setInterval should return delta time

[benbrimeyer]

Right now I have to calculate this manually. Would be nice to have the dt returned to the passed function in some way instead.

[gtraines]

I think he means something like:

	local startTime = time()
	return {
		getDeltaTime = function()
			return time() - startTime
		end,
		clear = function()
			timeout:clear()
		end
	}

Of course, he could be referring to the Delta timezone if he's a pilot or affiliated with NATO.

• Edit to add: Actually, now that I think of it, he's probably wanting to know the delta time since the last interval.

	local timeout
	local callTimeout
	local lastCallTime
	local function handleTimeout()
		lastCallTime = time()
		callTimeout()
		fn(timeout)
	end
	callTimeout = function()
		timeout = Functions.setTimeout(handleTimeout, intervalInSeconds)
	end
	if delayInSeconds ~= nil then
		timeout = Functions.setTimeout(handleTimeout, delayInSeconds)
	else
		callTimeout()
	end
	
	return {
		getDeltaTime = function()
			return time() - lastCallTime
		end,
		clear = function()
			timeout:clear()
		end
	}

[benbrimeyer]

@gtraines is spot on in their edit.

Since the interval parameter is not a fixed interval, the actual time between function invocations is entirely at the mercy of the lua task scheduler.

This means even if I set the interval to something like 0.1s, the task scheduler may actually invoke the function passed to it every 0.1111 seconds, or under heavy stress quite a bit more than 0.1s. This is because passing the interval is sending the instruction as "wait at least 0.1s", not exactly 0.1s.

The delta time between function invocations is important for writing frame rate independent code.

[gtraines]

@benbrimeyer that does make sense. @Voperak what do you think, how does that fit with what you intended for the function?

[Hugheth]

I was also considering passing the delta through as an argument. As you say there's two ways to define setInterval delta, from before the handler is called or after (consider in Lua particular it might yield)

Methods for the delta could make these differences explicit, so I think they're quite a neat way to solve this.