Understanding Flutter BLoC: A Comprehensive Guide

Hi Flutter Developers, this is an interesting topic: Why there are a lot of bloc-components: Bloc Provider, Builder, Listener and Consumer.

Flutter BLoC (Business Logic Component) is a design pattern used for state management in Flutter applications. It aims to separate business logic from UI code, making the application easier to maintain, scalable, and testable. In this article, we will explore key components of the BLoC pattern in Flutter — BlocProvider, BlocBuilder, BlocListener, and BlocConsumer—and understand their usage, differences, and when to use each of them. We will also cover side effects and build use-cases to understand how to structure your app effectively.

What is Flutter BLoC?

BLoC stands for Business Logic Component. It’s a design pattern in which:

• The app’s business logic is encapsulated in a BLoC (or multiple BLoCs).
• BLoC listens to streams of events and outputs states.
• UI components (widgets) listen to these states and rebuild themselves when necessary.

BLoC promotes a clear separation of concerns, with UI components focused solely on the view, while business logic resides within the BLoC, which is responsible for handling events, processing them, and yielding new states.

Key BLoC Components

1. BlocProvider

BlocProvider is used to provide a Bloc to the widget tree. It makes the bloc available to all the widgets below it in the tree, allowing them to access the bloc and listen to its state.

When to Use:

• Use BlocProvider when you want to create and expose a Bloc to a widget subtree.
• It is typically placed at a high level in the widget tree to provide access to the BLoC throughout the screen or app.

2. BlocBuilder

BlocBuilder is a widget that listens to a Bloc's state and rebuilds the UI whenever a new state is emitted.

When to Use:

• Use BlocBuilder when you need to rebuild parts of your UI based on new states emitted by the bloc.
• It helps ensure the UI updates when the state changes, such as showing lists, forms, or handling loading/error states.

Build Use-case Example:

• Displaying a list of items or data fetched from an API.
• Showing a loading spinner while fetching data.

3. BlocListener

BlocListener is used to listen for state changes and perform side effects, such as showing a snackbar, navigating, or triggering animations. Unlike BlocBuilder, it does not rebuild the UI; instead, it reacts to state changes with actions that should not trigger UI rebuilds.

When to Use:

• Use BlocListener when you need to trigger side effects like showing notifications, navigating, or logging.
• It is helpful when you want to react to a state change but don’t need the widget tree to rebuild.

4. BlocConsumer

BlocConsumer combines both BlocBuilder and BlocListener into a single widget. It listens for state changes and allows you to both rebuild the UI and perform side effects.

When to Use:

• Use BlocConsumer when you want to handle both UI updates and side effects in a single widget. It allows you to consume states with the builder and listen to states for actions in one place.

Differences Between BlocBuilder, BlocListener, and BlocConsumer

Use-Cases for Side Effects and Build

Side Effects Use-Cases (Where to use BlocListener or BlocConsumer for side effects)

BlocListener and BlocConsumer

Side effects are actions that don’t directly affect the UI but respond to state changes, like showing notifications, navigating to a new page, or calling external APIs.

Push Notifications (e.g., new message, update alert)

• Side effect action: Show a notification when a new message is received.
• When to Use: Use BlocListener or BlocConsumer to show a notification without rebuilding the UI.

API Calls (e.g., saving data after form submission)

• Side effect action: Make an API call when the form is successfully submitted.
• When to Use: Use BlocListener to trigger the API call once the form is successfully validated.

User Authentication (e.g., login, logout)

• Side effect action: Trigger navigation after successful login.
• When to Use: Use BlocListener to perform navigation or logout actions.

Error Alerts or Logging (e.g., showing an error message or logging an error)

• Side effect action: Show an error message in case of failure.
• When to Use: Use BlocListener to handle error states and show feedback to the user.

Build Use-Cases (Where to use BlocBuilder or BlocConsumer for UI updates)

BlocBuilder and BlocConsumer

Build operations focus on updating the UI whenever the state changes. These operations trigger UI rebuilds based on the data in the new state.

List Display (e.g., loading and displaying items)

• Build action: Fetch data from an API and display it in a list.
• When to Use: Use BlocBuilder to show a loading spinner while data is being fetched and update the list once it’s loaded.

Form State (e.g., enabling/disabling submit button based on validation)

• Build action: Show a form and update the UI based on the validation status.
• When to Use: Use BlocBuilder to update the button state (enabled/disabled) based on form validation.

Loading Indicators (e.g., showing a progress indicator while data is loading)

• Build action: Display a loading spinner while an operation is being performed.
• When to Use: Use BlocBuilder to show a progress indicator when data is being fetched.

Theme(dark, light) or Localization (e.g., switching themes or languages)

• Build action: Update the app’s theme or language dynamically.
• When to Use: Use BlocBuilder to update the theme or language settings based on the state.

Conclusion

Flutter BLoC is a powerful state management solution that promotes separation of concerns by keeping business logic separate from UI code. By using the right components — BlocProvider, BlocBuilder, BlocListener, and BlocConsumer—developers can efficiently manage UI states, trigger side effects, and build scalable applications. Understanding when to use each component will help you write clean, maintainable, and efficient Flutter applications.

References:

• https://bloclibrary.dev/
• https://pub.dev/packages/flutter_bloc
• https://github.com/felangel/bloc
• https://github.com/felangel/bloc/tree/master/examples
• https://www.dhiwise.com/post/flutter-bloc-tutorial-understanding-state-management
• https://www.biztechcs.com/blog/flutter-bloc-tutorial/
• https://hackernoon.com/the-ultimate-guide-to-flutter-bloc-state-management-and-testing
• https://www.freecodecamp.org/news/user-authentication-flow-in-flutter-with-firebase-and-bloc/

Source Code

Open source end-to-end Flutter BLoC advanced usage: https://github.com/cevheri/flutter-bloc-advanced