Test coverage in Flutter - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ ui_behavior

intermediate

What is the output of this Flutter widget test?

Consider this Flutter widget test code. What will the test find when it runs?

Flutter

testWidgets('Finds a Text widget with Hello', (WidgetTester tester) async {
  await tester.pumpWidget(MaterialApp(home: Scaffold(body: Text('Hello'))));
  final textFinder = find.text('Hello');
  expect(textFinder, findsOneWidget);
});

A The test fails because the Text widget is not found.

B The test passes because the Text widget with 'Hello' is found exactly once.

C The test fails because there are multiple Text widgets with 'Hello'.

D The test throws a runtime error due to missing MaterialApp.

💡 Hint

Remember that MaterialApp provides necessary context for Text widgets.

The test pumps a MaterialApp with a Scaffold containing a Text widget with 'Hello'. The find.text('Hello') finds exactly one widget, so the expect passes.

❓ lifecycle

intermediate

What is the value of counter after this test runs?

Given this Flutter widget test that increments a counter, what is the final value of counter?

Flutter

int counter = 0;

class CounterButton extends StatelessWidget {
  final VoidCallback onPressed;
  CounterButton({required this.onPressed});

  @override
  Widget build(BuildContext context) {
    return ElevatedButton(onPressed: onPressed, child: Text('Increment'));
  }
}

testWidgets('Counter increments on button tap', (WidgetTester tester) async {
  await tester.pumpWidget(MaterialApp(home: CounterButton(onPressed: () { counter++; })));
  await tester.tap(find.byType(ElevatedButton));
  await tester.pump();
});

A counter is 1 after the test runs.

B counter is 0 because the button was not tapped.

C counter is 2 because the button was tapped twice.

D counter is null due to uninitialized variable.

💡 Hint

Check how many times tester.tap is called.

The test taps the ElevatedButton once, triggering the onPressed callback which increments counter from 0 to 1.

📝 Syntax

advanced

What error does this Flutter test code produce?

Analyze this Flutter test code snippet. What error will it cause when run?

Flutter

testWidgets('Missing await on pumpWidget', (WidgetTester tester) {
  tester.pumpWidget(MaterialApp(home: Text('Test')));
  final textFinder = find.text('Test');
  expect(textFinder, findsOneWidget);
});

A No error; test passes successfully.

B Test fails with a MissingPluginException error.

C Test fails because pumpWidget is not awaited, causing a test failure.

D Test fails with a syntax error due to missing semicolon.

💡 Hint

pumpWidget returns a Future and should be awaited in async tests.

The testWidgets callback is not marked async and pumpWidget is not awaited, so the widget tree is not built before the find and expect calls, causing the test to fail.

🔧 Debug

advanced

Why does this widget test fail to find the button?

This test tries to find a button but fails. What is the cause?

Flutter

testWidgets('Finds button by key', (WidgetTester tester) async {
  await tester.pumpWidget(MaterialApp(home: Scaffold(body: ElevatedButton(key: Key('btn'), onPressed: () {}, child: Text('Tap me')))));
  final buttonFinder = find.byKey(Key('button'));
  expect(buttonFinder, findsOneWidget);
});

A The key used in find.byKey does not match the button's key.

B The ElevatedButton is not wrapped in a MaterialApp.

C The test does not await tester.pumpWidget.

D The button's child Text widget is missing.

💡 Hint

Check the key strings used in the widget and the finder.

The button's key is 'btn' but the finder looks for key 'button', so it does not find the widget.

🧠 Conceptual

expert

Which option best describes test coverage in Flutter?

Select the statement that correctly explains test coverage in Flutter development.

A Test coverage refers to the number of devices a Flutter app supports.

B Test coverage is the time taken to run all tests in a Flutter project.

C Test coverage is the number of UI widgets visible on the screen during testing.

D Test coverage measures the percentage of code executed by automated tests, helping identify untested parts.

💡 Hint

Think about what 'coverage' means in software testing.

Test coverage shows how much of the app's code is tested by automated tests, helping developers find untested code.

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