An introduction to unit testing
How can you ensure that your app continues to work as you add more features or change existing functionality? By writing tests.
Unit tests are handy for verifying the behavior of a single function, method, or class. The test
package provides the core framework for writing unit tests, and the flutter_test
package provides additional utilities for testing Widgets.
This recipe demonstrates the core features provided by the test
package. For more information about the test package, see the test package documentation.
Directions
- Add the
test
orflutter_test
dependency - Create a test file
- Create a class to test
- Write a
test
for our class - Combine multiple tests in a
group
- Run the tests
1. Add the test dependency
If you’re working on a Dart package that does not depend on Flutter, you can import the test
package. The test package provides the core functionality for writing tests in Dart. This is the best approach when writing packages that will be consumed by web, server, and Flutter apps.
dev_dependencies:
test: <latest_version>
2. Create a test file
In this example, create two files: counter.dart
and counter_test.dart
.
The counter.dart
file will contain a class that you want to test and resides in the lib
folder. The counter_test.dart
file will contain the tests themselves and lives inside the test
folder.
In general, test files should reside inside a test
folder located at the root of your Flutter application or package.
When you’re finished, the folder structure should look like this:
counter_app/
lib/
counter.dart
test/
counter_test.dart
3. Create a class to test
Next, you need a “unit” to test. Remember: “unit” is a fancy name for a function, method, or class. In this example, create a Counter
class inside the lib/counter.dart
file. It will be responsible for incrementing and decrementing a value
starting at 0
.
class Counter {
int value = 0;
void increment() => value++;
void decrement() => value--;
}
Note: For simplicity, this tutorial does not follow the “Test Driven Development” approach. If you’re more comfortable with that style of development, you can always go that route.
4. Write a test for our class
Inside the counter_test.dart
file, write the first unit test. Tests are defined using the top-level test
function, and you can check if the results are correct by using the top-level expect
function. Both of these functions come from the test
package.
// Import the test package and Counter class
import 'package:test/test.dart';
import 'package:counter_app/counter.dart';
void main() {
test('Counter value should be incremented', () {
final counter = Counter();
counter.increment();
expect(counter.value, 1);
});
}
5. Combine multiple tests in a group
If you have several tests that are related to one another, combine them using the group
function provided by the test
package.
import 'package:test/test.dart';
import 'package:counter_app/counter.dart';
void main() {
group('Counter', () {
test('value should start at 0', () {
expect(Counter().value, 0);
});
test('value should be incremented', () {
final counter = Counter();
counter.increment();
expect(counter.value, 1);
});
test('value should be decremented', () {
final counter = Counter();
counter.decrement();
expect(counter.value, -1);
});
});
}
6. Run the tests
Now that you have a Counter
class with tests in place, you can run the tests.
Run tests using IntelliJ or VSCode
The Flutter plugins for IntelliJ and VSCode support running tests. This is often the best option while writing tests because it provides the fastest feedback loop as well as the ability to set breakpoints.
- IntelliJ
- Open the
counter_test.dart
file - Select the
Run
menu - Click the
Run 'tests in counter_test.dart'
option - Alternatively, use the appropriate keyboard shortcut for your platform.
- Open the
- VSCode
- Open the
counter_test.dart
file - Select the
Debug
menu - Click the
Start Debugging
option - Alternatively, use the appropriate keyboard shortcut for your platform.
- Open the
Run tests in a terminal
You can also use a terminal to run the tests by executing the following command from the root of the project:
flutter test test/counter_test.dart