Widget 测试介绍

在单元测试介绍部分，我们学习了使用 test 这个 package 测试 Dart 类的方法。为了测试 widget 类，我们需要使用 flutter_test package 提供的额外工具，这些工具是跟 Flutter SDK 一起发布的。

In the introduction to unit testing recipe, you learned how to test Dart classes using the test package. To test widget classes, you need a few additional tools provided by the flutter_test package, which ships with the Flutter SDK.

flutter_test package 提供了以下工具用于 widget 的测试：

The flutter_test package provides the following tools for testing widgets:

WidgetTester，使用该工具可在测试环境下建立 widget 并与其交互。
The WidgetTester allows building and interacting with widgets in a test environment.
testWidgets() 函数，此函数会自动为每个测试创建一个 WidgetTester，用来代替普通的 test 函数。
The testWidgets() function automatically creates a new WidgetTester for each test case, and is used in place of the normal test() function.
Finder 类，可以方便我们在测试环境下查找 widgets。
The Finder classes allow searching for widgets in the test environment.
Widget-specific Matcher 常量，该常量在测试环境下帮助我们验证 Finder 是否定位到一个或多个 widgets。
Widget-specific Matcher constants help verify whether a Finder locates a widget or multiple widgets in the test environment.

如果觉得太复杂，别担心！让我们通过下面这些步骤把这些内容整合起来。

If this sounds overwhelming, don’t worry. Learn how all of these pieces fit together throughout this recipe, which uses the following steps:

步骤：

Directions

添加一个 flutter_test 依赖
Add the flutter_test dependency.
创建一个测试用的 widget
Create a widget to test.
创建一个 testWidgets 测试方法
Create a testWidgets test.
使用 WidgetTester 建立 widget
Build the widget using the WidgetTester.
使用 Finder 查找 widget
Search for the widget using a Finder.
使用 Matcher 验证 widget 是否正常工作
Verify the widget using a Matcher.

一. 添加一个 `flutter_test` 依赖

1. Add the `flutter_test` dependency

我们开始编写测试之前，需要先给 pubspec.yaml 文件的 dev_dependencies 段添加 flutter_test 依赖。如果使用命令行或编译器新建一个 Flutter 项目，那么依赖已经默认添加了。

Before writing tests, include the flutter_test dependency in the dev_dependencies section of the pubspec.yaml file. If creating a new Flutter project with the command line tools or a code editor, this dependency should already be in place.

dev_dependencies:
  flutter_test:
    sdk: flutter

2. Create a widget to test

接下来，我们需要创建一个可以测试的 widget！在此例中，我们创建了一个 widget 显示一个 标题 (title) 和 信息 (message)。

Next, create a widget for testing. For this recipe, create a widget that displays a title and message.

class MyWidget extends StatelessWidget {
  final String title;
  final String message;

  const MyWidget({
    Key key,
    @required this.title,
    @required this.message,
  }) : super(key: key);

  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      title: 'Flutter Demo',
      home: Scaffold(
        appBar: AppBar(
          title: Text(title),
        ),
        body: Center(
          child: Text(message),
        ),
      ),
    );
  }
}

三. 创建一个 `testWidgets` 测试方法

3. Create a `testWidgets` test

现在我们有了一个可以测试的 widget，可以开始编写第一个测试了！第一步，我们用 flutter_test 这个 package 提供的 testWidgets() 函数定义一个测试。 testWidgets 函数可以定义一个 widget 测试并创建一个可以使用的 WidgetTester。

With a widget to test, begin by writing your first test. Use the testWidgets() function provided by the flutter_test package to define a test. The testWidgets function allows you to define a widget test and creates a WidgetTester to work with.

我们的测试会验证 MyWidget 是否显示给定的标题和信息。

This test verifies that MyWidget displays a given title and message. It is titled accordingly, and it will be populated in the next section.

void main() {
  // Define a test. The TestWidgets function also provides a WidgetTester
  // to work with. The WidgetTester allows you to build and interact
  // with widgets in the test environment.
  testWidgets('MyWidget has a title and message', (WidgetTester tester) async {
    // Test code goes here.
  });
}

四. 使用 `WidgetTester` 建立 Widget

4. Build the widget using the `WidgetTester`

下一步，为了在测试环境中建立 MyWidget，我们可以使用 WidgetTester 提供的 pumpWidget()方法，pumpWidget 方法会建立并渲染我们提供的 widget。

Next, build MyWidget inside the test environment by using the pumpWidget() method provided by WidgetTester. The pumpWidget method builds and renders the provided widget.

在这个示例中，我们将创建一个显示标题“T”和信息“M”的 MyWidget 示例。

Create a MyWidget instance that displays “T” as the title and “M” as the message.

void main() {
  testWidgets('MyWidget has a title and message', (WidgetTester tester) async {
    // Create the widget by telling the tester to build it.
    await tester.pumpWidget(MyWidget(title: 'T', message: 'M'));
  });
}

pump() 方法相关提示

Notes about the pump() methods

初次调用 pumpWidget() 之后，WidgetTester 会提供其他方式来重建相同的 widget。这对使用 StatefulWidget 或者动画会非常有用。

After the initial call to pumpWidget(), the WidgetTester provides additional ways to rebuild the same widget. This is useful if you’re working with a StatefulWidget or animations.

例如，如果我们点击调用 setState() 的按钮，在测试环境中，Flutter 并不会自动重建你的 widget。我们需要用以下列举的方法来让 Flutter 再一次建立我们的 widget。

For example, tapping a button calls setState(), but Flutter won’t automatically rebuild your widget in the test environment. Use one of the following methods to ask Flutter to rebuild the widget.

tester.pump(Duration duration)
调度一帧以触发 widget 的重建。若指定了 Duration，那么则会让 clock 经过该时长之后调度一帧。它并不会因为持续时间大于一帧的时间而调度多帧。

tester.pump(Duration duration)
Schedules a frame and triggers a rebuild of the widget. If a Duration is specified, it advances the clock by that amount and schedules a frame. It does not schedule multiple frames even if the duration is longer than a single frame.

tester.pumpAndSettle()
在给定期间内不断重复调用 pump 直到完成所有绘制帧。一般需要等到所有动画全部完成。

tester.pumpAndSettle()
Repeatedly calls pump() with the given duration until there are no longer any frames scheduled. This essentially waits for all animations to complete.

这些方法在构建周期中保证细粒度控制，这在测试中非常有用。

These methods provide fine-grained control over the build lifecycle, which is particularly useful while testing.

五. 使用 `Finder` 查找 widget

5. Search for our widget using a `Finder`

现在让我们在测试环境中建立 widget。我们需要用 Finder 通过 widget 树来查找 标题 和 信息 Text widgets，这样可以验证这些 Widgets 是否正确显示。

With a widget in the test environment, search through the widget tree for the title and message Text widgets using a Finder. This allows verification that the widgets are being displayed correctly.

为了实现这个目的，我们使用 flutter_test 这个 package 提供的顶级 find() 方法来创建我们的 Finders。因为我们要查找的是 Text widgets，所以可以使用 find.text() 方法。

For this purpose, use the top-level find() method provided by the flutter_test package to create the Finders. Since you know you’re looking for Text widgets, use the find.text() method.

关于 Finder classes 的更多信息，请参阅定位到目标 Widgets 章节。

For more information about Finder classes, see the Finding widgets in a widget test recipe.

void main() {
  testWidgets('MyWidget has a title and message', (WidgetTester tester) async {
    await tester.pumpWidget(MyWidget(title: 'T', message: 'M'));

    // Create the Finders.
    final titleFinder = find.text('T');
    final messageFinder = find.text('M');
  });
}

六. 使用 `Matcher` 验证 widget 是否正常工作

6. Verify the widget using a `Matcher`

最后，让我们来用 flutter_test 提供的 Matcher 常量验证 Text widgets 显示的标题和信息。Matcher 类是 test 包里的核心部分，它提供一种通用方法来验证给定值是否符合我们的预期。

Finally, verify the title and message Text widgets appear on screen using the Matcher constants provided by flutter_test. Matcher classes are a core part of the test package, and provide a common way to verify a given value meets expectations.

在这个示例中，我们要确保 widget 只在屏幕中出现一次。因此，可以使用 findsOneWidget Matcher。

Ensure that the widgets appear on screen exactly one time. For this purpose, use the findsOneWidget Matcher.

void main() {
  testWidgets('MyWidget has a title and message', (WidgetTester tester) async {
    await tester.pumpWidget(MyWidget(title: 'T', message: 'M'));
    final titleFinder = find.text('T');
    final messageFinder = find.text('M');

    // Use the `findsOneWidget` matcher provided by flutter_test to verify
    // that the Text widgets appear exactly once in the widget tree.
    expect(titleFinder, findsOneWidget);
    expect(messageFinder, findsOneWidget);
  });
}

其他的 Matchers

Additional Matchers

除了 findsOneWidget，flutter_test 还为常见情况提供了其他的 matchers。

In addition to findsOneWidget, flutter_test provides additional matchers for common cases.

findsNothing
验证没有可被查找的 widgets。

findsNothing
Verifies that no widgets are found.

findsWidgets
验证一个或多个 widgets 被找到。

findsWidgets
Verifies that one or more widgets are found.

findsNWidgets
验证特定数量的 widgets 被找到。

findsNWidgets
Verifies that a specific number of widgets are found.

matchesGoldenFile: Verifies that a widget’s rendering matches a particular bitmap image (“golden file” testing).

完整样例

Complete example

import 'package:flutter/material.dart';
import 'package:flutter_test/flutter_test.dart';

void main() {
  // Define a test. The TestWidgets function also provides a WidgetTester
  // to work with. The WidgetTester allows building and interacting
  // with widgets in the test environment.
  testWidgets('MyWidget has a title and message', (WidgetTester tester) async {
    // Create the widget by telling the tester to build it.
    await tester.pumpWidget(MyWidget(title: 'T', message: 'M'));

    // Create the Finders.
    final titleFinder = find.text('T');
    final messageFinder = find.text('M');

    // Use the `findsOneWidget` matcher provided by flutter_test to
    // verify that the Text widgets appear exactly once in the widget tree.
    expect(titleFinder, findsOneWidget);
    expect(messageFinder, findsOneWidget);
  });
}

class MyWidget extends StatelessWidget {
  final String title;
  final String message;

  const MyWidget({
    Key key,
    @required this.title,
    @required this.message,
  }) : super(key: key);

  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      title: 'Flutter Demo',
      home: Scaffold(
        appBar: AppBar(
          title: Text(title),
        ),
        body: Center(
          child: Text(message),
        ),
      ),
    );
  }
}

步骤：

Directions

一. 添加一个 flutter_test 依赖

1. Add the flutter_test dependency

二. 创建一个测试用的 Widget

2. Create a widget to test

三. 创建一个 testWidgets 测试方法

3. Create a testWidgets test

四. 使用 WidgetTester 建立 Widget

4. Build the widget using the WidgetTester