mini-events

Simplified event stream and state management

Table of Contents

• Usage
• Timing guarantees
• variable
• asyncVariable
• Helper functions
  • filter
  • map
    • filterMap
  • merge
  • next
  • select
  • toAsync
  • fromAsyncIterable

Usage

Obtain an emit-subscribe pair using the event factory method

const [emit, subscribe] = event<[EventType]>()

Subscribe by passing a callback to the second function returned by event

const [emit, subscribe] = event<[EventType]>()
subscribe(e => ...)

Subscribe synchronously by passing true as the second argument. A synchronous handler will execute before emit would return. Use them sparingly, only attach very short handlers synchronously.

const [emit, subscribe] = event<[EventType]>()
subscribe(e => ..., true)

Emit an event by calling the first function returned by event

const [emit, subscribe] = event<[EventType]>()
emit(...)

Unsubscribe by calling the function returned by the subscribe call.

const [emit, subscribe] = event<[EventType]>()
const unsubscribe = subscribe(e => ...)
unsubscribe()

Multi-parameter calls

const [emit, subscribe] = event<[A, B]>()
const unsubscribe = subscribe((a, b) => ...)
emit(a, b)

Timing guarantees

• An event handler MAY receive events emitted before the subscribe call.
• An event handler MUST receive events emitted after the subscribe call.
• An event handler MUST NOT receive events emitted after the unsubscribe call.
• An event handler MAY NOT receive events emitted before the unsubscribe call.
• An event handler subscribed in a given microtask MUST receive all events that a handler unsubscribed on the same microtask would not receive.
• Flushing the microtask queue once MUST result in every event handler's completion.

variable

A little addition that I havve come to need quite often. changed is only fired if the new value doesn't === the previous value.

const [set, { get, changed }] = variable('default')
get() // ='default'
changed(console.log)
set('foo') // > 'foo', 'default'
get() // ='foo'

Same guarantees apply as with event

asyncVariable

Changing state with asynchronity. The API expects locking support

const [set, { get, changed }, lock] = asyncVariable('default')
await get() // ='default'
await set('foo')
await get() // ='foo'
const [release, value] = await lock() // resolve after the last call's release has been called 

Helper functions

filter

const [emit, subscribe] = event<[string]>()
const filtered = filter(subscribe, s => s.startsWith('.'))
emit('foo') // nothing
emit('.foo') // > '.foo'

map

const [emit, subscribe] = event<[string]>()
const mapped = map(subscribe, s => [Number.parseFloat(s)])
emit('0') // > 0
emit('0.5') // > 0.5
emit('meow') // > NaN

filterMap

Same as map except the function can return undefined to indicate that no event should be mapped. This functionality isn't included in map because accidentally returning undefined is a common JS mistake

merge

const [emit1, subscribe1] = event<[string]>()
const [emit2, subscribe2] = event<[string]>()
const merged = merge(subscribe1, subscribe2);
emit1('foo') // merged> 'foo'
emit2('bar') // merged> 'bar'

next

Take the next event from a stream

const [emit, subscribe] = event<[string]>()
(async function() {
  const [data] = await next(subscribe)
  console.log(data)
})()
emit("foo") // > "foo"

select

Derive a variable from another

const [set, original] = variable([1, 2])
const derived = select(original, t => t[1])
derived.get() // = 2
derived.changed(console.log)
set([3, 4]) // > 4
set([5, 4]) // nothing

toAsync

Compatibility method for passing synchronous variable abstractions to APIs that expect asynchronous ones. Note that the underlying variable is also used to store the semaphore value.

const [set, { get, changed }, lock] = toAsync(variable, 'foo')

fromAsyncIterable

Converts an AsyncIterable into an event

const changes = fromAsyncIterable(fs.watch('./foo.txt'))