Mastering Flutter Bloc: Changing State from Different Widgets

Learn how to effectively change state in Flutter using the Bloc pattern, providing tips on dependency injection and how to interact with your Bloc instance.
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This video is based on the question https://stackoverflow.com/q/66718822/ asked by the user 'Nishuthan S' ( https://stackoverflow.com/u/9074190/ ) and on the answer https://stackoverflow.com/a/66725440/ provided by the user 'Simon Sot' ( https://stackoverflow.com/u/13701546/ ) at 'Stack Overflow' website. Thanks to these great users and Stackexchange community for their contributions.

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Mastering Flutter Bloc: Changing State from Different Widgets

In the world of Flutter development, building responsive and dynamic applications can sometimes feel overwhelming, especially when it comes to state management. Developers often struggle with how to change the state of their application from different widgets, particularly when utilizing the popular flutter_bloc plugin. If you've ever faced the dilemma of managing state changes in your Flutter application, you’re not alone. In this post, we’ll explore how to effectively accomplish this using the Bloc pattern with clear and concise explanations.

Understanding the Need for State Management

Imagine you have a list view on your app's homepage that needs to rebuild every time a user clicks on a new page. How can you ensure that multiple widgets can interact with the same state seamlessly? This is where the flutter_bloc pattern comes in handy, allowing for efficient state management and facilitating communication between different parts of your UI.

Key Considerations for Changing State

When handling state changes with flutter_bloc, keep these two fundamental aspects in mind:

Dependency Injection (DI) of a Bloc

Interacting with Your Bloc Instance

1. Dependency Injection of a Bloc

When it comes to supplying a Bloc instance to various widgets, you can use the BlocProvider. This widget helps create and provide a Bloc instance that can be accessed throughout the widget subtree.

Case 1: Providing a Bloc within One Route

Use the BlocProvider to create a Bloc instance for child widgets as shown below:

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Note: By default, the BlocProvider is created with the property lazy: true, meaning the Bloc instance is created only when it is requested. If you want it to be immediate, set lazy: false.

Case 2: Providing a Bloc across Different Routes

If you need to pass a Bloc instance to widgets in a different context or route, utilize BlocProvider.value. This method allows the Bloc instance to be shared, but be cautious that it only works with existing instances:

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Important: Avoid creating a new Bloc instance using this method; it should only provide already instantiated Blocs.

2. Interaction with Your Bloc Instance

Given changes in the latest versions of the Bloc library, context.bloc and context.repository are now deprecated. Instead, you should use context.read and context.watch. Here’s how:

Accessing Bloc State

Using context.watch(): If you want a widget to rebuild when the bloc state changes, employ context.watch() or BlocBuilder:

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Using BlocBuilder: If only certain parts of a widget need to be rebuilt, wrap those parts in a BlocBuilder:

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Adding Events to the Bloc

To add an event to your Bloc, simply use context.read():

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Conclusion

Mastering how to change state from different widgets in Flutter using the Bloc pattern is essential for building efficient applications. By understanding dependency injection and how to properly interact with your Bloc, your Flutter applications can achieve a new level of responsiveness. Whether you're a seasoned developer or just starting, applying these principles will surely elevate your app development experience.

If you have any questions or additional tips regarding state management in Flutter, feel free to share in the comments! Happy coding!