Add Effect Queue to State

This skill adds an EffectQueue field to a state class for sequenced one-time UI effects.

What This Skill Does

Adds EffectQueue<T> to state for triggering multiple side effects in order:

•Show toast, then dialog, then navigate
•Execute a sequence of UI actions
•Coordinate multiple side effects from a single Cubit action

This pattern eliminates the need for BlocListener. The Cubit declares what should happen (the effects), while the widget determines how to execute them (the UI logic).

When to Use

Use EffectQueue when:

•You need to trigger multiple effects in sequence
•Order matters (show message before navigating)
•Multiple effects should execute from one Cubit action

Use simple Effect<T> when:

•You only need one effect at a time
•Order doesn't matter

Instructions

Step 1: Define Effect Types

Create a sealed class hierarchy for your effects:

dart

sealed class UiEffect {}

class ShowToast extends UiEffect {
  final String message;
  ShowToast(this.message);
}

class ShowDialog extends UiEffect {
  final String title;
  final String content;
  ShowDialog(this.title, this.content);
}

class Navigate extends UiEffect {
  final String route;
  Navigate(this.route);
}

class ClearForm extends UiEffect {}

Step 2: Add EffectQueue to State

Add an EffectQueue<UiEffect> field. Always initialize as spent:

dart

import 'package:bloc_superpowers/bloc_superpowers.dart';

class AppState {
  final User? user;
  final EffectQueue<UiEffect> effectQueue;

  AppState({
    this.user,
    EffectQueue<UiEffect>? effectQueue,
  }) : effectQueue = effectQueue ?? EffectQueue.spent();

  AppState copyWith({
    User? user,
    EffectQueue<UiEffect>? effectQueue,
  }) {
    return AppState(
      user: user ?? this.user,
      effectQueue: effectQueue ?? this.effectQueue,
    );
  }
}

Step 3: Emit Effect Queue from Cubit

Create an EffectQueue with a list of effects and a callback for remaining effects:

dart

class AppCubit extends Cubit<AppState> {
  AppCubit() : super(AppState());

  void onPurchaseComplete() {
    emit(state.copyWith(
      effectQueue: EffectQueue<UiEffect>(
        [
          ShowToast('Purchase successful!'),
          ShowDialog('Thank You', 'Your order has been placed.'),
          Navigate('/orders'),
        ],
        // Callback to emit remaining effects
        (remaining) => emit(state.copyWith(effectQueue: remaining)),
      ),
    ));
  }
}

Step 4: Consume Queue in Widget

Use context.effectQueue() in the build method:

dart

class AppScreen extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    context.effectQueue<AppCubit, UiEffect>(
      // Select the queue
      (cubit) => cubit.state.effectQueue,

      // Handle each effect
      (context, effect) => switch (effect) {
        ShowToast(:final message) =>
          ScaffoldMessenger.of(context).showSnackBar(
            SnackBar(content: Text(message)),
          ),

        ShowDialog(:final title, :final content) =>
          showDialog(
            context: context,
            builder: (_) => AlertDialog(
              title: Text(title),
              content: Text(content),
              actions: [
                TextButton(
                  onPressed: () => Navigator.pop(context),
                  child: const Text('OK'),
                ),
              ],
            ),
          ),

        Navigate(:final route) =>
          Navigator.of(context).pushNamed(route),

        ClearForm() =>
          _formKey.currentState?.reset(),
      },
    );

    return Scaffold(
      body: MyContent(),
    );
  }
}

Execution Modes

One Per Frame (Default)

Effects execute one at a time, with a rebuild between each:

dart

context.effectQueue<AppCubit, UiEffect>(
  (cubit) => cubit.state.effectQueue,
  onePerFrame: true,  // Default
  (context, effect) => ...,
);

All At Once

All effects execute in a single frame:

dart

context.effectQueue<AppCubit, UiEffect>(
  (cubit) => cubit.state.effectQueue,
  onePerFrame: false,  // Execute all immediately
  (context, effect) => ...,
);

Common Effect Patterns

Onboarding Flow

dart

sealed class OnboardingEffect {}
class ShowWelcome extends OnboardingEffect {}
class RequestPermissions extends OnboardingEffect {}
class ShowTutorial extends OnboardingEffect {}
class NavigateToHome extends OnboardingEffect {}

void startOnboarding() {
  emit(state.copyWith(
    effectQueue: EffectQueue<OnboardingEffect>(
      [
        ShowWelcome(),
        RequestPermissions(),
        ShowTutorial(),
        NavigateToHome(),
      ],
      (remaining) => emit(state.copyWith(effectQueue: remaining)),
    ),
  ));
}

Form Submission

dart

sealed class FormEffect {}
class ShowSaving extends FormEffect {}
class ShowSuccess extends FormEffect {
  final String message;
  ShowSuccess(this.message);
}
class ClearForm extends FormEffect {}
class NavigateBack extends FormEffect {}

void submitForm(FormData data) => mix(
  key: this,
  () async {
    await api.submit(data);
    emit(state.copyWith(
      effectQueue: EffectQueue<FormEffect>(
        [
          ShowSuccess('Form submitted!'),
          ClearForm(),
          NavigateBack(),
        ],
        (remaining) => emit(state.copyWith(effectQueue: remaining)),
      ),
    ));
  },
);

Error with Recovery Options

dart

sealed class ErrorEffect {}
class ShowError extends ErrorEffect {
  final String message;
  ShowError(this.message);
}
class OfferRetry extends ErrorEffect {
  final VoidCallback onRetry;
  OfferRetry(this.onRetry);
}
class LogError extends ErrorEffect {
  final Object error;
  LogError(this.error);
}

void onError(Object error) {
  emit(state.copyWith(
    effectQueue: EffectQueue<ErrorEffect>(
      [
        LogError(error),
        ShowError('Something went wrong'),
        OfferRetry(() => loadData()),
      ],
      (remaining) => emit(state.copyWith(effectQueue: remaining)),
    ),
  ));
}

Complete Example

dart

// Effects
sealed class CheckoutEffect {}

class ShowProcessing extends CheckoutEffect {}

class ShowSuccess extends CheckoutEffect {
  final String orderId;
  ShowSuccess(this.orderId);
}

class SendConfirmationEmail extends CheckoutEffect {
  final String email;
  SendConfirmationEmail(this.email);
}

class NavigateToOrder extends CheckoutEffect {
  final String orderId;
  NavigateToOrder(this.orderId);
}

// State
class CheckoutState {
  final Cart cart;
  final EffectQueue<CheckoutEffect> effectQueue;

  CheckoutState({
    required this.cart,
    EffectQueue<CheckoutEffect>? effectQueue,
  }) : effectQueue = effectQueue ?? EffectQueue.spent();

  CheckoutState copyWith({
    Cart? cart,
    EffectQueue<CheckoutEffect>? effectQueue,
  }) => CheckoutState(
    cart: cart ?? this.cart,
    effectQueue: effectQueue ?? this.effectQueue,
  );
}

// Cubit
class CheckoutCubit extends Cubit<CheckoutState> {
  CheckoutCubit(Cart cart) : super(CheckoutState(cart: cart));

  void placeOrder(String email) => mix(
    key: this,
    () async {
      final order = await api.placeOrder(state.cart);

      emit(state.copyWith(
        cart: Cart.empty(),
        effectQueue: EffectQueue<CheckoutEffect>(
          [
            ShowSuccess(order.id),
            SendConfirmationEmail(email),
            NavigateToOrder(order.id),
          ],
          (remaining) => emit(state.copyWith(effectQueue: remaining)),
        ),
      ));
    },
  );
}

// Widget
class CheckoutScreen extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    context.effectQueue<CheckoutCubit, CheckoutEffect>(
      (cubit) => cubit.state.effectQueue,
      onePerFrame: true,
      (context, effect) => switch (effect) {
        ShowProcessing() =>
          showDialog(
            context: context,
            barrierDismissible: false,
            builder: (_) => const AlertDialog(
              content: CircularProgressIndicator(),
            ),
          ),

        ShowSuccess(:final orderId) =>
          ScaffoldMessenger.of(context).showSnackBar(
            SnackBar(content: Text('Order $orderId placed!')),
          ),

        SendConfirmationEmail(:final email) =>
          emailService.sendConfirmation(email),

        NavigateToOrder(:final orderId) =>
          Navigator.pushReplacementNamed(
            context,
            '/order/$orderId',
          ),
      },
    );

    return CheckoutForm();
  }
}

User Preferences

Ask the user:

•What effects are needed? (toast, dialog, navigation, etc.)
•What's the sequence? (order matters)
•One per frame or all at once?