Test coverage in Flutter - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is test coverage in Flutter testing?

Test coverage measures how much of your app's code is executed when tests run. It helps you see which parts of your code are tested and which are not.

beginner

Why is high test coverage important?

High test coverage means more code is checked by tests, reducing bugs and making your app more reliable and easier to maintain.

intermediate

How do you generate a test coverage report in Flutter?

Run flutter test --coverage in your project folder. This creates a coverage file showing which lines of code were tested.

intermediate

What file format does Flutter use for coverage reports?

Flutter generates coverage reports in lcov.info format, which can be viewed with tools like lcov or online coverage viewers.

advanced

Can 100% test coverage guarantee a bug-free app?

No. 100% coverage means all code runs during tests, but tests must also check correct behavior. Coverage shows tested code, not test quality.

Which command generates a test coverage report in Flutter?

A flutter test --coverage

B flutter build coverage

C flutter analyze coverage

D flutter run --coverage

The correct command to generate coverage data is flutter test --coverage.

What does test coverage measure?

A How many tests you wrote

B How fast tests run

C How many bugs tests find

D How much code runs during tests

Test coverage measures how much of your code is executed when tests run.

Which file contains Flutter's coverage report data?

A lcov.info

B coverage.json

C coverage.txt

D report.xml

Flutter outputs coverage data in the lcov.info file.

Does 100% test coverage mean your app has no bugs?

A Yes, always

B Only if tests are written in Dart

C No, coverage shows tested code but not test quality

D Only if tests run fast

100% coverage means all code runs during tests, but tests must also check correct behavior to catch bugs.

Why should you check test coverage regularly?

A To make tests run slower

B To find untested code that might have bugs

C To reduce app size

D To increase app startup time

Checking coverage helps find code not tested, which might hide bugs.

Explain what test coverage is and why it matters in Flutter app development.

Think about how much of your code your tests actually run.

Describe the steps to generate and view a test coverage report in Flutter.

Start with running tests with coverage enabled.

Practice

1.

What does test coverage measure in a Flutter app?

easy

A. How much of the app's code is executed by tests

B. The number of tests written

C. The speed of test execution

D. The size of the app's binary

Solution

1. Step 1: Understand test coverage meaning

   Test coverage shows what percentage of your code runs when tests are executed.

2. Step 2: Compare options to definition

   Only How much of the app's code is executed by tests matches this meaning by describing code executed by tests.

3. Final Answer:

   How much of the app's code is executed by tests -> Option A

4. Quick Check:

   Test coverage = code executed by tests [OK]

Hint: Coverage means code tested, not test count or speed [OK]

Common Mistakes:

• Confusing coverage with number of tests
• Thinking coverage measures test speed
• Assuming coverage relates to app size

2.

Which command runs Flutter tests and collects coverage data?

?

easy

A. flutter test --coverage

B. flutter run --coverage

C. flutter build --coverage

D. flutter analyze --coverage

Solution

1. Step 1: Recall Flutter test commands

   The command to run tests and collect coverage is flutter test --coverage.

2. Step 2: Eliminate incorrect commands

   flutter run runs the app, flutter build builds it, and flutter analyze checks code style, none collect coverage.

3. Final Answer:

   flutter test --coverage -> Option A

4. Quick Check:

   Run tests with coverage = flutter test --coverage [OK]

Hint: Use flutter test --coverage to get coverage data [OK]

Common Mistakes:

• Using flutter run instead of flutter test
• Confusing build or analyze commands with testing
• Omitting the --coverage flag

3.

Given this test code snippet, what is the coverage result for increment() method?

class Counter {
  int value = 0;
  void increment() {
    value++;
  }
}

void main() {
  final counter = Counter();
  test('value starts at 0', () {
    expect(counter.value, 0);
  });
}

medium

A. 100% coverage including increment()

B. Partial coverage of increment() method

C. 0% coverage for increment() method

D. Syntax error prevents coverage

Solution

1. Step 1: Analyze test coverage of increment()

   The test only checks initial value; it never calls increment().

2. Step 2: Determine coverage for increment()

   Since increment() is never executed, coverage for it is 0%.

3. Final Answer:

   0% coverage for increment() method -> Option C

4. Quick Check:

   Method not called = 0% coverage [OK]

Hint: Only called code counts as covered [OK]

Common Mistakes:

• Assuming all methods are covered by default
• Confusing test presence with code execution
• Ignoring that increment() is never called

4.

Identify the error in this test setup that causes coverage to be incomplete:

void main() {
  final counter = Counter();
  test('increment test', () {
    counter.increment;
    expect(counter.value, 1);
  });
}

medium

A. Test name is missing

B. Missing parentheses to call increment() method

C. expect() syntax is incorrect

D. Counter class is not imported

Solution

1. Step 1: Check method call syntax

   The code uses counter.increment; which references the method but does not call it.

2. Step 2: Understand impact on coverage

   Because increment() is not called, coverage misses this method.

3. Final Answer:

   Missing parentheses to call increment() method -> Option B

4. Quick Check:

   Method call needs () to execute [OK]

Hint: Method calls need () to run and count for coverage [OK]

Common Mistakes:

• Forgetting () after method name
• Assuming method reference calls it
• Ignoring test name importance (not required)

5.

You want to improve test coverage for this class. Which test addition best increases coverage?

class Calculator {
  int add(int a, int b) => a + b;
  int subtract(int a, int b) => a - b;
  int multiply(int a, int b) => a * b;
}

Current tests only call add().

hard

A. Add tests checking only Calculator class existence

B. Add tests calling add() multiple times

C. Add tests that print results without assertions

D. Add tests calling subtract() and multiply() methods

Solution

1. Step 1: Identify untested methods

   Only add() is tested; subtract() and multiply() are not covered.

2. Step 2: Add tests for untested methods

   Writing tests that call subtract() and multiply() will increase coverage.

3. Final Answer:

   Add tests calling subtract() and multiply() methods -> Option D

4. Quick Check:

   Test all methods to increase coverage [OK]

Hint: Test all methods, not just one, to boost coverage [OK]

Common Mistakes:

• Testing only one method repeatedly
• Testing class existence without method calls
• Printing results without assertions doesn't count