An introduction to widget testing

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.

The flutter_test package provides the following tools for testing widgets:

  • The WidgetTester allows building and interacting with widgets in a test environment.
  • The testWidgets() function automatically creates a new WidgetTester for each test case, and is used in place of the normal test() function.
  • The Finder classes allow searching for widgets in the test environment.
  • Widget-specific Matcher constants help verify whether a Finder locates a widget or multiple widgets in the test environment.

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

  1. Add the flutter_test dependency.
  2. Create a widget to test.
  3. Create a testWidgets test.
  4. Build the widget using the WidgetTester.
  5. Search for the widget using a Finder.
  6. Verify the widget using a Matcher.

1. Add the flutter_test dependency

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

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),
        ),
      ),
    );
  }
}

3. Create a testWidgets test

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.

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.
  });
}

4. Build the widget using the WidgetTester

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

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'));
  });
}

Notes about the pump() methods

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.

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

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

5. Search for our widget using a Finder

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.

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.

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');
  });
}

6. Verify the widget using a Matcher

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.

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);
  });
}

Additional Matchers

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

findsNothing
Verifies that no widgets are found.
findsWidgets
Verifies that one or more widgets are found.
findsNWidgets
Verifies that a specific number of widgets are found.

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),
        ),
      ),
    );
  }
}