Stream Unit Test in Flutter with Mokito

I’m creating a new app with Flutter and I tested units with Mokito.
I can test Future functions with await.

How can I test stream?

Mocking stream

First of all I should mock stream. I asked to GPT and he answered following.

// Set up the behavior of the mock controller
when(mockController.stream).thenAnswer((_) => Stream.fromIterable([1, 2, 3]));

// Expectations: we expect the stream to produce three numbers
expectLater(producer.numberStream, emitsInOrder([1, 2, 3]));

It seemed like to mock by returning the stream created by fromIterable. Therefore I attempted the mocking like this.

final Iterable<Message> messages = [message];

when(chatMessageDatabaseService.streamMessages(chatId))
    .thenAnswer((value) => Stream.fromIterable(messages));

However compile had been failed with this error.

It’s mean I should give [[Message]] instead of [Message].

So I defined one more variable messageEvents.

final Iterable<Iterable<Message>> mesaageEvents = [messages];

when(chatMessageDatabaseService.streamMessages(chatId))
    .thenAnswer((value) => Stream.fromIterable(mesaageEvents));

expectLater

When I tested Future functions, expect is enough with await. How can we expect streamed data? There is expectLater instead of expect.

Usage is very simple. Give stream and matcher like below.

expectLater(messageStream, emitsInOrder(mesaageEvents));

emitsInOrder means we expect messageStream will emit event by ordering with same sequence of messageEvents.

Full Source

test('streamMessages method should return Message Stream', () async {
  final Iterable<Message> messages = [message];

  when(chatMessageDatabaseService.streamMessages(chatId))
      .thenAnswer((value) => Stream.value(messages));

  final messageStream = service.streamMessages(chatId);

  expectLater(messageStream, emitsThrough(messages));
});

Troubleshootings

References