Flutter 布局基础教程

目录

欢迎来到 Flutter 布局 codelab！你将在这里学到如何构建 Flutter UI，更棒的是这一切都不需要安装 Flutter 或者 Dart！

Welcome to the Flutter layout codelab, where you learn how to build a Flutter UI without downloading and installing Flutter or Dart!

Flutter 与其他框架有着明显的差异，原因在于它使用代码来构建 UI，而不是 XML 或其他东西。其中，widget 是构建 Flutter UI 的基本单元。当你逐渐深入这个 codelab，你将会发现在 Flutter 中几乎所有的东西都是 widget。 widget 是一个不会改变的对象，它是 UI 中一个特定部分的描述。你还会学到 Flutter 的 widget 非常容易组合，这意味着你能够通过组合已有的 widgets 来创造更多复杂的 widgets。到这篇文章的最后，你会运用这里所学的知识构建一个显示名片的 Flutter UI。

Flutter is different from other frameworks because its UI is built in code, not (for example) in an XML file or similar. widgets are the basic building blocks of a Flutter UI. As you progress through this codelab, you’ll learn that almost everything in Flutter is a widget. A widget is an immutable object that describes a specific part of a UI. You’ll also learn that Flutter widgets are composable, meaning that you can combine existing widgets to make more sophisticated widgets. At the end of this codelab, you’ll get to apply what you’ve learned into building a Flutter UI that displays a business card.

本 codelab 的预期完成时间约为 45 - 60 分钟

Estimated time to complete this codelab: 45-60 minutes.

Row 和 Column 类

Row and Column classes

Row 和 Column 是两个用来容纳和布局 widgets 的类。在它们内部的 widgets 我们称为 children， Row 和 Column 就作为它们的父级。 Row 将会让 widgets 水平排列，而 Column 则会让其竖直排列。

Row and Column are classes that contain and lay out widgets. widgets inside of a Row or Column are called children, and Row and Column are referred to as parents. Row lays out its widgets horizontally, and Column lays out its widgets vertically.

样例：创建一个 Column

Example: Creating a Column

{$ begin main.dart $}
import 'dart:async';
import 'package:flutter/material.dart';
import 'package:flutter_test/flutter_test.dart';

class MyWidget extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return Row(
      children: [
        BlueBox(),
        BlueBox(),
        BlueBox(),
      ],
    );
  }
}

class BlueBox extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return Container(
      width: 50,
      height: 50,
      decoration: BoxDecoration(
        color: Colors.blue,
        border: Border.all(),
      ),
    );
  }
}
{$ end main.dart $}
{$ begin test.dart $}
class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return Directionality(
      textDirection: TextDirection.ltr,
      child: Container(
        color: Color(0xffeeeeee),
        child: Center(
          child: Container(
            child: MyWidget(),
            color: Color(0xffcccccc),
          ),
        ),
      ),
    );
  }
}

Future<void> main() async {
  final completer = Completer<void>();
  
  runApp(MyApp());

  WidgetsFlutterBinding.ensureInitialized()
      .addPostFrameCallback((timestamp) async {
    completer.complete();
  });
  
  await completer.future;
  
  runApp(MyApp());

  final controller = LiveWidgetController(WidgetsBinding.instance!);

  final columns = controller.widgetList(find.byType(Column));

  if (columns.length == 0) {
    _result(false, ['The Row contains three BlueBox widgets and lays them out horizontally.']);
    return;
  }

  if (columns.length > 1) {
    _result(false, ['Found ${columns.length} Rows, rather than just one.']);
    return;
  }

  final column = columns.first as Column;

  if (column.children.length != 3 || column.children.any((w) => w is! BlueBox)) {
    _result(false, ['Row/Column should contain three children, all BlueBox widgets.']);
    return;
  }

  _result(true, ['The Column contains three BlueBox widgets and lays them out vertically.']);
}
{$ end test.dart $}

轴大小和对齐方式

Axis size and alignment

至此，BlueBox widget 已经在一起被压扁了 (在界面的左边或者上面 )。你可以通过轴大小和对齐属性来改变 BlueBox widget 的间距。

So far, the BlueBox widgets have been squished together (either to the left or at the top of the UI Output). You can change how the BlueBox widgets are spaced out using the axis size and alignment properties.

mainAxisSize 属性

mainAxisSize property

Row 和 Column分别占据了不同的主轴。Row 的主轴是水平的。 mainAxisSize 决定了 Row 和 Column 能够在主轴上占据多大空间。 mainAxisSize 有两个可选属性：

Row and Column occupy different main axes. A Row’s main axis is horizontal, and a Column’s main axis is vertical. The mainAxisSize property determines how much space a Row and Column can occupy on their main axes. The mainAxisSize property has two possible values:

MainAxisSize.max
Row 和 Column 占据它们主轴上所有空间。如果子 widget 的总宽度小于主轴上的空间，它们就会充满剩余的空间。

MainAxisSize.max
Row and Column occupy all of the space on their main axes. If the combined width of their children is less than the total space on their main axes, their children are laid out with extra space.

MainAxisSize.min
Row 和 Column 仅占据它的 children 在主轴上所需的空间，它的 children 在主轴之间将没有额外空间。

MainAxisSize.min
Row and Column only occupy enough space on their main axes for their children. Their children are laid out without extra space and at the middle of their main axes.

样例：自定义轴大小

Example: Modifying axis size

{$ begin main.dart $}
import 'dart:async';
import 'package:flutter/material.dart';
import 'package:flutter_test/flutter_test.dart';

class MyWidget extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return Row(
      mainAxisSize: MainAxisSize.max,
      children: [
        BlueBox(),
        BlueBox(),
        BlueBox(),
      ],
    );
  }
}

class BlueBox extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return Container(
      width: 50,
      height: 50,
      decoration: BoxDecoration(
        color: Colors.blue,
        border: Border.all(),
      ),
    );
  }
}
{$ end main.dart $}
{$ begin test.dart $}
class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return Directionality(
      textDirection: TextDirection.ltr,
      child: Container(
        color: Color(0xffeeeeee),
        child: Center(
          child: Container(
            child: MyWidget(),
            color: Color(0xffcccccc),
          ),
        ),
      ),
    );
  }
}

Future<void> main() async {
  final completer = Completer<void>();

  runApp(MyApp());

  WidgetsFlutterBinding.ensureInitialized()
      .addPostFrameCallback((timestamp) async {
    completer.complete();
  });

  await completer.future;

  final controller = LiveWidgetController(WidgetsBinding.instance!);

  final rows = controller.widgetList(find.byType(Row));

  if (rows.length == 0) {
    _result(false, ['Couldn\'t find Row!']);
    return;
  }

  if (rows.length > 1) {
    _result(false, ['Found ${rows.length} Rows, rather than just one.']);
    return;
  }

  final row = rows.first as Row;

  if (row.mainAxisSize != MainAxisSize.min) {
    _result(false, ['Row lays out the BlueBox widgets with extra space. Change MainAxisSize.max to MainAxisSize.min']);
    return;
  }

  if (row.children.length != 3 || row.children.any((w) => w is! BlueBox)) {
    _result(false, ['There should only be three children, all BlueBox widgets.']);
    return;
  }

  _result(true, ['Row lays out the BlueBox widgets without extra space, and the BlueBox widgets are positioned at the middle of Row\'s main axis.']);
}
{$ end test.dart $}

mainAxisAlignment 属性

mainAxisAlignment property

当 mainAxisSize 被设为 MainAxisSize.max, Row 和 Column 将会使用额外空间来对齐它的 children。 mainAxisAlignment 属性决定了 Row 和 Column 将会在额外空间中如何对齐它的 children。 mainAxisAlignment 有以下六个可选属性：

When mainAxisSize is set to MainAxisSize.max, Row and Column might lay out their children with extra space. The mainAxisAlignment property determines how Row and Column can position their children in that extra space. mainAxisAlignment has six possible values:

MainAxisAlignment.start
将其 children 从主轴起点处开始对齐。 (Row 的起点在左边，Column 的起点在顶部 )

MainAxisAlignment.start
Positions children near the beginning of the main axis. (Left for Row, top for Column)

MainAxisAlignment.end
将其 children 从主轴终点处开始对齐。 (Row 的终点在右边，Column 的终点在底部 )

MainAxisAlignment.end
Positions children near the end of the main axis. (Right for Row, bottom for Column)

MainAxisAlignment.center
将其 children 置于主轴中心。

MainAxisAlignment.center
Positions children at the middle of the main axis.

MainAxisAlignment.spaceBetween
在 children 之间平均分配额外空间。

MainAxisAlignment.spaceBetween
Divides the extra space evenly between children.

MainAxisAlignment.spaceEvenly
在 children 之间，以及第一个 children 之前和最后一个 children 之后，平均分配额外空间。

MainAxisAlignment.spaceEvenly
Divides the extra space evenly between children and before and after the children.

MainAxisAlignment.spaceAround
与 MainAxisAlignment.spaceEvenly 相似，但在第一个 child 之前以及最后一个孩子之后减少了一半的空间，让其 children 之间宽度缩减一半。

MainAxisAlignment.spaceAround
Similar to MainAxisAlignment.spaceEvenly, but reduces half of the space before the first child and after the last child to half of the width between the children.

样例：自定义主轴对齐方式

Example: Modifying main axis alignment

{$ begin main.dart $}
import 'dart:async';
import 'package:flutter/material.dart';
import 'package:flutter_test/flutter_test.dart';

class MyWidget extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return Row(
      mainAxisAlignment: MainAxisAlignment.start,
      children: [
        BlueBox(),
        BlueBox(),
        BlueBox(),
      ],
    );
  }
}

class BlueBox extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return Container(
      width: 50,
      height: 50,
      decoration: BoxDecoration(
        color: Colors.blue,
        border: Border.all(),
      ),
    );
  }
}
{$ end main.dart $}
{$ begin test.dart $}
class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return Directionality(
      textDirection: TextDirection.ltr,
      child: Container(
        color: Color(0xffeeeeee),
        child: Center(
          child: Container(
            child: MyWidget(),
            color: Color(0xffcccccc),
          ),
        ),
      ),
    );
  }
}

Future<void> main() async {
  final completer = Completer<void>();

  runApp(MyApp());

  WidgetsFlutterBinding.ensureInitialized()
      .addPostFrameCallback((timestamp) async {
    completer.complete();
  });

  await completer.future;
  final controller = LiveWidgetController(WidgetsBinding.instance!);

  final rows = controller.widgetList(find.byType(Row));

  if (rows.length == 0) {
    _result(false, ['Couldn\'t find a Row!']);
    return;
  }

  if (rows.length > 1) {
    _result(false, ['Found ${rows.length} Rows, rather than just one.']);
    return;
  }

  final row = rows.first as Row;

  if (row.mainAxisSize != MainAxisSize.max) {
    _result(false, ['It\'s best to leave the mainAxisSize set to MainAxisSize.max, so there\'s space for the alignments to take effect.']);
    return;
  }

  if (row.children.length != 3 || row.children.any((w) => w is! BlueBox)) {
    _result(false, ['The Row should have three children, all BlueBox widgets.']);
    return;
  }

  if (row.mainAxisAlignment == MainAxisAlignment.start) {
    _result(false, ['MainAxisAlignment.start positions the BlueBox widgets on the left of the main axis. Change the value to MainAxisAlignment.end.']);
  } else if (row.mainAxisAlignment == MainAxisAlignment.end) {
    _result(true, ['MainAxisAlignment.end positions the BlueBox widgets on the right of the main axis.']);
  } else if (row.mainAxisAlignment == MainAxisAlignment.center) {
    _result(true, ['MainAxisAlignment.center positions the BlueBox widgets at the middle of the main axis.']);
  } else if (row.mainAxisAlignment == MainAxisAlignment.spaceBetween) {
    _result(true, ['The extra space is divided between the BlueBox widgets.']);
  } else if (row.mainAxisAlignment == MainAxisAlignment.spaceEvenly) {
    _result(true, ['The extra space is divided evenly between the BlueBox widgets and before and after them.']);
  } else if (row.mainAxisAlignment == MainAxisAlignment.spaceAround) {
    _result(true, ['Similar to MainAxisAlignment.spaceEvenly, but reduces half of the space before the first BlueBox widget and after the last BlueBox widget to half of the width between the BlueBox widgets.']);
  }
}
{$ end test.dart $}

crossAxisAlignment 属性

crossAxisAlignment property

crossAxisAlignment 属性决定了 Row 和 Column 能够如何在其横轴上定位 children。 Row 的横轴是竖直的，而 Column 则是水平的， crossAxisAlignment 属性有五个可选属性：

The crossAxisAlignment property determines how Row and Column can position their children on their cross axes. A Row’s cross axis is vertical, and a Column’s cross axis is horizontal. The crossAxisAlignment property has five possible values:

CrossAxisAlignment.start
将其 children 横轴顶部对齐。 (顶部是 Row，左侧是 Column )

CrossAxisAlignment.start
Positions children near the start of the cross axis. (Top for Row, Left for Column)

CrossAxisAlignment.end
将其 children 横轴底部对齐。 (底部是 Row，右侧是 Column )

CrossAxisAlignment.end
Positions children near the end of the cross axis. (Bottom for Row, Right for Column)

CrossAxisAlignment.center
将其 children 横轴中心对齐。 (中间是 Row，中心是 Column )

CrossAxisAlignment.center
Positions children at the middle of the cross axis. (Middle for Row, Center for Column)

CrossAxisAlignment.stretch
沿横轴延伸 children。 (在 Row 中是从顶至底，Column 则是从左至右 )

CrossAxisAlignment.stretch
Stretches children across the cross axis. (Top-to-bottom for Row, left-to-right for Column)

CrossAxisAlignment.baseline
根据 children 的基线对子节点。 (仅限Text类，并要求 textBaseline 属性设置为 TextBaseline.alphabetic。在 Text class 小节中查看样例。

CrossAxisAlignment.baseline
Aligns children by their character baselines. (Text class only, and requires that the textBaseline property is set to TextBaseline.alphabetic. See the Text widget section for an example.)

样例：自定义横轴对齐方式

Example: Modifying cross axis alignment

{$ begin main.dart $}
import 'dart:async';
import 'package:flutter/material.dart';
import 'package:flutter_test/flutter_test.dart';

class MyWidget extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return Row(
      mainAxisAlignment: MainAxisAlignment.spaceAround,
      crossAxisAlignment: CrossAxisAlignment.center,
      children: [
        BlueBox(),
        BiggerBlueBox(),
        BlueBox(),
      ],
    );
  }
}

class BlueBox extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return Container(
      width: 50,
      height: 50,
      decoration: BoxDecoration(
        color: Colors.blue,
        border: Border.all(),
      ),
    );
  }
}

class BiggerBlueBox extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return Container(
      width: 50,
      height: 100,
      decoration: BoxDecoration(
        color: Colors.blue,
        border: Border.all(),
      ),
    );
  }
}
{$ end main.dart $}
{$ begin test.dart $}
class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return Directionality(
      textDirection: TextDirection.ltr,
      child: Container(
        color: Color(0xffeeeeee),
        child: Center(
          child: Container(
            child: MyWidget(),
            color: Color(0xffcccccc),
          ),
        ),
      ),
    );
  }
}

Future<void> main() async {
  final completer = Completer<void>();
  
  runApp(MyApp());

  WidgetsFlutterBinding.ensureInitialized()
      .addPostFrameCallback((timestamp) async {
    completer.complete();
  });
  
  await completer.future;

  final controller = LiveWidgetController(WidgetsBinding.instance!);

  final rows = controller.widgetList(find.byType(Row));

  if (rows.length == 0) {
    _result(false, ['Couldn\'t find a Row!']);
    return;
  }

  if (rows.length > 1) {
    _result(false, ['Found ${rows.length} Rows, rather than just one.']);
    return;
  }

  final row = rows.first as Row;

  if (row.children.length != 3 || row.children.any((w) => w is! BlueBox && w is! BiggerBlueBox)) {
    _result(false, ['The Row should have three children, all BlueBox or BiggerBlueBox widgets.']);
    return;
  }

  if (row.crossAxisAlignment == CrossAxisAlignment.start) {
    _result(true, ['The BlueBox and BiggerBlueBox widgets are positioned at the top of the cross axis.']);
  } else if (row.crossAxisAlignment == CrossAxisAlignment.end) {
    _result(true, ['The BlueBox and BiggerBlueBox widgets are positioned at the bottom of the cross axis']);
  } else if (row.crossAxisAlignment == CrossAxisAlignment.center) {
    _result(false, ['The widgets are positioned at the middle of the cross axis. Change CrossAxisAlignment.center to CrossAxisAlignment.start.']);
  } else if (row.crossAxisAlignment == CrossAxisAlignment.stretch) {
    _result(true, ['The BlueBox and BiggerBlueBox widgets are stretched across the cross axis. Change the Row to a Column, and run again.']);
  } else if(row.crossAxisAlignment == CrossAxisAlignment.baseline) {
    _result(false, ['Couldn\t find a text class.']);
  }
}
{$ end test.dart $}

Flexible widget

正如你所看到，mainAxisAlignment 和 crossAxisAlignment 属性决定了 Row 和 Column在各个轴上如何布局 widget。 Row 和 Column 首先布置固定大小的 widget。固定大小的小部件被认为是 不灵活的 因为它们布局后无法自我调整大小。

As you’ve seen, the mainAxisAlignment and crossAxisAlignment properties determine how Row and Column position widgets along both axes. Row and Column first lay out widgets of a fixed size. Fixed size widgets are considered inflexible because they can’t resize themselves after they’ve been laid out.

Flexible widget 包裹一个 widget 让这个 widget 变得可以调整大小。当 Flexible widget 包裹 widget 时，这个 widget 就成为 Flexible widget 的子节点，并被视为 flexible 的。在布置固定大小的 widget 后， Flex 的 widget 根据其 flex 和 fit 属性调整大小：

The Flexible widget wraps a widget, so the widget becomes resizable. When the Flexible widget wraps a widget, the widget becomes the Flexible widget’s child and is considered flexible. After inflexible widgets are laid out, the widgets are resized according to their flex and fit properties.:

flex
将自身的 flex 因子与其他的比较，以决定自身占剩余空间的比例。

flex
Compares itself against other flex properties before determining what fraction of the total remaining space each Flexible widget receives.

fit
决定 Flexible 的 widget 是否能够填充所有剩余空间。

fit
Determines whether a Flexible widget fills all of its extra space.

样例：改变 fit 属性

Example: Changing fit properties

{$ begin main.dart $}
import 'dart:async';
import 'package:flutter/material.dart';
import 'package:flutter_test/flutter_test.dart';

class MyWidget extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return Row(
      children: [
        BlueBox(),
        Flexible(
          fit: FlexFit.loose,
          flex: 1,
          child: BlueBox(),
        ),
        Flexible(
          fit: FlexFit.loose,
          flex: 1,
          child: BlueBox(),
        ),
      ],
    );
  }
}

class BlueBox extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return Container(
      width: 50,
      height: 50,
      decoration: BoxDecoration(
        color: Colors.blue,
        border: Border.all(),
      ),
    );
  }
}
{$ end main.dart $}
{$ begin test.dart $}
class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return Directionality(
      textDirection: TextDirection.ltr,
      child: Container(
        color: Color(0xffeeeeee),
        child: Center(
          child: Container(
            child: MyWidget(),
            color: Color(0xffcccccc),
          ),
        ),
      ),
    );
  }
}

Future<void> main() async {
  final completer = Completer<void>();
  
  runApp(MyApp());

  WidgetsFlutterBinding.ensureInitialized()
      .addPostFrameCallback((timestamp) async {
    completer.complete();
  });
  
  await completer.future;

  final controller = LiveWidgetController(WidgetsBinding.instance!);

  final rows = controller.widgetList(find.byType(Row));

  if (rows.length == 0) {
    _result(false, ['Couldn\'t find a Row!']);
    return;
  }

  if (rows.length > 1) {
    _result(false, ['Found ${rows.length} Rows, rather than just one.']);
    return;
  }

  final row = rows.first as Row;

  if (row.mainAxisSize != MainAxisSize.max) {
    _result(false, ['It\'s best to leave the mainAxisSize set to MainAxisSize.max, so there\'s space for the alignments to take effect.']);
    return;
  }
  
  if (row.children.length != 3) {
    _result(false, ['The Row should have three children, all BlueBox or Flexible widgets.']);
    return;
  }

  if (row.children[0] is! BlueBox) {
    _result(false, ['Row\'s first child should be a BlueBox.']);
    return;
  }

  if (row.children[1] is! Flexible) {
    _result(false, ['Row\'s second child should be a Flexible class.']);
    return;
  }

  if (row.children[2] is! Flexible) {
    _result(false, ['Row\'s third child should be a Flexible class.']);
    return;
  }

  final flexibleWidget = row.children[2] as Flexible;
  
  if (flexibleWidget.child is! BlueBox) {
    _result(false, ['The Flexible classes should have BlueBox widgets as their children.']);
    return;
  }

  if (flexibleWidget.fit != FlexFit.tight) {
    _result(false, ['The fit properties set the Flexible widgets to their preferred size. Change both fit values to FlexFit.tight.']);
    return;
  }

  _result(true, ['The Flexible widgets now occupy the space determined by their flex values.']);
}
{$ end test.dart $}

样例：测试 flex 值

Example: Testing flex values

{$ begin main.dart $}
import 'dart:async';
import 'package:flutter/material.dart';
import 'package:flutter_test/flutter_test.dart';

class MyWidget extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return Row(
      children: [
        BlueBox(),
        Flexible(
          fit: FlexFit.tight,
          flex: 1,
          child: BlueBox(),
        ),
        Flexible(
          fit: FlexFit.tight,
          flex: 1,
          child: BlueBox(),
        ),
      ],
    );
  }
}

class BlueBox extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return Container(
      width: 50,
      height: 50,
      decoration: BoxDecoration(
        color: Colors.blue,
        border: Border.all(),
      ),
    );
  }
}
{$ end main.dart $}
{$ begin test.dart $}
class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return Directionality(
      textDirection: TextDirection.ltr,
      child: Container(
        color: Color(0xffeeeeee),
        child: Center(
          child: Container(
            child: MyWidget(),
            color: Color(0xffcccccc),
          ),
        ),
      ),
    );
  }
}

Future<void> main() async {
  final completer = Completer<void>();
  
  runApp(MyApp());

  WidgetsFlutterBinding.ensureInitialized()
      .addPostFrameCallback((timestamp) async {
    completer.complete();
  });
  
  await completer.future;

  final controller = LiveWidgetController(WidgetsBinding.instance!);

  final rows = controller.widgetList(find.byType(Row));

  if (rows.length == 0) {
    _result(false, ['Couldn\'t find a Row!']);
    return;
  }

  if (rows.length > 1) {
    _result(false, ['Found ${rows.length} Rows, rather than just one.']);
    return;
  }

  final row = rows.first as Row;

  if (row.children.length != 3) {
    _result(false, ['The Row should have three children, all BlueBlox or Flexible widgets.']);
    return;
  }

  if (row.children[0] is! BlueBox) {
    _result(false, ['The Row\'s first child should be a BlueBox widget.']);
    return;
  }

  if (row.children[1] is! Flexible) {
    _result(false, ['The Row\'s second child should be a Flexible widget.']);
    return;
  }

  if (row.children[2] is! Flexible) {
    _result(false, ['The Row\'s third child should be a Flexible widget.']);
    return;
  }

  final flexibleWidget = row.children[1] as Flexible;
  
  if (flexibleWidget.child is! BlueBox) {
    _result(false, ['The Flexible should have a BlueBox widget as its child.']);
    return;
  }

  if (flexibleWidget.flex != 1) {
    _result(false, ['Notice how the flex properties divide the extra space between the two Flexible widgets.']);
    return;
  }

  _result(true, ['Both Flexible widgets receive half of the total remaining space.']);
}
{$ end test.dart $}

Expanded widget

Expanded widget 能够包裹一个 widget 并强制其填满剩余空间，与 Flexible 非常相似。

Similar to Flexible, the Expanded widget can wrap a widget and force the widget to fill extra space.

样例：填补额外空间

Example: Filling extra space

{$ begin main.dart $}
import 'dart:async';
import 'package:flutter/material.dart';
import 'package:flutter_test/flutter_test.dart';

class MyWidget extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return Row(
      children: [
        BlueBox(),
        BlueBox(),
        BlueBox(),
      ],
    );
  }
}

class BlueBox extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return Container(
      width: 50,
      height: 50,
      decoration: BoxDecoration(
        color: Colors.blue,
        border: Border.all(),
      ),
    );
  }
}
{$ end main.dart $}
{$ begin test.dart $}
class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return Directionality(
      textDirection: TextDirection.ltr,
      child: Container(
        color: Color(0xffeeeeee),
        child: Center(
          child: Container(
            child: MyWidget(),
            color: Color(0xffcccccc),
          ),
        ),
      ),
    );
  }
}

Future<void> main() async {
  final completer = Completer<void>();
  
  runApp(MyApp());

  WidgetsFlutterBinding.ensureInitialized()
      .addPostFrameCallback((timestamp) async {
    completer.complete();
  });
  
  await completer.future;

  final controller = LiveWidgetController(WidgetsBinding.instance!);

  final rows = controller.widgetList(find.byType(Row));

  if (rows.length == 0) {
    _result(false, ['Couldn\'t find a Row!']);
    return;
  }

  if (rows.length > 1) {
    _result(false, ['Found ${rows.length} Rows, rather than just one.']);
    return;
  }

  final row = rows.first as Row;
  
  if (row.children.length != 3) {
    _result(false, ['The Row should have three children, all BlueBox widgets.']);
    return;
  }

   if (row.children[0] is! BlueBox) {
    _result(false, ['The Row\'s first child should be a BlueBox widget.']);
    return;
  }

  if (row.children[1] is! Expanded) {
    _result(false, ['Notice how Row contains extra space on its main axis. Wrap the second BlueBox widget in an Expanded widget.']);
    return;
  }

  if (row.children[2] is! BlueBox) {
    _result(false, ['The Row\'s third child should be a Flexible widget.']);
    return;
  }
  
  _result(true, ['Expanded forces second BlueBox widget to fill the extra space.']);
}
{$ end test.dart $}

SizedBox widget

SizedBox widget 的两种用途之一就是创建精确的尺寸。当 SizedBox 包裹了一个 widget 时，它会使用 height 和 width 调整其大小。如果它没有包裹 widget，它可以使用height和width属性创造空的空间。

The SizedBox widget can be used in one of two ways when creating exact dimensions. When SizedBox wraps a widget, it resizes the widget using the height and width properties. When it doesn’t wrap a widget, it uses the height and width properties to create empty space.

样例：调整一个 widget

Example: Resizing a widget

{$ begin main.dart $}
import 'dart:async';
import 'package:flutter/material.dart';
import 'package:flutter_test/flutter_test.dart';

class MyWidget extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return Row(
      mainAxisSize: MainAxisSize.max,
      children: [
        BlueBox(),
        SizedBox(
          width: 100,
          child: BlueBox(),
        ),
        BlueBox(),
      ],
    );
  }
}

class BlueBox extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return Container(
      width: 50,
      height: 50,
      decoration: BoxDecoration(
        color: Colors.blue,
        border: Border.all(),
      ),
    );
  }
}
{$ end main.dart $}
{$ begin test.dart $}
class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return Directionality(
      textDirection: TextDirection.ltr,
      child: Container(
        color: Color(0xffeeeeee),
        child: Center(
          child: Container(
            child: MyWidget(),
            color: Color(0xffcccccc),
          ),
        ),
      ),
    );
  }
}

Future<void> main() async {
  final completer = Completer<void>();
  
  runApp(MyApp());

  WidgetsFlutterBinding.ensureInitialized()
      .addPostFrameCallback((timestamp) async {
    completer.complete();
  });
  
  await completer.future;

  final controller = LiveWidgetController(WidgetsBinding.instance!);

  final rows = controller.widgetList(find.byType(Row));

  if (rows.length == 0) {
    _result(false, ['Couldn\'t find a Row!']);
    return;
  }

  if (rows.length > 1) {
    _result(false, ['Found ${rows.length} Rows, rather than just one.']);
    return;
  }

  final row = rows.first as Row;

  if (row.mainAxisSize != MainAxisSize.max) {
    _result(false, ['It\'s best to leave the mainAxisSize set to MainAxisSize.max, so there\'s space for the alignments to take effect.']);
    return;
  }
  
  
  if (row.children.length != 3) {
    _result(false, ['The Row should end up with three children.']);
    return;
  }

  if (row.children[0] is! BlueBox) {
    _result(false, ['The Row\'s first child should be a BlueBox widget.']);
    return;
  }

  if (row.children[1] is! SizedBox) {
    _result(false, ['The Row\'s second child should be a SizedBox widget.']);
    return;
  }

  if (row.children[2] is! BlueBox) {
    _result(false, ['The Row\'s third child should be a BlueBox widget.']);
    return;
  }

  final sizedBox = row.children[1] as SizedBox;
  
  if (sizedBox.width != 100) {
    _result(false, ['The SizedBox should have a width of 100.']);
    return;
  }
  
  if (sizedBox.height != 100) {
    _result(false, ['The SizedBox widget resizes the BlueBox widget to 100 logical pixels wide. Add a height property inside SizedBox equal to 100 logical pixels.']);
    return;
  }
  
  _result(true, ['The SizedBox widget resizes the BlueBox widget to 100 logical pixels wide and tall.']);
}
{$ end test.dart $}

样例：创建空间

Example: Creating space

{$ begin main.dart $}
import 'dart:async';
import 'package:flutter/material.dart';
import 'package:flutter_test/flutter_test.dart';

class MyWidget extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return Row(
      children: [
        BlueBox(),
        SizedBox(width: 50),
        BlueBox(),
        BlueBox(),
      ],
    );
  }
}

class BlueBox extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return Container(
      width: 50,
      height: 50,
      decoration: BoxDecoration(
        color: Colors.blue,
        border: Border.all(),
      ),
    );
  }
}
{$ end main.dart $}
{$ begin test.dart $}
class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return Directionality(
      textDirection: TextDirection.ltr,
      child: Container(
        color: Color(0xffeeeeee),
        child: Center(
          child: Container(
            child: MyWidget(),
            color: Color(0xffcccccc),
          ),
        ),
      ),
    );
  }
}

Future<void> main() async {
  final completer = Completer<void>();
  
  runApp(MyApp());

  WidgetsFlutterBinding.ensureInitialized()
      .addPostFrameCallback((timestamp) async {
    completer.complete();
  });
  
  await completer.future;

  final controller = LiveWidgetController(WidgetsBinding.instance!);

  final rows = controller.widgetList(find.byType(Row));

  if (rows.length == 0) {
    _result(false, ['Couldn\'t find a Row!']);
    return;
  }

  if (rows.length > 1) {
    _result(false, ['Found ${rows.length} Rows, rather than just one.']);
    return;
  }

  final row = rows.first as Row;

  if (row.mainAxisSize != MainAxisSize.max) {
    _result(false, ['It\'s best to leave the mainAxisSize set to MainAxisSize.max, so there\'s space for the alignments to take effect.']);
    return;
  }
  
  if (row.mainAxisAlignment == MainAxisAlignment.spaceAround 
      || row.mainAxisAlignment == MainAxisAlignment.spaceBetween 
      || row.mainAxisAlignment == MainAxisAlignment.spaceEvenly) {
    _result(false, ['It\'s best to use MainAxisAlignment.start, MainAxisAlignment.end, or MainAxisAlignment.center to see how the SizedBox widgets work in a Row.']);
    return;
  }
  
  if (row.children.length != 5) {
    _result(false, ['The SizedBox widget creates space at 50 logical pixels wide. Add another SizedBox class between the second and third BlueBox widgets with a width property equal to 25 logical pixels.']);
    return;
  }

  if (row.children[0] is! BlueBox) {
    _result(false, ['The Row\'s first child should be a BlueBox widget.']);
    return;
  }

  if (row.children[1] is! SizedBox) {
    _result(false, ['The Row\'s second child should be a SizedBox widget.']);
    return;
  }

  if (row.children[2] is! BlueBox) {
    _result(false, ['The Row\'s third child should be a BlueBox widget.']);
    return;
  }

  if (row.children[3] is! SizedBox) {
    _result(false, ['The Row\'s fourth child should be a SizedBox widget.']);
    return;  
  }
  
   if (row.children[4] is! BlueBox) {
    _result(false, ['The Row\'s fifth child should be a BlueBox widget.']);
    return;
  }
  
  final sizedBox = row.children[1] as SizedBox;
  
  if (sizedBox.width != 50) {
    _result(false, ['The SizedBox should have a width of 50.']);
    return;
  }
  
  final sizedBox2 = row.children[3] as SizedBox;
  
  if (sizedBox2.width != 25) {
    _result(false, ['SizedBox should have a width of 25.']);
    return;
  }
  
  _result(true, ['The SizedBox widgets create space between the BlueBox widgets, one space at 50 logical pixels and one at 25 logical pixels.']);
}
{$ end test.dart $}

Spacer widget

与 SizedBox 相似，Spacer widget 也能在 widgets 之间创建空间。

Similar to SizedBox, the Spacer widget also can create space between widgets.

Example：创建更多空间

Example: Creating more space

{$ begin main.dart $}
import 'dart:async';
import 'package:flutter/material.dart';
import 'package:flutter_test/flutter_test.dart';

class MyWidget extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return Row(
      children: [
        BlueBox(),
        Spacer(flex: 1),
        BlueBox(),
        BlueBox(),
      ],
    );
  }
}

class BlueBox extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return Container(
      width: 50,
      height: 50,
      decoration: BoxDecoration(
        color: Colors.blue,
        border: Border.all(),
      ),
    );
  }
}
{$ end main.dart $}
{$ begin test.dart $}
class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return Directionality(
      textDirection: TextDirection.ltr,
      child: Container(
        color: Color(0xffeeeeee),
        child: Center(
          child: Container(
            child: MyWidget(),
            color: Color(0xffcccccc),
          ),
        ),
      ),
    );
  }
}

Future<void> main() async {
  final completer = Completer<void>();
  
  runApp(MyApp());

  WidgetsFlutterBinding.ensureInitialized()
      .addPostFrameCallback((timestamp) async {
    completer.complete();
  });
  
  await completer.future;

  final controller = LiveWidgetController(WidgetsBinding.instance!);

  final rows = controller.widgetList(find.byType(Row));

  if (rows.length == 0) {
    _result(false, ['Couldn\'t find a Row!']);
    return;
  }

  if (rows.length > 1) {
    _result(false, ['Found ${rows.length} Rows, rather than just one.']);
    return;
  }

  final row = rows.first as Row;

  if (row.mainAxisSize != MainAxisSize.max) {
    _result(false, ['It\'s best to leave the mainAxisSize set to MainAxisSize.max, so there\'s space for the alignments to take effect.']);
    return;
  }
  
  if (row.mainAxisAlignment == MainAxisAlignment.spaceAround 
      || row.mainAxisAlignment == MainAxisAlignment.spaceBetween 
      || row.mainAxisAlignment == MainAxisAlignment.spaceEvenly) {
    _result(false, ['It\'s best to use MainAxisAlignment.start, MainAxisAlignment.end, or MainAxisAlignment.center to see how the SizedBox widgets work in a Row.']);
    return;
  }
  
  if (row.children.length != 5) {
    _result(false, ['What do you think would happen if you added another Spacer widget with a flex value of 1 between the second and third BlueBox widgets?']);
    return;
  }

  if (row.children[0] is! BlueBox ||
     row.children[1] is! Spacer ||
     row.children[2] is! BlueBox ||
     row.children[3] is! Spacer ||
     row.children[4] is! BlueBox) {
    _result(false, ['Not quite. Row should contain five children in this order: BlueBox, Spacer, BlueBox, Spacer, BlueBox.']);
    return;
  }
  
    final spacer = row.children[3] as Spacer;
  
    if (spacer.flex != 1) {
    _result(false, ['The Spacer class should have a flex equal to 1.']);
    return;
  }
  
  _result(true, ['Both Spacer widgets create equal amounts of space between all three BlueBox widgets.']);
}
{$ end test.dart $}

Text widget

Text widget 不仅能够显示文字，并能够配置不同的字体，大小和颜色。

The Text widget displays text and can be configured for different fonts, sizes, and colors.

样例：文字对齐

Example: Aligning text

{$ begin main.dart $}
import 'dart:async';
import 'package:flutter/material.dart';
import 'package:flutter_test/flutter_test.dart';

class MyWidget extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return Row(
      crossAxisAlignment: CrossAxisAlignment.center,
      textBaseline: TextBaseline.alphabetic,
      children: [
        Text(
          'Hey!',
          style: TextStyle(
            fontSize: 30,
            fontFamily: 'Futura',
            color: Colors.blue,
          ),
        ),
        Text(
          'Hey!',
          style: TextStyle(
            fontSize: 50,
            fontFamily: 'Futura',
            color: Colors.green,
          ),
        ),
        Text(
          'Hey!',
          style: TextStyle(
            fontSize: 40,
            fontFamily: 'Futura',
            color: Colors.red,
          ),
        ),
      ],
    );
  }
}
{$ end main.dart $}
{$ begin test.dart $}
class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return Directionality(
      textDirection: TextDirection.ltr,
      child: Container(
        color: Color(0xffeeeeee),
        child: Center(
          child: Container(
            child: MyWidget(),
            color: Color(0xffcccccc),
          ),
        ),
      ),
    );
  }
}

Future<void> main() async {
  final completer = Completer<void>();
  
  runApp(MyApp());

  WidgetsFlutterBinding.ensureInitialized()
      .addPostFrameCallback((timestamp) async {
    completer.complete();
  });
  
  await completer.future;

  final controller = LiveWidgetController(WidgetsBinding.instance!);

  final rows = controller.widgetList(find.byType(Row));

  if (rows.length == 0) {
    _result(false, ['Couldn\'t find a Row!']);
    return;
  }

  if (rows.length > 1) {
    _result(false, ['Found ${rows.length} Rows, rather than just one.']);
    return;
  }

  final row = rows.first as Row;

  if (row.mainAxisSize != MainAxisSize.max) {
    _result(false, ['It\'s best to leave the mainAxisSize set to MainAxisSize.max, so there\'s space for the alignments to take effect.']);
    return;
  }
  
  if (row.children.length != 3 || row.children.any((w) => w is! Text)) {
    _result(false, ['The Row should have three children, all Text widgets.']);
    return;
  }

  if (row.textBaseline == null) {
    _result(false, ['To use CrossAxisAlignment.baseline, you need to set the Row\'s textBaseline property.']);
    return;
  } 
  
  if (row.crossAxisAlignment != CrossAxisAlignment.baseline) {
    _result(false, ['The Text widgets are positioned at the middle of the cross axis. Change CrossAxisAlignment.center to CrossAxisAlignment.baseline.']);
		return;
  }  

  _result(true, ['The Text widgets are now aligned by their character baselines.']);
  
}
{$ end test.dart $}

Icon widget

Icon widget 能够显示图形符号，这代表了 UI 的一个方面。 Flutter 将会为 Material 和 Cupertino 的应用提前加载 icon packages。

The Icon widget displays a graphical symbol that represents an aspect of the UI. Flutter is preloaded with icon packages for Material and Cupertino applications.

样例：创建一个 Icon

Example: Creating an Icon

{$ begin main.dart $}
import 'dart:async';
import 'package:flutter/material.dart';
import 'package:flutter_test/flutter_test.dart';

class MyWidget extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return Row(
      crossAxisAlignment: CrossAxisAlignment.center,
      textBaseline: TextBaseline.alphabetic,
      children: [
        Icon(
          Icons.widgets,
          size: 50,
          color: Colors.blue,
        ),
        Icon(
          Icons.widgets,
          size: 50,
          color: Colors.red,
        ),
      ],
    );
  }
}
{$ end main.dart $}
{$ begin test.dart $}
class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return Directionality(
      textDirection: TextDirection.ltr,
      child: Container(
        color: Color(0xffeeeeee),
        child: Center(
          child: Container(
            child: MyWidget(),
            color: Color(0xffcccccc),
          ),
        ),
      ),
    );
  }
}

Future<void> main() async {
  final completer = Completer<void>();
  
  runApp(MyApp());

  WidgetsFlutterBinding.ensureInitialized()
      .addPostFrameCallback((timestamp) async {
    completer.complete();
  });
  
  await completer.future;

  final controller = LiveWidgetController(WidgetsBinding.instance!);

  final rows = controller.widgetList(find.byType(Row));

  if (rows.length == 0) {
    _result(false, ['Couldn\'t find a Row!']);
    return;
  }

  if (rows.length > 1) {
    _result(false, ['Found ${rows.length} Rows, rather than just one.']);
    return;
  }

  final row = rows.first as Row;

  if (row.mainAxisSize != MainAxisSize.max) {
    _result(false, ['It\'s best to leave the mainAxisSize set to MainAxisSize.max, so there\'s space for the alignments to       take effect.']);
    return;
  }
  
    if (row.children.length != 3 || row.children.any((w) => w is! Icon)) {
    _result(false, ['Row should have three children, all Icon widgets.']);
    return;
  }

    final icon = row.children[2] as Icon;
  
  if (icon.color != Colors.amber) {
    _result(false, ['Add a third Icon. Give the Icon a size of 50 and a color of Colors.amber.']);
    return;
  }
  
  _result(true, ['The code displays three Icons in blue, red, and amber.']);
  
}
{$ end test.dart $}

Image widget

Image widget 显示了一张图片。你还能够直接引用图片 URL，或是你的应用 package 中的图片。但是由于 DartPad 无法引用包图片，所以下面的样例将会使用网络上的图片。

The Image widget displays an image. You either can reference images using a URL, or you can include images inside your app package. Since DartPad can’t package an image, the following example uses an image from the network.

样例：显示一张图片

Example: Displaying an image

{$ begin main.dart $}
import 'dart:async';
import 'package:flutter/material.dart';
import 'package:flutter_test/flutter_test.dart';

class MyWidget extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return Row(
      mainAxisAlignment: MainAxisAlignment.center,
      children: [
        Image.network('[Place an image link here!]'),
      ],
    );
  }
}
{$ end main.dart $}
{$ begin test.dart $}
class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return Directionality(
      textDirection: TextDirection.ltr,
      child: Container(
        color: Color(0xffeeeeee),
        child: Center(
          child: Container(
            child: MyWidget(),
            color: Color(0xffcccccc),
          ),
        ),
      ),
    );
  }
}

Future<void> main() async {
  final completer = Completer<void>();
  
  runApp(MyApp());

  WidgetsFlutterBinding.ensureInitialized()
      .addPostFrameCallback((timestamp) async {
    completer.complete();
  });
  
  await completer.future;

  final controller = LiveWidgetController(WidgetsBinding.instance!);

  final rows = controller.widgetList(find.byType(Row));

  if (rows.length == 0) {
    _result(false, ['Couldn\'t find a Row!']);
    return;
  }

  if (rows.length > 1) {
    _result(false, ['Found ${rows.length} Rows, rather than just one.']);
    return;
  }

  final row = rows.first as Row;

  if (row.mainAxisSize != MainAxisSize.max) {
    _result(false, ['It\'s best to leave the mainAxisSize set to MainAxisSize.max, so there\'s space for the alignments to take effect.']);
    return;
  }
 
}
{$ end test.dart $}

综合练习

Putting it all together

你就要完成这个 codelab 了！如果你想要检验你刚学的知识，为何不讲这些结合起来，构建一个显示名片的 Flutter UI 呢！

You’re almost at the end of this codelab. If you’d like to test your knowledge of the techniques that you’ve learned, why not apply those skills into building a Flutter UI that displays a business card!

Completed business card

你将会把 Flutter 的布局分解成几个部分，这就是如何在实际开发中构建 Flutter UI 方式！

You’ll break down Flutter’s layout into parts, which is how you’d create a Flutter UI in the real world.

在第一部分, 你将会实现包含姓名和标题的 Column。然后你将会在 Column 包裹一个含有 icon 的 Row，它将会被放在姓名和标题的左边。

In Part 1, you’ll implement a Column that contains the name and title. Then you’ll wrap the Column in a Row that contains the icon, which is positioned to the left of the name and title.

Completed business card

在第二部分中，你将会在 Row 外包裹一个 Column，所以你的代码中就包含了一个 Column (Row (Column) )。然后你将调整最外面的Column的布局，所以它看起来不错。最后，您将添加联系信息到最外面的Column的 children 中，所以它将显示在名称，标题和图标下方。

In Part 2, you’ll wrap the Row in a Column, so the code contains a Column within a Row within a Column. Then you’ll tweak the outermost Column’s layout, so it looks nice. Finally, you’ll add the contact information to the outermost Column’s list of children, so it’s displayed below the name, title, and icon.

Completed business card

在第三部分，你将会完成添加了更多图标的名片，它们会被放在联系信息的下方。

In Part 3, you’ll finish building the business card display by adding four more icons, which are positioned below the contact information.

Completed business card

第一部分

Part 1

练习：创建 name 和 title

Exercise: Create the name and title

{$ begin main.dart $}
import 'dart:async';
import 'package:flutter/material.dart';
import 'package:flutter_test/flutter_test.dart';

class MyWidget extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    TODO('Begin implementing the Column here.');
  }
}
{$ end main.dart $}
{$ begin solution.dart $}
import 'dart:async';
import 'package:flutter/material.dart';
import 'package:flutter_test/flutter_test.dart';

class MyWidget extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return Column(
      mainAxisSize: MainAxisSize.min,
      crossAxisAlignment: CrossAxisAlignment.start,
      children: [
        Text(
          'Flutter McFlutter', 
          style: Theme.of(context).textTheme.headline5,
        ),
        Text('Experienced App Developer'),
      ],
    );
  }
}
{$ end solution.dart $}
{$ begin test.dart $}
class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      debugShowCheckedModeBanner: false,
      theme: ThemeData(
        scaffoldBackgroundColor: Color(0xffeeeeee),
        textTheme: TextTheme(
          bodyText2: TextStyle(
            fontSize: 16,
          ),
        ),
      ),
      home: Scaffold(
        body: Padding(
          padding: const EdgeInsets.all(16.0),
          child: Center(
            child: Container(
              decoration: BoxDecoration(
                color: Color(0xffffffff),
                border: Border.all(),
                boxShadow: [
                  BoxShadow(
                    blurRadius: 10,
                    color: Color(0x80000000),
                  ),
                ],
              ),
              padding: const EdgeInsets.all(8.0),
              child: MyWidget(),
            ),
          ),
        ),
      ),
    );
  }
}

Future<void> main() async {
  final completer = Completer<void>();
  
  runApp(MyApp());

  WidgetsFlutterBinding.ensureInitialized()
      .addPostFrameCallback((timestamp) async {
    completer.complete();
  });
  
  await completer.future;

  final controller = LiveWidgetController(WidgetsBinding.instance!);

  // Check MyWidget starts with one Column

  final myWidgetElement = controller.element(find.byType(MyWidget));

  final myWidgetChildElements = <Element>[];
  myWidgetElement.visitChildElements((e) => myWidgetChildElements.add(e));

  if (myWidgetChildElements.length != 1 ||
      myWidgetChildElements[0].widget is! Column) {
    _result(false, ['The root widget in MyWidget\'s build method should be a Column.']);
    return;
  }

  // Check Column has correct properties

  final innerColumnElement = myWidgetChildElements[0];
  final innerColumnWidget = innerColumnElement.widget as Column;

  if (innerColumnWidget.crossAxisAlignment != CrossAxisAlignment.start) {
    _result(false, ['The Column that contains the name and title should use CrossAxisAlignment.start as its CrossAxisAlignment value.']);
    return;
  }

  if (innerColumnWidget.mainAxisSize != MainAxisSize.min) {
    _result(false, ['The Column that contains the name and title should use MainAxisSize.min as its MainAxisSize value.']);
    return;
  }

  // Check inner Column has two Text children

  if (innerColumnWidget.children.any((w) => w is! Text)) {
    _result(false, ['The Column that contains the name and title should have two children, both Text widgets.']);
    return;
  }

  // Check first Text has headline style

  final nameText = innerColumnWidget.children[0] as Text;

  if (nameText.style?.fontSize != 24) {
    _result(false, ['The Text widget for the name should use the "headline5" textStyle.']);
    return;
  }

  _result(true);
}
{$ end test.dart $}

练习：在 Column 外包裹一个 Row

Exercise: Wrap the Column in a Row