Why does withFrameNanos return inconsistent values...

# coroutines

[myanmarking]

Why does withFrameNanos return inconsistent values (details in thread)

[myanmarking]

I have this game loop:

suspend fun loop(
    speed: GameSpeed,
    action: (motion: Float) -> Unit
) {
    withFrameNanos { time ->
        val sinceLast = time - lastFrame
        val motion = speed.value / (sinceLast)
        lastFrame = time
        action(motion)
    }
}

whenever i restart this loop, the value produced by ‘motion’ is very different from the previous ones

[myanmarking]

i cannot figure out why

[kevin.cianfarini]

Is this a compose thing? withFrameNanos is also a function on monotonic frame clocks.

[myanmarking]

not compose. I’m building a game loop with GraphicsSurface

[kevin.cianfarini]

What is your definition of withFrameNanos then? Or a link to documentation if you didn’t write it?

[myanmarking]

i cannot get the function to return relatively stable inputs

[myanmarking]

yes, its compose. sorry

[kevin.cianfarini]

Which platform are you running on?

[myanmarking]

android

[kevin.cianfarini]

Time based things really aren’t guaranteed to be very precise

[kevin.cianfarini]

just in general

[kevin.cianfarini]

There might be more specific, better time synchronization with compose, but I am unsure.

[myanmarking]

ya. i might need to tune this impl to a specific fps, so i get constant output

[kevin.cianfarini]

For example, here’s an impl of monotonic frame clock that I wrote for mac os. You’re dependent on when the platform says there should be another frame rendered. https://github.com/cashapp/molecule/blob/trunk/molecule-runtime/src/quartzCoreMain/kotlin/app/cash/molecule/DisplayLinkClock.kt

[kevin.cianfarini]

I am unsure of the android frame clock.

[Zach Klippenstein (he/him) [MOD]]

There are no guarantees about the clock values on compose’s side. We just pass through whatever we get from Choreographer. E.g, If you’re on a device that dynamically adjusts refresh rate, they could change.