Testing

Bloc was designed to be extremely easy to test. In this section, we’ll walk through how to unit test a bloc.

For the sake of simplicity, let’s write tests for the CounterBloc we created in Core Concepts.

To recap, the CounterBloc implementation looks like:

counter_bloc.dart

sealed class CounterEvent {}

final class CounterIncrementPressed extends CounterEvent {}

final class CounterDecrementPressed extends CounterEvent {}

class CounterBloc extends Bloc<CounterEvent, int> {
  CounterBloc() : super(0) {
    on<CounterIncrementPressed>((event, emit) => emit(state + 1));
    on<CounterDecrementPressed>((event, emit) => emit(state - 1));
  }
}

Setup

Before we start writing our tests we’re going to need to add a testing framework to our dependencies.

We need to add test and bloc_test to our project.

Dart

dart pub add dev:test dev:bloc_test

Flutter

flutter pub add dev:test dev:bloc_test

Testing

Let’s get started by creating the file for our CounterBloc Tests, counter_bloc_test.dart and importing the test package.

counter_bloc_test.dart

import 'package:test/test.dart';
import 'package:bloc_test/bloc_test.dart';

Next, we need to create our main as well as our test group.

counter_bloc_test.dart

void main() {
  group(CounterBloc, () {

  });
}

Note

Groups are for organizing individual tests as well as for creating a context in which you can share a common setUp and tearDown across all of the individual tests.

Let’s start by creating an instance of our CounterBloc which will be used across all of our tests.

counter_bloc_test.dart

group(CounterBloc, () {
  late CounterBloc counterBloc;

  setUp(() {
    counterBloc = CounterBloc();
  });
});

Now we can start writing our individual tests.

counter_bloc_test.dart

group(CounterBloc, () {
  late CounterBloc counterBloc;

  setUp(() {
    counterBloc = CounterBloc();
  });

  test('initial state is 0', () {
    expect(counterBloc.state, equals(0));
  });
});

Note

We can run all of our tests with the dart test command.

At this point we should have our first passing test! Now let’s write a more complex test using the bloc_test package.

counter_bloc_test.dart

blocTest(
  'emits [1] when CounterIncrementPressed is added',
  build: () => counterBloc,
  act: (bloc) => bloc.add(CounterIncrementPressed()),
  expect: () => [1],
);

blocTest(
  'emits [-1] when CounterDecrementPressed is added',
  build: () => counterBloc,
  act: (bloc) => bloc.add(CounterDecrementPressed()),
  expect: () => [-1],
);

We should be able to run the tests and see that all are passing.

That’s all there is to it, testing should be a breeze and we should feel confident when making changes and refactoring our code.

You can refer to the Weather App for an example of a fully tested application.