constraintLayout"/>

</android.support.constraint.ConstraintLayout>

Measure

纵横比

ConstraintLayout支持使用constraintDimensionRatio设置宽高的纵横比, 把宽(layout_width)或者高(layout_height)设置为0dp, 则根据另一个属性值和比例, 计算当前属性值.

<?xml version="1.0" encoding="utf-8"?>
<android.support.constraint.ConstraintLayout
    android:id="@+id/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="200dp"
        android:background="@color/colorAccent"
        android:src="@drawable/total_large"
        app:layout_constraintBottom_toBottomOf="@+id/constraintLayout"
        app:layout_constraintLeft_toLeftOf="@+id/constraintLayout"
        app:layout_constraintRight_toRightOf="@+id/constraintLayout"
        app:layout_constraintTop_toTopOf="@+id/constraintLayout"
        app:layout_constraintDimensionRatio="16:9"/>

    <ImageView
        android:layout_width="0dp"
        android:layout_height="200dp"
        android:background="@color/colorAccent"
        android:contentDescription="@null"
        android:src="@drawable/total_large"
        app:layout_constraintBottom_toBottomOf="@+id/constraintLayout"
        app:layout_constraintDimensionRatio="4:3"
        app:layout_constraintLeft_toLeftOf="@+id/constraintLayout"
        app:layout_constraintRight_toRightOf="@+id/constraintLayout"/>

</android.support.constraint.ConstraintLayout>

Ratio

链样式

ConstraintLayout同时支持链样式, 这与LinearLayout的layout_weight属性非常类似, 通过设置不同的样式排列元素.

Chain

app:layout_constraintHorizontal_chainStyle设置水平链.

<TextView
    android:id="@+id/property_tv_cst_aries"
    style="@style/MemberWidget.Constellation"
    android:layout_marginLeft="@dimen/spacing_big"
    android:layout_marginTop="@dimen/spacing_medium"
    android:onClick="@{listener::onConstellationSelected}"
    android:text="白"
    app:layout_constraintHorizontal_chainStyle="spread_inside"
    app:layout_constraintLeft_toLeftOf="parent"
    app:layout_constraintRight_toLeftOf="@+id/property_tv_cst_taurus"
    app:layout_constraintTop_toBottomOf="@id/property_tv_constellation" />

通过不同的链式组合, 生成复杂的视图样式.

Chains

ConstraintLayout的基本使用方式就是这些, 兼顾LinearLayout与RelativeLayout的优点, 非常适合构建复杂布局, 降低布局的层级, 加快渲染速度.