BLoC pattern basics in Flutter

Introduction

The BLoC pattern helps separate your app's logic from its user interface. This makes your app easier to manage and test.

When you want to keep your UI code simple and clean.

When your app has multiple screens sharing data.

When you want to handle user actions and update UI reactively.

When you want to write tests for your app's logic without UI.

When you want to organize your app for easier maintenance.

Syntax

Flutter

import 'dart:async';

class CounterBloc {
  final _counterController = StreamController<int>.broadcast();
  int _counter = 0;

  Stream<int> get counterStream => _counterController.stream;

  void increment() {
    _counter++;
    _counterController.sink.add(_counter);
  }

  void dispose() {
    _counterController.close();
  }
}

The BLoC class holds your app's logic and data.

Use Streams to send data updates to the UI.

Examples

This example shows how to use the BLoC to increment a counter and listen for updates.

Flutter

final bloc = CounterBloc();
bloc.increment();
bloc.counterStream.listen((count) {
  print('Count is $count');
});

This shows how to connect the BLoC stream to the UI using StreamBuilder.

Flutter

StreamBuilder<int>(
  stream: bloc.counterStream,
  builder: (context, snapshot) {
    final count = snapshot.data ?? 0;
    return Text('Count: $count');
  },
);

Sample App

This app shows a number on screen that increases each time you press the button. The BLoC handles the counting logic and updates the UI through a stream.

Flutter

import 'dart:async';
import 'package:flutter/material.dart';

class CounterBloc {
  final _counterController = StreamController<int>.broadcast();
  int _counter = 0;

  Stream<int> get counterStream => _counterController.stream;

  void increment() {
    _counter++;
    _counterController.sink.add(_counter);
  }

  void dispose() {
    _counterController.close();
  }
}

void main() {
  runApp(MyApp());
}

class MyApp extends StatelessWidget {
  final CounterBloc bloc = CounterBloc();

  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      home: Scaffold(
        appBar: AppBar(title: Text('BLoC Pattern Example')),
        body: Center(
          child: StreamBuilder<int>(
            stream: bloc.counterStream,
            initialData: 0,
            builder: (context, snapshot) {
              return Text('Count: ${snapshot.data}', style: TextStyle(fontSize: 30));
            },
          ),
        ),
        floatingActionButton: FloatingActionButton(
          onPressed: bloc.increment,
          child: Icon(Icons.add),
          tooltip: 'Increment',
        ),
      ),
    );
  }
}

Important Notes

Always close your StreamControllers by calling dispose() to avoid memory leaks.

Use StreamBuilder in the UI to listen and rebuild widgets when data changes.

BLoC helps keep your UI code simple and your logic reusable.

Summary

BLoC separates app logic from UI using streams.

Use StreamController to manage data flow.

StreamBuilder connects streams to UI widgets.

Practice

1. What is the main purpose of the BLoC pattern in Flutter?
BLoC stands for Business Logic Component.

easy

A. To separate app logic from the user interface using streams

B. To create animations easily

C. To manage database connections

D. To style widgets with themes

Solution

1. Step 1: Understand BLoC purpose

   BLoC is designed to separate business logic from UI to keep code clean and testable.

2. Step 2: Identify correct function

   Among options, only separation of logic and UI using streams matches BLoC's goal.

3. Final Answer:

   To separate app logic from the user interface using streams -> Option A

4. Quick Check:

   BLoC = Separate logic and UI [OK]

Hint: Remember BLoC means separating logic from UI with streams [OK]

Common Mistakes:

• Confusing BLoC with UI styling or animations
• Thinking BLoC manages databases directly
• Assuming BLoC is only for UI widgets

2. Which of the following is the correct way to create a StreamController in Dart for BLoC?

easy

A. var controller = Stream();

B. var controller = StreamController<int>();

C. var controller = ControllerStream();

D. var controller = StreamBuilder();

Solution

1. Step 1: Recall StreamController syntax

   In Dart, StreamController is created by calling its constructor: StreamController<int>().

2. Step 2: Check options for correct syntax

   Only var controller = StreamController<int>(); uses correct class and constructor syntax.

3. Final Answer:

   var controller = StreamController<int>(); -> Option B

4. Quick Check:

   StreamController() constructor = var controller = StreamController(); [OK]

Hint: StreamController is a class, so use its constructor with parentheses [OK]

Common Mistakes:

• Using Stream() instead of StreamController()
• Confusing StreamBuilder with StreamController
• Misspelling class names

3. Given this BLoC code snippet:

final _controller = StreamController<int>();
void addNumber(int n) {
  _controller.sink.add(n);
}

Stream<int> get stream => _controller.stream;

// In UI:
StreamBuilder<int>(
  stream: bloc.stream,
  builder: (context, snapshot) {
    return Text(snapshot.data?.toString() ?? 'No data');
  },
);

What will the UI show after calling addNumber(5)?

medium

A. 5

B. No data

C. null

D. Error

Solution

1. Step 1: Understand data flow

   Calling addNumber(5) adds 5 to the stream sink, which emits 5 to listeners.

2. Step 2: StreamBuilder updates UI

   StreamBuilder listens to stream and rebuilds Text widget with snapshot.data = 5.

3. Final Answer:

   5 -> Option A

4. Quick Check:

   Stream emits 5, UI shows 5 [OK]

Hint: StreamBuilder shows latest stream data in UI [OK]

Common Mistakes:

• Thinking UI shows 'No data' after adding a number
• Confusing snapshot.data with snapshot.hasData
• Assuming null is shown instead of actual data

4. Identify the error in this BLoC snippet:

class CounterBloc {
  final _controller = StreamController<int>();

  void increment() {
    _controller.sink.add(_counter + 1);
  }

  Stream<int> get stream => _controller.stream;
}

Assuming _counter is not declared anywhere, what is the issue?

medium

A. increment() should return a value

B. StreamController is not closed properly

C. Stream getter should be a method, not a property

D. _counter is not defined, causing a compile error

Solution

1. Step 1: Check variable usage

   _counter is used but never declared or initialized, causing an error.

2. Step 2: Review other options

   While closing StreamController is good practice, missing _counter causes immediate compile error.

3. Final Answer:

   _counter is not defined, causing a compile error -> Option D

4. Quick Check:

   Undefined variable = compile error [OK]

Hint: Undefined variables cause compile errors immediately [OK]

Common Mistakes:

• Ignoring missing variable declarations
• Confusing Stream getter syntax
• Thinking StreamController must always be closed immediately

5. You want to create a BLoC that filters out even numbers and only emits odd numbers to the UI. Which code snippet correctly implements this behavior?

hard

A. final _controller = StreamController<int>();
Stream<int> get stream => _controller.stream.skip(1);

B. final _controller = StreamController<int>();
Stream<int> get stream => _controller.stream.map((n) => n * 2);

C. final _controller = StreamController<int>();
Stream<int> get stream => _controller.stream.where((n) => n % 2 != 0);

D. final _controller = StreamController<int>();
Stream<int> get stream => _controller.stream.where((n) => n % 2 == 0);

Solution

1. Step 1: Understand filtering logic

   We want to emit only odd numbers, so filter with condition n % 2 != 0.

2. Step 2: Check each option's stream transformation

   Only the transformation using where((n) => n % 2 != 0) correctly filters odd numbers. The others either skip the first element, double the numbers with map, or filter even numbers with where((n) => n % 2 == 0).

3. Final Answer:

   final _controller = StreamController<int>();\nStream<int> get stream => _controller.stream.where((n) => n % 2 != 0); -> Option C

4. Quick Check:

   Filter odd numbers with where(n % 2 != 0) [OK]

Hint: Use where() with condition to filter stream data [OK]

Common Mistakes:

• Using map() instead of where() for filtering
• Filtering even numbers instead of odd
• Using skip() which removes elements by count