event-emitters


Typesafe event emitters.

EventEmitter<T>

Similar to node's EventEmitter but:

- Only emits a single event of type T.
- Throw an exception when a client registers the same listener more than once.
- Throw an exception when a client tries to remove a listener that is not listening.
- Support async iteration.

``typescript
const emitter = new EventEmitter()
const listener = (n: number): void => {
console.log(n)
}

emitter.subscribe(listener)
emitter.emit(42)

try {
emitter.subscribe(listener)
} catch (e) {
console.log('same listener')
}

emitter.unsubscribe(listener)
try {
emitter.unsubscribe(() => {})
} catch (e) {
console.log('listener not found')
}
`

The above will log:

`
42
same listener
listener not found
`

`typescript
const emitter = new EventEmitter()

async function logEmitterValues(emitter: EventEmitter) {
for await (const val of emitter) {
console.log(val)
}
}

logEmitterValues(emitter)

emitter.emit(42)
emitter.emit(43)
`

The above will log:

`
42
43
`

`typescript
const emitter = new EventEmitter()
const listener = emitter.subscribe((val: number) => {
console.log(val)
})
emitter.unsubscribe(listener)
`

subscribe returns the callback passed to it which can be useful for unsubscribing later.

EventEmitterWithCurrent<T>

Provides the same API as EventEmitter but:

- Is initialized with the current message.
- Emits the current message to each listener as soon as it subscribes.

`typescript
const emitter = new EventEmitterWithCurrent(42)
emitter.subscribe((n: number): void => {
console.log(n)
})
emitter.emit(43)
`

The above will log:

`
42
43
`

EventEmitterWithOptionalCurrent<T>

Provides the same API as EventEmitterWithCurrent but:

- Can optionally be initialized with the current message.
- Emits the current message to each listener as soon as it subscribes only when the current message is available.

`typescript
const emitter = new EventEmitterWithCurrent()
emitter.subscribe((n: number): void => {
console.log(n)
})
emitter.emit(43)
`

The above will log:

`
43
`

QueueingEventEmitter<T>

Provides the same API as EventEmitter but:

- Queues messages when there are no subscribers.
- Delivers queued message to the first subscriber, then drains the queue.

`typescript
const emitter = new EventEmitterWithCurrent(42)
emitter.emit(44)
emitter.emit(45)
emitter.subscribe((n: number): void => {
console.log(n)
})
emitter.emit(46)
`

The above will log:

`
44
45
46
``