feat: Add `reactive.delay`

[schloerke]

Goal: I'd like to have a reactive event or calculation run after a set amount of time. I'd like for the reactive graph to not be blocked while waiting.

Related to debounce / throttle issue: #564 . It could be argued that a single use of delay is throttle and throttle would de a better job at cancelling previous jobs to deter concurrent delays at the same time.

While debounce and throttle both have cancellation policies, delay could be a third option where there is no cancellation policy... only a delay between the trigger and execution.

(Either way, this is more motivation for any of these methods to be added!)

---

Naive implementation:

trigger = reactive.val(False)
result = reactive.val()

# Start
@reactive.effect
def _():
	trigger.set(True)

# Update
@reactive.effect
@reactive.event(triggger)
async def _():
	await asyncio.sleep(4)
    result.set(new_value)

Extended task impl:

result = reactive.value()

# Sleep for `k` seconds
@reactive.extended_task
async def trigger(delay: int = 1) -> int:
    await asyncio.sleep(delay)
    return delay

# Start
@reactive.effect
def _():
    trigger(4)

# Update only if `trigger` has completed
@reactive.effect
async def _():
    trigger.result()

	result.set(new_value)

---

Goal API

@reactive.effect
@reactive.event(triggger, delay = 4)
def _():
    result.set(new_value)

@reactive.calc
@reactive.event(triggger, delay = 4)
def result():
	return new_value

[gadenbuie]

Could you explain a bit more about why you'd want to delay the reactive evaluation?

[schloerke]

My current use case is "on modal close, wait 1 second, update chatlas user input".

After submitting the modal close, many other reactives could be run within the 1 second wait. If the UI isn't being constructed, then the wait is unnecessary and timing is bad.

[schloerke]

For my particular use case, it could be resolved with ui.Chat() setting client data input hidden values from the JavaScript.

Since ui.Chat() is an output, this value should be exposed.

[gadenbuie]

Yeah, I've learned to be very suspicious of any kind of magical delays, to me it almost always indicates a missing step in the reactive graph or a missing event client-side.