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Originally started to provide implementation of a State Machine using Redux design pattern, this library now includes a Redux Store which can be used as a regular state store or a state machine.

This library implements simplified action dispatch flow:

  1. User dispatches an action
  2. Store executes corresponding reducer function, synchronously.
  3. Store publishes a StoreEvent as a result into events stream.
  4. Side-effects are handled in stream listeners subscribed to events stream.

Since reducers are pure functions and there is no middleware layer - dispatching an action is always side-effect free and always results in an event published to events stream (unless reducer function resulted in an error).

Any middleware-like logic should be put in events stream listeners.

Upgrading from 0.1.x? See UPGRADING.md for instructions.

Usage

TL;DR see full source code of this example in the example/ folder.

Redux requires three things: state, actions and reducers.

We start by defining our state object. Below is an example of a coin-operated turnstile (from Wikipedia):

class Turnstile {
  final bool isLocked;
  final int coinsCollected;
  final int visitorsPassed;

  Turnstile(this.isLocked, this.coinsCollected, this.visitorsPassed);

  /// Convenience method to use in reducers.
  Turnstile copyWith({
    bool isLocked,
    int coinsCollected,
    int visitorsPassed,
  }) {
    return new Turnstile(
      isLocked ?? this.isLocked,
      coinsCollected ?? this.coinsCollected,
      visitorsPassed ?? this.visitorsPassed,
    );
  }
}

Next, actions:

abstract class Actions {
  /// Put coin to unlock turnstile
  static const putCoin = const VoidActionBuilder('putCoin');

  /// Push turnstile to pass through
  static const push = const VoidActionBuilder('push');
}

There are 4 action builder classes provided for most common use cases. Above example shows usage of VoidActionBuilder which creates Actions with no (void) payload. Here are examples of using all 4 builders:

abstract class Actions {
  /// The same example as above.
  static const putCoin = const VoidActionBuilder('putCoin');

  /// Regular action builder with [String] payload. Payload is required.
  static const getUser = const ActionBuilder<String>('getUser');

  /// The same as [VoidActionBuilder] but creates [AsyncAction] with no payload.
  /// Read more on async actions below in this document.
  static const clearCache = const AsyncVoidActionBuilder('clearCache');

  /// Similarly creates [AsyncAction] with required payload of type [String].
  static const deleteUser = const AsyncActionBuilder<String>('deleteUser');
}

/// Using builders
Future<void> main() async {
  final Store<MyState> store = getStore();
  /// All builders implement [call] method and can be invoked as a regular
  /// function. [call] method of `void` builders has zero arguments.
  store.dispatch(Actions.putCoin());

  /// `Async*` builders create [AsyncAction]s which allow dispatching side
  /// to know when action is completed via [AsyncAction.done] Future.
  final clearCache = Actions.clearCache();
  store.dispatch(clearCache);
  await clearCache.done;

  /// [ActionBuilder] and [AsyncActionBuilder] implement `call` method with
  /// single argument - payload for the created action:
  store.dispatch(Actions.getUser('user-id'));
  /// Or:
  final deleteUser = Actions.deleteUser('user-id');
  store.dispatch(deleteUser);
  await deleteUser.done;
}

Next step, reducers:

Turnstile putCoinReducer(Turnstile state, Action<void> action) {
  int coinsCollected = state.coinsCollected + 1;
  print('Coins collected: $coinsCollected');
  return state.copyWith(isLocked: false, coinsCollected: coinsCollected);
}

Turnstile pushReducer(Turnstile state, Action<void> action) {
  int visitorsPassed = state.visitorsPassed;
  if (!state.isLocked) {
    visitorsPassed++;
    print('Visitors passed: ${visitorsPassed}');
  }
  return state.copyWith(isLocked: true, visitorsPassed: visitorsPassed);
}

Combining everything together:

void main() {
  // Create Redux store and register reducers using provided builder class:
  final builder = new StoreBuilder<Turnstile>(
    initialState: new Turnstile(true, 0, 0),
  );
  builder
    ..bind(Actions.putCoin, putCoinReducer)
    ..bind(Actions.push, pushReducer);
  final store = builder.build();

  // Try triggering some actions
  machine.dispatch(Actions.push());
  machine.dispatch(Actions.putCoin());
  // .. etc.
  // Make sure to dispose the machine in the end:
  machine.dispose();
}

Chaining actions

Sometimes it is useful to trigger another action from inside current reducer. It is possible via Action.next() method:

State exampleReducer(State state, Action<void> action) {
  // do work here
  // ...
  final newState = state.copyWith(exampleField: 'value');
  // State store will dispatch `otherAction` and pass `newState` as an input
  // state argument.
  return action.next(newState, Actions.otherAction());
}

Note that Action.next does not perform actual dispatch so calling it multiple times within a reducer function has no chaining effect. Only action passed to the last invocation of Action.next will be dispatched by the state store.

Middleware example 1: logging

Store class exposes events stream which contains all dispatched actions and their results. Logging middleware becomes a simple stream subscription. Below is simplistic printing to stdout of all events:

final Store<MyState> store = getStore();
// Print all events to stdout:
store.events.listen(print);

Middleware example 2: error reporting

Any unhandled errors in reducers are forwarded to the errors stream if there is an active listener on it. If there is no active listener all errors are simply rethrown during dispatch.

Note that Store.errors stream contains instances of StoreError which provide details about the failed action and current state. In case there is no listener on this stream the unhandled error from reducer is rethrown as-is (not wrapped with StoreError) to preserve original stack trace.

To log all unhandled errors listen on the "errors" stream.

final Store<MyState> store = getStore();
// Print all events to stdout:
store.errors.listen(null, onError: errorHandler, cancelOnError: false);

void errorHandler(error, stackTrace) {
  print(error);
  print(stackTrace);
}

Actions which resulted in an error do not publish a StoreEvent to the events stream.

Middleware example 3: making HTTP request

In below example we use Store.eventsFor stream which returns a stream of events produced by the same action type (in this case all "fetchUser" events).

final Store<MyState> store = getStore();
// Note that async is allowed in event listeners.
store.eventsFor(Actions.fetchUser).listen((StoreEvent<MyState, String> event) async {
  try {
    // assuming action payload is the ID of a user to fetch.
    String userId = event.action.payload;
    final user = await fetchUserFromHttpApi(userId);
    store.dispatch(Actions.userFetched(user));
  } catch (error) {
    store.dispatch(Actions.userFetchFailed(error));
  }
});

store.dispatch(Actions.fetchUser('user-id-here'));

Async actions (experimental)

AsyncAction is like regular Redux Action except it also carries a Future. In many cases it can be a simpler alternative to traditional trio of doFoo, doFooSuccess and doFooFailed actions.

Common use case for async actions is when no explicit UI interaction is expected with the user after the action is done. For intance, deleting content or swiping list items left or right.

Using async actions

Async actions assume there are side-effects involved so they are normally handled by an event stream listener where side-effects are allowed:

abstract class Actions {
  /// Action payload is an integer ID of the note to delete.
  static const deleteNote = const AsyncActionBuilder<int>('deleteNote');
}

// Subscribing to deleteNote events.
Store buildStore() {
  final builder = new StoreBuilder<AppState>();
  // ...bind reducers
  final store = builder.build();
  store.eventsFor(Actions.deleteNote).listen(_deleteNote);
}

/// Listener for deleteNote events
_deleteNote(StoreEvent<AppState, int> event) async {
  AsyncAction<int> action = event.action;
  int noteId = action.payload;
  try {
    var result = await httpClient.send('DELETE', '/notes/$noteId');
    // Delete successful, mark the action as done
    action.complete();
  } catch (error) {
    action.completeError(error);
  }
}

// Somewhere on the client side where the action is dispatched
deleteNoteButtonPressed(int noteId) async {
  final action = Actions.deleteNote(noteId);
  store.dispatch(action);
  // refresh UI to show loading state, pseudo-code
  setState(isLoading: true);
  try {
    await action.done;
    setState(isLoading: false); // refresh UI, delete successful
  } catch (error) {
    // failed to delete, show the error.
    setState(errorMessage: error.toString());
  }
}

Features and bugs

Please file feature requests and bugs at the issue tracker.

Libraries

redux_machine
Redux Store implementation which can also be used as a state machine.