Testing Dependent Events in Flutter Bloc: Seeding State & Handling Multiple Events

In Flutter Bloc testing, there are two main approaches to test the dependent Event in scenarios where the Event in test depends on the outcome of a previous event: seeding the state or calling a pre-event with skip. Both methods have their own advantages and disadvantages.

Seeding State

Advantages

1. Direct Control: You can directly set the Bloc's state to a desired state without having to dispatch any events. This provides a precise starting point for your tests.
2. Simplicity: Seeding is straightforward and easy to understand, as it eliminates the need for additional event handling and state transitions.

Disadvantages

1. Unrealistic Testing: Directly seeding a state might not accurately reflect the real-world usage of your Bloc, where states are typically reached through event dispatching and state transitions.
2. Initialization Complexity: Creating and passing the correct initial state can be complex, especially when the state has multiple parameters or requires specific configurations.
3. Limited Coverage: This approach may not cover all the transitions and side effects that occur when events are dispatched, potentially missing bugs in event handling logic.

Calling Pre-Event with Skip

Advantages

1. Realistic Testing: By dispatching events to reach a desired state, you mimic the actual behavior of your application, ensuring that all transitions and side effects are covered.
2. Efficient Event Handling: Often, setting up the initial state by dispatching a pre-event is straightforward and may be simpler than manually constructing a complex state.
3. Thorough Coverage: This approach tests the entire flow, from event dispatch to state transition, providing a more comprehensive test of your Bloc's logic.

Disadvantages

1. Event Dependency: If your tests require many pre-events to reach a desired state, it can make the test setup more verbose, although each individual pre-event is typically simple to add.
2. Performance: Dispatching multiple events to reach the desired state can slow down your tests, especially if there are many intermediate states. However, this is usually a minor issue for most test cases.

Example in Flutter Bloc Test

Seeding State

blocTest(
  'CounterBloc emits [10] when seeded with 9',
  build: () => CounterBloc(),
  seed: () => 9,
  act: (bloc) => bloc.add(CounterEvent.increment),
  expect: () => [10],
);

Calling Pre-Event with Skip

blocTest<MyBloc, MyBlocState>(
  'emits [MyBlocState] when SearchItemEvent is added',
  build: () => MyBloc(),
  act: (bloc) {
    bloc.add(ListItemsEvent()); // Start with loading the list
    bloc.add(SearchItemEvent(item: 'Apple')); // Start searching loaded list
  },
  skip: 2, // Skip emissions for intermediate states if not needed
  expect: () => [
    MyBlocState(filteredList: ['Green Apple', 'Red Apple'])
  ],
);

Summary

Use Seeding State When: You need to quickly set up a simple initial state, or when you are testing scenarios where event-driven transitions are either not needed or not practical like in above example.

Avoid Seeding State When: Your focus is on understanding how your Bloc reacts to sequences of events, how it transitions between states, or when you want to ensure that all parts of the Bloc’s behavior are tested as they would occur in a live application.

In practice, a combination of both methods might be useful depending on the scenario. Seeding can be a useful tool, but it’s important to ensure that it aligns with the objectives of your tests and does not bypass critical event-driven logic.

Feel free to share your thoughts and experiences in the comments below!