Dart Test Fundamentals

Dart Test Fundamentals

When to use this skill

Use this skill when:

• Writing new test files.
• Structuring test suites with group.
• Configuring test execution via dart_test.yaml.
• Understanding test lifecycle methods.

When NOT to use (Abstention Guardrails)

Do NOT apply this skill or refactor existing tests when:

• Legacy Single-Group Churn: Do NOT remove or reformat existing group hierarchies in untouched existing tests unless explicitly asked, as this causes unwanted diff churn.
• Alternative Assertion Frameworks: The package has migrated to package:checks or a specialized testing framework; do not revert tests back to legacy package:matcher idioms.
• Trivial Tests with Zero Setup: Simple standalone tests with no shared state or resources do not need group, setUp, or addTearDown. Do not add ceremonial wrapper boilerplate.

Discovery

To find candidates for improving test structure:

try-finally Cleanup

Search for tests that use try-finally for cleanup instead of addTearDown:

• Regex: \bfinally\s*\{ (Check if this is used for resource cleanup inside a test).

Core Concepts

1. Test Structure (test and group)

• test: The fundamental unit of testing.

  test('description', () {
    // assertions
  });

• group: Used to organize tests into logical blocks.

  • Groups can be nested.
  • Descriptions are concatenated (e.g., "Group Description Test Description").
  • Helps scope setUp and tearDown calls.
  • Naming: Use PascalCase for groups that correspond to a class name (e.g., group('MyClient', ...)).
  • Avoid Single Groups: Do not wrap all tests in a file with a single group call if it's the only one.
    • NOTE: DO NOT remove groups when doing a cleanup on existing code you didn't create unless explicitly asked to. This can cause a LOT of churn in the DIFF that most engineers won't want!

• Naming Tests test('test name here',:

  • Avoid redundant "test" prefixes. Use group instead.
  • Include the expected behavior or outcome in the description (e.g., 'throws StateError' or 'adds API key to URL').
  • Descriptions should read well when concatenated with their group name.

• Named Parameters Placement:

  • For test and group calls, place named parameters (e.g., testOn, timeout, skip) immediately after the description string, before the callback closure. This improves readability by keeping the test logic last.

    test('description', testOn: 'vm', () {
      // assertions
    });

2. Lifecycle Methods (setUp, tearDown)

• setUp: Runs before every test in the current group (and nested groups).
• tearDown: Runs after every test in the current group.
• setUpAll: Runs once before any test in the group.
• tearDownAll: Runs once after all tests in the group.

Best Practice:

• Use setUp for resetting state to ensure test isolation.
• Avoid sharing mutable state between tests without resetting it.

3. Cleaning Up Resources

• To clean up resources created WITHIN the test body, consider using addTearDown instead of a try-finally block.

Avoid:

test('can create and delete a file', () {
  final file = File('temp.txt');
  try {
    file.writeAsStringSync('hello');
    expect(file.readAsStringSync(), 'hello');
  } finally {
    if (file.existsSync()) file.deleteSync();
  }
});

Prefer:

test('can create and delete a file', () {
  final file = File('temp.txt');
  // Register teardown immediately after resource creation intent
  addTearDown(() {
    if (file.existsSync()) file.deleteSync();
  });

  file.writeAsStringSync('hello');
  expect(file.readAsStringSync(), 'hello');
});

4. Configuration (dart_test.yaml)

The dart_test.yaml file configures the test runner. Common configurations include: