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bloc_library sample

This sample makes use of the bloc and flutter_bloc libraries to manage state.

Check out the bloc library documentation for more details and tutorials.

Key Concepts

  • Lift State Up
    • If a bloc is needed by multiple widgets provide it using BlocProvider at the greatest common ancestor
  • Events are the input to a bloc.
    • They are commonly dispatched in response to user interactions such as button presses or lifecycle events like page loads.
  • States are the output of a bloc and represent a part of your application's state.
    • components can be notified of states and redraw portions of themselves based on the current state.
  • The change from one state to another is called a Transition.
    • A Transition consists of the current state, the event, and the next state.
  • A bloc converts a Stream of incoming Events into a Stream of outgoing States

Updating App State

Every bloc has a dispatch method. Dispatch takes an event and triggers mapEventToState. Dispatch notifies the bloc of a new event which will then trigger a state change.

Sharing Data between Blocs

In this implementation, the FilteredTodosBloc depends on the TodosBloc and will update the list of filtered todos in response to changes in the TodosBloc. This approach demonstrates how we can build a reactive application by composing blocs from other blocs. On app start, the TodosBloc is hydrated with todos from the repository and from that moment forward, all mutations to todos are executed through the TodosBloc which persists the changes asynchronously. The FilteredTodosBloc listens for changes in the TodosBloc state and will update it's list of filtered todos. The result is, a hierarchy of states with all todos being managed by the TodosBloc and a subset of those todos (filtered todos) being managed by the FilteredTodosBloc.

Updating UI

BlocBuilder is a Flutter widget which requires a bloc and a builder function. BlocBuilder handles building the widget in response to new states. BlocBuilder is very similar to StreamBuilder but has a simplified API to reduce the amount of boilerplate code needed and is also optimized to avoid unnecessary rebuilds.


Generally, this app conforms the "Testing Pyramid": Lots of Unit tests, fewer Widget tests, and fewer integration tests.

  • Unit tests
    • Blocs can easily be tested by mocking dependencies with Mockito. Events can be dispatched and we can assert that the blocs' state has changed correctly.
  • Widget Tests
    • Widgets can be tested by passing in fake data and making assertions against the Widget rendered with that data.
  • Integration Tests
    • Run the app and drive it using flutter_driver flutter drive --target test_driver/todo_app.dart.
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