FSA Emitter

EventEmitter in FSA style.

When dealing with event, I find myself need to remember the name of the event, and the parameters each event has.

The person who defines the events and the person who consumes the events can be two different person.

This implicit knowledge coupling relies on communication and documentation that are prone to error, and are not convenient.

This library addresses this issue by emitting and consuming events using a standard format, and provides IDE type support so they can be consumed easily.

Best practice

One of the benefits of using events is decoupling. Here is one way to organize your code:

1// app.ts
2import { doWork } from './doWork'
3
4const emitter = new Emitter()
5
6doWork({ emitter })
7
8// doWork.ts
9import { createEvent, Emitter } from 'fsa-emitter'
10
11export const count = createEvent<number>('count')
12
13export function doWork({ emitter }) {
14  emitter.emit(count(1, undefined))
15}
16
17// in UI
18import { count } from './doWork'
19
20emitter.on(count, payload => {
21  console.log('payload is typed and is a number: ', payload)
22})

Installation

1npm install fsa-emitter

Emitter

Emitter uses FluxStandardAction as the standard event format.

Another key difference between Emitter and NodeJS EventEmitter is that Emitter will capture any error thrown in listener and send it to console.error(). This avoid any listener code throws error and break the event emitting logic.

To create event, use one of the helper methods below:

1import {
2  createEvent,
3  createScopedCreateEvent,
4  createEventAction,
5  createScopedCreateEventAction,
6  Emitter
7} from 'fsa-emitter'
8
9const count = createEvent<number>('count')
10
11// create scoped event
12const createMyModuleEvent = createScopedCreateEvent('myModule')
13// `scopedCount` will emit FSA with `type: 'myModule/count'`
14const scopedCount = createMyModuleEvent<number>('count')
15
16const add = createAction<
17  /* input */ { a: number, b: number },
18  /* payload */ number>('add', ({ a, b }) => emit => emit(a + b))
19
20// create scoped action
21const createMyModuleAction = createScopedCreateEventAction('myModule')
22// `scopedCountAction` will emit FSA with `type: 'myModule/add'`
23const scopedAdd = createMyModuleAction<
24  /* input */ { a: number, b: number },
25  /* payload */ number>('add', ({ a, b }) => emit => emit(a + b))
26
27const emitter = new Emitter()
28
29emitter.emit(count(1, undefined))
30emitter.emit(scopedCount(2, undefined))
31add(emitter, { a: 1, b: 2 }, undefined)
32scopedAdd(emitter, { a: 1, b: 2 }, undefined)

Note that due to https://github.com/Microsoft/TypeScript/issues/12400, you need to supply undefined at where they expect meta.

To consume the event, use one of the following methods:

1import { createEvent, Emitter } from 'fsa-emitter'
2
3const count = createEvent<number>('count')
4const emitter = new Emitter()
5
6emitter.addListener(count, value => console.info(value))
7emitter.on(count, value => console.info(value))
8
9// Will only listen to the event once
10emitter.once(count, value => console.info(value))
11
12// line up in queue to listen for the event once
13emitter.queue(count, value => console.info(value))
14
15// listen to all events
16emitter.onAny(fsa => console.info(fsa.type, fsa.payload))
17
18// listen to any event that does not have a listener
19emitter.onMissed(fsa => console.info(fsa.type, fsa.payload))

Command

Command is a simple command pattern that provides a simple way to manage dependencies.

1import { Command, createEvent, Emitter } from 'fsa-emitter'
2
3const count = createEvent<{ n: number }>('count')
4
5class CountCommand extends Command {
6  run(n: number) {
7    while (n) {
8      this.emitter.emit(count({ n }, undefined))
9      n--
10    }
11  }
12}
13
14const emitter = new Emitter()
15const cmd = new CountCommand({ emitter })
16cmd(10)

1import { Command, createEvent, Emitter } from 'fsa-emitter'
2
3const changeHeight = createEvent<{ height: number }>('changeHeight')
4const landed = createEvent('landed')
5
6class FlyCommand extends Command<{ fuel: number }> {
7  fuel: number
8  height = 0
9  run() {
10    while (this.fuel > 0) {
11      this.height += 100
12      this.fuel--
13      this.emitter.emit(changeHeight({ height: this.height }, undefined))
14    }
15    const gliding = setInterval(() => {
16      this.height -= 50
17      this.emitter.emit(changeHeight({ height: this.height }, undefined))
18      if (this.height <= 0) {
19        this.emitter.emit(landed(undefined, undefined))
20        clearInterval(gliding)
21      }
22    }, 100)
23  }
24}
25
26// `fuel` is in the constructor context
27const fly = new FlyCommand({ emitter: new Emitter(), fuel: 10 })
28fly.run()

TestEmitter

TestEmitter can be used as a drop in replacement as Emitter during test.

The difference between TestEmitter and Emitter is that TestEmitter will not capture errors thrown in listeners. This make it more suitable to use during testing so that you can detect any error thrown during the test.

Also, it provides some additional methods:

listenerCalled(event: TypedEvent | string): boolean

1import { TestEmitter, createEvent } from 'fsa-emitter'
2
3const emitter = new TestEmitter()
4const count = createEvent<number>('count')
5emitter.on(count, () => ({}))
6t.false(emitter.listenerCalled(count))
7
8emitter.emit(count(1, undefined))
9t.true(emitter.listenerCalled(count))
10t.true(emitter.listenerCalled(count.type))

allListenersCalled(): boolean

1import { TestEmitter, createEvent } from 'fsa-emitter'
2
3const emitter = new TestEmitter()
4const count = createEvent<number>('count')
5const bound = createEvent<number>('bound')
6
7emitter.on(count, () => ({}))
8emitter.on(bound, () => ({}))
9
10emitter.emit(count(1, undefined))
11
12t.false(emitter.allListenersCalled())
13
14emitter.emit(bound(1, undefined))
15
16t.true(emitter.allListenersCalled())

listenedTo(events: (TypedEvent | string)[] | { [k]: TypedEvent })

1import { TestEmitter, createEvent } from 'fsa-emitter'
2
3const emitter = new TestEmitter()
4const count = createEvent<number>('count')
5const bound = createEvent<number>('bound')
6
7emitter.on(count, () => ({}))
8
9emitter.listenedTo([count]) // true
10emitter.listenedTo({ count }) // true
11emitter.listenedTo(['count']) // true
12
13emitter.listenedTo([ bound ]) // false
14emitter.listenedTo({ bound }) // false
15emitter.listenedTo([ 'bound' ]) // false

setupCommandTest(Command, context?)

setupCommandTest() is a simple helper to create a TestEmitter for the command to run with. The same completion support is available as in the Command constructor.

1import { Command, createEvent, setupCommandTest } from 'fsa-emitter'
2
3const count = createEvent<{ n: number }>('count')
4
5class CountCommand extends Command {
6  ...
7}
8
9const { command, emitter } = setupCommandTest(CountCommand)
10
11class FlyCommand extends Command<{ fuel: number }> {
12  ...
13}
14
15// completion is available for `fuel`
16const { command, emitter } = setupCommandTest(FlyCommand, { fuel: 10 })

Contribute

1# after fork and clone
2npm install
3
4# begin making changes
5git checkout -b <branch>
6npm run watch
7
8# after making change(s)
9git commit -m "<commit message>"
10git push
11
12# create PR