EventBus Arch

event_bus_arch is an event-driven architecture toolkit for Dart and Flutter. Components talk to each other through events addressed by a topic — a type plus a path — so they stay decoupled and can be moved between isolates without changing their logic.

On top of the bus the package ships a finite state machine, an isolate bridge, multi-isolate "layers", and a scope/command wrapper for stateful values.

Detailed documentation: doc/
Full API reference: doc/06-api-reference.md
Upgrading from 2.x: doc/07-caveats.md
Load benchmark: benchmark/README.md

Features

Two publish modes: fire (one-way, listeners only) and request (one handler, returns a result) — see Two modes: fire and request
Send and receive events between components (listen / subscribe)
Handlers that return a result to request
Two bus flavours: common (EventBus()) and model (EventBus(isModelBus: true)) — the model bus keeps the last value per topic even when nobody is listening
Topic-based routing: event type + paths, plus arguments as call parameters
EventBusIsolate — the same API, but the events are processed in a worker isolate
Layers — chains of isolates: main ↔ isolate₂ ↔ isolate₁ ↔ isolate₀
FSM — finite state machine driven by bus events
Scope / Command — stateful value with call history
EventBusLogger — trace every event that crosses the bus
Type-safe handlers, no code generation

Installation

dependencies:
  event_bus_arch: ^3.0.0

dart pub get      # or: flutter pub get

Dart SDK >=3.6.0 <4.0.0. The package re-exports rxdart, so BehaviorSubject, debounceTime, doOnCancel and friends are available without an extra import.

Quick start

import 'package:event_bus_arch/event_bus_arch.dart';

Future<void> main() async {
  final bus = EventBus();

  // Receive: every event of that topic, current and future
  final sub = bus.subscribe<String>(onData: (event) => print('got: $event'));

  // Handle: the handler returns the response; request waits for it.
  // lastData is the PREVIOUS value of this topic, dto.data is the current one.
  bus.handlers.setHandler<String>(handler: (dto, lastData) async {
    return 'processed: ${dto.data}';
  });

  final result = await bus.request<String>('hello');
  print(result);                 // processed: hello

  // by now the listener has already received the value: request() is a sync point
  await sub.cancel();
  await bus.dispose();
}

Two modes: `fire` and `request`

There are exactly two ways to publish an event, and they have different contracts:

	`fire<T>(...)`	`request<T>(...)`
Purpose	one-way event	ask a handler for a result
Invokes `setHandler` responders	no — listeners only	yes — exactly one handler
Return value	`bool` — was the event accepted	`Future<dynamic>` — the handler's result
No listener / no handler	`false`, not an error	throws `NoHandlerException`
Waits	no, returns immediately	yes, until the handler returns; optional `timeout`
Call arguments	`arguments` reach listeners in `dto.arguments`	the same, plus the handler
Wildcard paths	rejected — the call names a concrete topic	rejected — the call names a concrete topic

final bus = EventBus();

// Listeners can be registered on a concrete topic or on a pattern.
bus.subscribe<int>(paths: ['sensor', '*'], onData: (v) => print('event: $v'));

// A handler is a request responder; fire never calls it.
bus.handlers.setHandler<int>(
  paths: ['sensor', 'temp'],
  handler: (dto, lastData) async => dto.data * 2,
);

// fire: one-way, returns whether the event was accepted.
final delivered = bus.fire<int>(21, paths: ['sensor', 'temp']);   // true
// the wildcard listener printed "event: 21" (asynchronously)

// request: waits for the handler and returns its result.
final doubled = await bus.request<int>(21, paths: ['sensor', 'temp']);   // 42
// the wildcard listener also saw the request: "event: 21"

fire and request both notify listeners; the difference is the handler call and the result channel. A request without a handler is an error, not a silently ignored event.

Topics

A topic is the static type argument of the call plus an optional path:

OrderPlaced.warehouse.in
     │            │
     │            └── paths: ['warehouse', 'in']
     └─────────────── type: the type parameter T of the call

bus.fire<Order>(order, paths: ['warehouse', 'in'], arguments: {'priority': '1'});

bus.subscribe<Order>(paths: ['warehouse', 'in'], onData: (order) => print(order));

paths selects the topic; arguments are delivered to the handler as dto.arguments but do not affect routing. Because the type is part of the address, request<int> and a handler registered with setHandler<num> are different topics — always be explicit with type arguments.

A path token must not contain the separator (. by default): paths: ['a.b'] and paths: ['a', 'b'] build the same address. Pass separator: '/' to address such a topic, or change the fallback for a whole bus:

final bus = EventBusImpl(false)..defaultTopicPathSeparator = '/';

Wildcards

listen, subscribe and setHandler accept wildcard path tokens:

* matches exactly one path token: int.sensor.*;
> matches one or more trailing tokens and must be the last token: int.sensor.>.

`fire` through a wildcard

fire always names a concrete topic; a wildcard subscription receives every matching event:

final sub = bus.subscribe<int>(
  paths: ['sensor', '*'],                       // one path token after "sensor"
  onData: (value) => print('sensor: $value'),
);

// Concrete events; no wildcard in the fire() call itself.
bus.fire<int>(21, paths: ['sensor', 'temp']);       // true → sensor: 21
bus.fire<int>(22, paths: ['sensor', 'humidity']);   // true → sensor: 22
bus.fire<int>(23, paths: ['sensor', 'room', 'x']);  // false — '*' matches one token, not two

// '>' matches one or more trailing tokens.
bus.subscribe<int>(paths: ['sensor', '>'], onData: (v) => print('subtree: $v'));
bus.fire<int>(24, paths: ['sensor', 'room', 'temp']);   // true → subtree: 24

The same pattern can answer requests when it is registered as a handler:

bus.handlers.setHandler<int>(paths: ['sensor', '*'],
    handler: (dto, lastData) async => dto.data * 2);
await bus.request<int>(21, paths: ['sensor', 'temp']);   // 42

A fire or request call itself must name a concrete topic: wildcard paths throw. A request may be answered by a handler registered on a pattern (exact handler first, then the most specific pattern). fire delivers to the exact node and to every matching pattern node. currentData(pattern) returns the last matching value; currentData(concrete) reads only the exact node.

Model bus vs common bus

	`EventBus()`	`EventBus(isModelBus: true)`
Creates a node on `fire` to an unknown topic	no	yes
Keeps `currentData` after the last listener unsubscribes	no	yes
`fire` to a topic with no node	returns `false`, event goes to `allEventStream` with flag `false`	returns `true`, node is created
`request` without a handler	throws `NoHandlerException`	throws `NoHandlerException`

A handler registered with setHandler is a request responder: request invokes it, while fire only notifies listeners.

Use the model bus when the bus is a store of current state, and the common bus when it is a message pipe.

Isolates

import 'dart:isolate';
import 'package:event_bus_arch/event_bus_arch.dart';

Future<void> main() async {
  final iso = EventBusIsolate(onInit: _worker, initialData: 'worker');
  await iso.waitInit;                                  // true when onInit has completed

  iso.subscribe<String>(onData: (e) => print('from worker: $e'));
  final doubled = await iso.request<int>(21, arguments: {'multiplier': '2'});
  print(doubled);                                      // 42

  await iso.dispose();
}

// runs inside the worker isolate — must be a top-level or static function
Future<void> _worker(EventBus bus, Object? initialData) async {
  await _heavyInit();                                  // async init is fine
  bus.handlers.setHandler<int>(handler: (dto, lastData) async {
    final m = int.tryParse(dto.arguments?['multiplier'] ?? '1') ?? 1;
    return dto.data * m;
  });
}

EventBusIsolate behaves like a normal bus on both sides: events sent from the worker reach main-isolate listeners, and request from main returns the worker's handler result. Paths, separator and arguments cross the boundary with the event.

Good to know:

Only sendable objects can cross the isolate boundary.
waitInit completes when onInit has finished (including its awaits), so events sent right after it are never lost. Errors from onInit surface as an EventBusException from waitInit.
Every request is answered exactly once and correlated by request id, so concurrent requests of the same type each get their own result. A topic the worker has no handler for fails with NoHandlerException; a handler error is rethrown at the sender; an in-flight request fails with EventBusException if the isolate is disposed.
ping() only checks liveness ("is the worker alive"), not readiness.

Layers

final layer = await Layers.createLayers([
  (_initMiddle, null, true),   // (init function, initial data, isModelBus)
  (_initBottom, null, true),
]);
// main ↔ isolate[middle] ↔ isolate[bottom]

// createLayers completes only when every layer in the chain is initialized,
// so events can be sent right away
layer.bottom!.subscribe<Ping>(onData: (e) => print(e.count));
layer.bottom!.fire(Ping(0));
await layer.close();

Inside a layer, layer.top sends to the layer above, layer.bottom to the layer below, and layer.current is the layer's own bus. Init functions may be async and may live in any layer; Layer.close() works from any layer and is safe to call twice. See doc/03-isolate-and-layers.md and example/event_bus_layers_example.dart.

Finite state machine

enum State { idle, loading, ready }

class Loader with FSM<State> {
  Loader(EventBus bus) {
    initFSM(bus, State.idle);
    addFSMTransition<Start>(State.idle, State.loading,
      eventPaths: ['api', 'items'],            // the transition listens on Start.api.items
      filter: (current, last, event) => event.url.isNotEmpty,
      handler: (current, last, event, ignored) async {
        print('$last -> $current ($event)');
        return null;
      });
  }
}

bus.fire(Start('/api/items'), paths: ['api', 'items']);   // State.idle -> State.loading

Events arriving while a transition handler is running are collected into the ignoredEvents argument and are not replayed afterwards. FSM.dispose() unsubscribes the machine from the bus.

Scope and Command

final scope = Scope<int>();
await scope.initScope(
  bus,
  paths: ['counter'],
  initialData: 0,
  maxLenForUndo: 10,                 // history depth; undo needs at least 2 entries
  onUpdate: (value) => print('value: $value'),
);

await scope.call(1);                 // goes through the bus
print(scope.lastData);               // 1  (filled by the bus subscription)
await scope.undo();                  // re-sends the PREVIOUS value from the history
scope.disposeScope();

Command keeps the call history (maxLen) and undo() sends the previous value back through the bus. undo() returns null when there is nothing to roll back to.

Documentation

Detailed guides live in doc/:

Document	Content
01-architecture.md	Topics, DTOs, nodes, the full send/receive flow
02-event-bus.md	Practical bus usage, handlers, addressing, recipes
03-isolate-and-layers.md	`EventBusIsolate`, `Layers`, exchange limits
04-fsm.md	State machine on top of the bus
05-scope-and-command.md	State with history, command pattern
06-api-reference.md	Every public type and member
07-caveats.md	What 3.0.0 changed, what remains by design, checklist
08-transports.md	Running the bus over NATS / Redis / NanoMQ: analysis
09-event-bus-nats.md	The implemented NATS transport: usage, architecture, limitations
10-event-bus-mqtt.md	The implemented MQTT 5 transport: usage, architecture, limitations

Version 3.0.0 changes

The address model changed; this is a breaking release.

Topic is no longer URI-like: scheme, target and fragment are gone
path: '/in' became paths: ['in'] — a list of tokens joined by a configurable separator (. by default, EventBus.defaultTopicPathSeparator)
arguments moved from Topic to EventDTO: dto.topic.arguments is now dto.arguments, both locally and across an isolate
send is split into fire and request: fire is one-way and returns bool; request invokes a handler and waits for its result (or NoHandlerException)
Handlers return their response; EventDTO.completer was removed, and fire no longer invokes handlers
lastData was renamed currentData (bus.currentData<T>(), EventNode.currentData)
Local wildcards: * matches one path token, > one or more trailing tokens; listen/subscribe/setHandler accept patterns, fire/request require concrete topics
The type in the address is '$T' normalised to a subject-safe token — no lowercasing, no percent-encoding (List<int> → List(int))
topic.topic is the single routing key (fullTopic/topicWithFragment are gone)
packages/event_bus_arch_nats adds a NATS transport as a separate package; the core stays broker-free (see doc/09-event-bus-nats.md)
packages/event_bus_arch_transport holds the broker-agnostic connection, mapping, bus and node shared by the NATS and MQTT 5 transports
packages/event_bus_arch_mqtt adds an MQTT 5 transport (see doc/10-event-bus-mqtt.md)
The deprecated queueLenght, initalData, initialDataNeedExecute and shema aliases were removed

The full migration table is in doc/07-caveats.md.

Version 2 changes

EventBusIsolate, Layers, FSM, Scope/Command, subscribe, dispose were added
Reduced API surface; the bus always returns a result or null (the old call method is gone)
EventBusMaster was removed — buses no longer need names
Handlers return values through EventDTO.completer (removed in 3.0.0 — handlers return the value directly)

Version 2.3.0 changes

EventBus now exposes handlers (no more bus as EventBusHandlers) and an idempotent dispose()
Handler failures reach the sender instead of hanging it; listeners receive values before await request(...) returns
EventBusIsolate correlates responses by request id, waitInit means "onInit completed", and ping() checks liveness; in-flight requests fail instead of hanging on dispose
Layers.createLayers completes only when the whole chain is initialized; Layer.close() works from any layer
Scope/Command undo() sends the previous value; Scope.maxLenForUndo defaults to 10
EventDTO.toJson no longer encodes data; EventBusException is exported
Typo'd members got correct names (queueLength, initialData, scheme, initialDataNeedExecute) — those aliases were removed again in 3.0.0

The full list with reproductions is in doc/07-caveats.md.

See CHANGELOG.md for the full history.

License

MIT — see LICENSE.

EventBus Arch

Features

Installation

Quick start

Two modes: `fire` and `request`

Topics

Wildcards

`fire` through a wildcard

Model bus vs common bus

Isolates

Layers

Finite state machine

Scope and Command

Documentation

Version 3.0.0 changes

Version 2 changes

Version 2.3.0 changes

License

Libraries

event_bus_arch package

EventBus Arch

Features

Installation

Quick start

Two modes: fire and request

Topics

Wildcards

fire through a wildcard

Model bus vs common bus

Isolates

Layers

Finite state machine

Scope and Command

Documentation

Version 3.0.0 changes

Version 2 changes

Version 2.3.0 changes

License

Libraries

event_bus_arch package

Two modes: `fire` and `request`

`fire` through a wildcard