command_history

Generic immutable undo/redo ring for Dart. Type-safe CommandHistory<S> with a Command<S> interface — no Flutter dependency, works in any Dart environment including pure Dart, Flutter (Riverpod, BLoC, etc.), and server-side.

Features

  • Immutable — every operation returns a new CommandHistory<S>; the original is never mutated
  • Generic — works with any state type S
  • No-op safe — execute skips the stack push if the command returns an identical state
  • Framework-agnostic — no Flutter, no Riverpod, no external dependencies
  • Tiny — two files, zero dependencies beyond the Dart SDK

Installation

dependencies:
  command_history: ^0.1.0

Core concepts

Command<S>

An interface with a single method. Implement it for each action your app can undo:

abstract interface class Command<S> {
  S execute(S state);
}

CommandHistory<S>

An immutable snapshot of the current state plus the undo/redo stacks:

final class CommandHistory<S> {
  S get state;
  bool get canUndo;
  bool get canRedo;

  CommandHistory<S> execute(Command<S> command);
  CommandHistory<S> undo();
  CommandHistory<S> redo();
  CommandHistory<S> replaceCurrent(S next, {S Function(S)? mapUndo, S Function(S)? mapRedo});
}

Usage

Basic counter example

import 'package:command_history/command_history.dart';

// 1. Define your state (can be any type — int, a record, an immutable class).
// 2. Implement Command<S> for each action.

class Add implements Command<int> {
  const Add(this.amount);
  final int amount;

  @override
  int execute(int state) => state + amount;
}

class Reset implements Command<int> {
  const Reset();

  @override
  int execute(int state) => 0;
}

void main() {
  var history = CommandHistory.initial(0);
  print(history.state);   // 0

  history = history.execute(const Add(5));
  print(history.state);   // 5

  history = history.execute(const Add(3));
  print(history.state);   // 8

  history = history.undo();
  print(history.state);   // 5

  history = history.redo();
  print(history.state);   // 8

  print(history.canUndo); // true
  print(history.canRedo); // false
}

With an immutable state class

import 'package:command_history/command_history.dart';

class TodoList {
  const TodoList({this.items = const []});
  final List<String> items;

  TodoList add(String item) => TodoList(items: [...items, item]);
  TodoList remove(int index) => TodoList(
    items: [...items.sublist(0, index), ...items.sublist(index + 1)],
  );
}

class AddTodo implements Command<TodoList> {
  const AddTodo(this.text);
  final String text;

  @override
  TodoList execute(TodoList state) => state.add(text);
}

class RemoveTodo implements Command<TodoList> {
  const RemoveTodo(this.index);
  final int index;

  @override
  TodoList execute(TodoList state) => state.remove(index);
}

void main() {
  var history = CommandHistory.initial(const TodoList());

  history = history.execute(const AddTodo('Buy milk'));
  history = history.execute(const AddTodo('Walk the dog'));
  print(history.state.items); // [Buy milk, Walk the dog]

  history = history.undo();
  print(history.state.items); // [Buy milk]
}

With Riverpod

class TodoNotifier extends Notifier<TodoList> {
  late CommandHistory<TodoList> _history;

  @override
  TodoList build() {
    _history = CommandHistory.initial(const TodoList());
    return _history.state;
  }

  void execute(Command<TodoList> command) {
    _history = _history.execute(command);
    state = _history.state;
  }

  void undo() {
    _history = _history.undo();
    state = _history.state;
  }

  void redo() {
    _history = _history.redo();
    state = _history.state;
  }

  bool get canUndo => _history.canUndo;
  bool get canRedo => _history.canRedo;
}

API

CommandHistory.initial(S initialState)

Creates a new history with the given initial state and empty stacks.

execute(Command<S> command) → CommandHistory<S>

Runs command.execute(state) and, if the result differs from the current state, pushes the current state onto the undo stack and clears the redo stack. Returns this unchanged when the command produces an identical state.

undo() → CommandHistory<S>

Pops the top of the undo stack as the new state. Returns this when canUndo is false.

redo() → CommandHistory<S>

Pops the top of the redo stack as the new state. Returns this when canRedo is false.

replaceCurrent(S next, {mapUndo, mapRedo}) → CommandHistory<S>

Replaces the current state without modifying the stacks. The optional mapUndo / mapRedo callbacks let you transform existing stack entries — useful when an out-of-band update (e.g. server sync) must be reflected in history so that undo/redo don't restore stale states.

// Server patched a field — keep history valid.
history = history.replaceCurrent(
  serverState,
  mapUndo: (old) => old.copyWith(serverPatchedField: serverState.serverPatchedField),
);

License

MIT — see LICENSE.

Libraries

command_history
Generic immutable undo/redo ring for Dart.