Add Optimistic Command

This skill adds optimisticCommand for one-time actions with immediate UI updates and automatic rollback on failure.

What This Skill Does

Implements optimistic updates for blocking, one-time operations like:

•Creating items (add todo, post message)
•Deleting items
•Form submissions
•File uploads
•Payments

The UI updates immediately, the server operation runs, and on failure the UI rolls back.

Instructions

Step 1: Identify the Operation

Use optimisticCommand when:

•The action should run once (not coalesce rapid calls)
•UI should update immediately for responsiveness
•Failure should revert the change
•The operation is a discrete command, not a continuous sync

Step 2: Implement optimisticCommand

dart

import 'package:bloc_superpowers/bloc_superpowers.dart';

class TodoCubit extends Cubit<TodoState> {
  TodoCubit() : super(const TodoState());

  void addTodo(Todo newTodo) => optimisticCommand(
    key: (AddTodo, newTodo.id),

    // 1. Return the optimistic value (new list with item added)
    optimisticValue: () => state.todoList.add(newTodo),

    // 2. Extract the value from current state
    getValueFromState: (state) => state.todoList,

    // 3. Apply a value back to state
    applyValueToState: (state, value) =>
        state.copyWith(todoList: value as List<Todo>),

    // 4. Send the command to the server
    sendCommandToServer: (optimisticValue) async {
      await api.saveTodo(newTodo);
      return null;  // Return server response if needed
    },
  );
}

Step 3: Understand the Flow

•Non-reentrant check: Prevents duplicate execution
•Capture initial state: For potential rollback
•Apply optimistic update: UI updates immediately
•Execute server command: Async operation
•On success: Optionally apply server response
•On failure: Rollback to initial state (if safe)

Rollback Safety: Rollback only occurs if the current state value still matches the optimistic value. This prevents overwriting concurrent changes made by other operations. If another update changed the value while the command was running, rollback is skipped to preserve that newer change.

Required Parameters

Parameter	Purpose
`key`	Identifies the operation for state tracking and non-reentrant
`optimisticValue`	Returns the immediate UI value
`getValueFromState`	Extracts the relevant value from state
`applyValueToState`	Applies a value back to state
`sendCommandToServer`	Executes the server operation

Optional Parameters

Apply Server Response

When the server returns data that should update the state:

dart

void addTodo(Todo newTodo) => optimisticCommand(
  key: (AddTodo, newTodo.id),
  optimisticValue: () => state.todoList.add(newTodo),
  getValueFromState: (state) => state.todoList,
  applyValueToState: (state, value) =>
      state.copyWith(todoList: value as List<Todo>),
  sendCommandToServer: (optimisticValue) async {
    final savedTodo = await api.saveTodo(newTodo);
    return savedTodo;  // Return server response
  },
  // Replace client-side todo with server-side todo
  applyServerResponseToState: (state, serverResponse) {
    final savedTodo = serverResponse as Todo;
    return state.copyWith(
      todoList: state.todoList
          .where((t) => t.id != newTodo.id)
          .toList()
        ..add(savedTodo),
    );
  },
);

Reload from Server

Reload data after success or failure:

dart

void deleteTodo(String todoId) => optimisticCommand(
  key: (DeleteTodo, todoId),
  optimisticValue: () =>
      state.todoList.where((t) => t.id != todoId).toList(),
  getValueFromState: (state) => state.todoList,
  applyValueToState: (state, value) =>
      state.copyWith(todoList: value as List<Todo>),
  sendCommandToServer: (optimisticValue) async {
    await api.deleteTodo(todoId);
    return null;
  },
  // Reload after operation
  reloadFromServer: () async {
    return await api.loadTodoList();
  },
  // Control when to reload
  shouldReload: ({
    required currentValue,
    required lastAppliedValue,
    required optimisticValue,
    required rollbackValue,
    required error,
  }) => error != null,  // Only reload on failure
);

Custom Rollback

Customize how rollback works:

dart

void addTodo(Todo newTodo) => optimisticCommand(
  key: (AddTodo, newTodo.id),
  // ... required params ...

  // Control whether to rollback
  shouldRollback: ({
    required currentValue,
    required initialValue,
    required optimisticValue,
    required error,
  }) => true,  // Default: always rollback on error

  // Custom rollback state (e.g., show failed status instead of removing)
  rollbackState: ({
    required state,
    required initialValue,
    required optimisticValue,
    required error,
  }) => state.copyWith(
    todoList: state.todoList.map((t) =>
      t.id == newTodo.id ? t.copyWith(status: TodoStatus.failed) : t
    ).toList(),
  ),
);

Separate Non-Reentrant Key

Track loading state globally but prevent duplicates per item:

dart

void addTodo(Todo newTodo) => optimisticCommand(
  key: AddTodo,  // isWaiting(AddTodo) shows any add in progress
  nonReentrantKey: (AddTodo, newTodo.id),  // Prevents duplicate for same todo
  // ... rest of params ...
);

// Widget
if (context.isWaiting(AddTodo)) {
  // Any add operation is in progress
}

Common Patterns

Add Item

dart

void addItem(Item newItem) => optimisticCommand(
  key: (AddItem, newItem.id),
  optimisticValue: () => [...state.items, newItem],
  getValueFromState: (state) => state.items,
  applyValueToState: (state, value) =>
      state.copyWith(items: value as List<Item>),
  sendCommandToServer: (optimisticValue) async {
    await api.createItem(newItem);
    return null;
  },
);

Delete Item

dart

void deleteItem(String itemId) => optimisticCommand(
  key: (DeleteItem, itemId),
  optimisticValue: () =>
      state.items.where((i) => i.id != itemId).toList(),
  getValueFromState: (state) => state.items,
  applyValueToState: (state, value) =>
      state.copyWith(items: value as List<Item>),
  sendCommandToServer: (optimisticValue) async {
    await api.deleteItem(itemId);
    return null;
  },
);

Update Item

dart

void updateItem(Item updatedItem) => optimisticCommand(
  key: (UpdateItem, updatedItem.id),
  optimisticValue: () => state.items
      .map((i) => i.id == updatedItem.id ? updatedItem : i)
      .toList(),
  getValueFromState: (state) => state.items,
  applyValueToState: (state, value) =>
      state.copyWith(items: value as List<Item>),
  sendCommandToServer: (optimisticValue) async {
    await api.updateItem(updatedItem);
    return null;
  },
);

Widget Integration

dart

Widget build(BuildContext context) {
  final isSaving = context.isWaiting((AddTodo, todoId));

  return ListTile(
    title: Text(todo.title),
    trailing: isSaving
        ? const CircularProgressIndicator()
        : const Icon(Icons.check),
  );
}

When to Use optimisticCommand vs optimisticSync

Scenario	Use
Add/Delete/Update item	`optimisticCommand`
Form submission	`optimisticCommand`
Toggle (like/favorite) with fast taps	`optimisticSync`
Slider/rating value	`optimisticSync`
Settings that change rapidly	`optimisticSync`

Rule: Use optimisticCommand for discrete actions, optimisticSync for values that may change rapidly.

User Preferences

Ask the user:

•What type of operation? (add, delete, update)
•Need server response in state? (use applyServerResponseToState)
•Should reload on failure? (use reloadFromServer)
•Custom rollback behavior? (use rollbackState)