BLoC pattern basics in Flutter - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to create a BLoC class that extends the correct base class.

Flutter

class CounterBloc extends [1] {
  // BLoC logic here
}

Drag options to blanks, or click blank then click option'

AStatelessWidget

BStreamBuilder

CCubit<int>

DStatefulWidget

Attempts:

3 left

💡 Hint

Common Mistakes

Using widget classes instead of BLoC base classes.

Confusing StreamBuilder with BLoC base class.

The Cubit<int> class is the base for simple BLoC classes managing integer state.

2fill in blank

medium

Complete the code to emit a new state value in the BLoC.

Flutter

void increment() {
  emit([1] + 1);
}

Drag options to blanks, or click blank then click option'

Astate

Bcontext

Cwidget

Dthis

Attempts:

3 left

💡 Hint

Common Mistakes

Using context or widget instead of state to get current value.

Trying to call emit without a value.

The state property holds the current state value in a Cubit or BLoC.

3fill in blank

hard

Fix the error in the BLoC provider code to correctly provide the BLoC to the widget tree.

Flutter

return BlocProvider(
  create: (context) => [1](),
  child: MyHomePage(),
);

Drag options to blanks, or click blank then click option'

AMaterialApp

BMyHomePage

CBlocBuilder

DCounterBloc

Attempts:

3 left

💡 Hint

Common Mistakes

Returning a widget instead of a BLoC instance.

Forgetting to call the constructor with parentheses.

The create function must return an instance of the BLoC class, here CounterBloc().

4fill in blank

hard

Fill both blanks to build a widget that listens to BLoC state changes and rebuilds accordingly.

Flutter

return BlocBuilder<CounterBloc, [1]>(
  builder: (context, [2]) {
    return Text('Count: $[2]');
  },
);

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Aint

Bstate

Ccontext

DString

Attempts:

3 left

💡 Hint

Common Mistakes

Using wrong state type in BlocBuilder generic.

Using context instead of state in builder arguments.

The BlocBuilder listens to CounterBloc with state type int. The builder function receives the current state as second argument.

5fill in blank

hard

Fill both blanks to define a BLoC event class and handle it in the BLoC.

Flutter

abstract class CounterEvent {}

class IncrementEvent extends CounterEvent {}

class CounterBloc extends Bloc<CounterEvent, int> {
  CounterBloc() : super(0) {
    on<[1]>((event, emit) {
      emit(state [2] 1);
    });
  }
}

Drag options to blanks, or click blank then click option'

AIncrementEvent

B+

C-

DCounterEvent

Attempts:

3 left

💡 Hint

Common Mistakes

Using wrong event class in on<>.

Using subtraction instead of addition.

The on<IncrementEvent> listens for the increment event. The state is updated by adding 1.

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