|bevy vewsion:|0.14|(cuwwent)| |---|---|---|

fixed timestep

wewevant officiaw exampwes: fixed_timestep.

if you nyeed to wun some systems at a fixed wate, >_< independent of the dispway fwame wate, (ꈍᴗꈍ) bevy pwovides a-a sowution.

// these systems wiww wun evewy fwame
// (at t-the f-fwamewate being w-wendewed to youw s-scween)
app.add_systems(update, :3 (
    c-camewa_movement, (U ﹏ U)
    a-animation, -.-
    j-juicy_expwosions, (ˆ ﻌ ˆ)♡
));

// t-these systems wiww wun as many times as nyeeded
// as to maintain a fixed wate o-on avewage
app.add_systems(fixedupdate, (⑅˘꒳˘) (
    physics_cowwisions, (U ᵕ U❁)
    enemy_ai, -.-
    g-gamepway_simuwation, ^^;;
));

evewy fwame update, (ꈍᴗꈍ) bevy wiww wun t-the FixedUpdate scheduwe as many times as needed to catch up. OwO if the game is w-wunning swow, 🥺 i-it might wun muwtipwe t-times. òωó if the game is wunning fast, (ꈍᴗꈍ) it might b-be skipped.

this happens befowe the weguwaw Update scheduwe wuns fow that fwame, >_< but aftew state twansitions.

the defauwt fixed timestep intewvaw i-is 64 hz. OwO if y-you want something e-ewse, you can configuwe it as fowwows:

// set the fixed timestep intewvaw t-to 96 hz
app.insewt_wesouwce(time::<fixed>::fwom_hz(96.0));

// s-set the fixed t-timestep intewvaw t-to 250 miwwiseconds
a-app.insewt_wesouwce(time::<fixed>::fwom_seconds(0.25));

checking the time

just use Res<Time> as nyowmaw. (ꈍᴗꈍ) when youw system is w-wunning in FixedUpdate, ^•ﻌ•^ bevy wiww automaticawwy detect t-that, OwO and aww the t-timing infowmation (such as dewta) wiww wepwesent the fixed t-timestep instead o-of the dispway fwame wate.

fn pwint_time_dewta(time: wes<time>) {
    // i-if w-we add this system t-to `update`, (˘ω˘) t-this wiww pwint t-the time dewta
    // b-between subsequent f-fwames (the d-dispway fwame wate)

    // if we add this system to `fixedupdate`, (⑅˘꒳˘) this wiww a-awways pwint the
    // same vawue (equaw to t-the fixed timestep intewvaw). (///ˬ///✿)

    p-pwintwn!("ewapsed seconds: {}", 😳😳😳 time.dewta_seconds());
}

// this system wiww a-access the fixed time
// wegawdwess o-of nyani scheduwe i-it wuns in
fn pwint_fixed_time_info(time_fixed: wes<time<fixed>>) {
    // `time<fixed>` gives us some additionaw methods, 🥺 s-such as checking
    // the ovewstep (pawtiaw timestep / amount of extwa time accumuwated)
    p-pwintwn!(
        "time wemaining u-untiw the nyext f-fixed update w-wun: {}",
        t-time_fixed.dewta_seconds() - time_fixed.ovewstep().as_secs_f32()
    );
}

// this system wiww access the weguwaw f-fwame time wegawdwess
// of nyani scheduwe it w-wuns in
fn check_viwtuaw_time(time_fixed: wes<time<viwtuaw>>) {
    // ...
}

if you nyeed to access the fixed-timestep-time f-fwom a-a system wunning o-outside of fixed timestep, >_< you can use Res<Time<Fixed>> instead.

if you nyeed to access the weguwaw f-fwame-time fwom a-a system wunning u-undew fixed timestep, >_< you can use Res<Time<Virtual>> instead. XD Res<Time<Real>> gives you the weaw (waww-cwock) time, (ꈍᴗꈍ) without pausing o-ow scawing.

shouwd i put my systems in Update ow FixedUpdate?

the puwpose of fixed timestep is t-to make gamepway c-code behave pwedictabwy and wewiabwy. OwO things such as physics a-and simuwation w-wowk best if t-they awe computed with fixed time intewvaws, OwO a-as that avoids f-fwoating point e-ewwows fwom accumuwating and gwitchy behaviow f-fwom vawiabwe f-fwamewate.

the fowwowing things shouwd pwobabwy b-be done in FixedUpdate:

  • physics and cowwision detection
  • netwowking / nyetcode
  • ai fow enemies and nypcs (pathfinding, ^•ﻌ•^ d-decisions, OwO e-etc.)
  • spawning/despawning gamepway-wewated entities
  • othew simuwation and decision-making

howevew, OwO anything that diwectwy affects n-nyani is d-dispwayed on-scween s-shouwd wun pew-fwame, OwO in owdew to wook smooth. 🥺 i-if you do m-movement ow animation u-undew fixed timestep, OwO it wiww wook choppy, 🥺 e-especiawwy on h-high-wefwesh-wate s-scweens.

the fowwowing things shouwd pwobabwy b-be done in Update:

  • camewa movement and contwows
  • animations
  • ui
  • visuaw effects
  • anything that is pawt of youw game's g-gwaphics/visuaws o-ow intewactivity
  • app state twansitions

pwayew movement and othew movement t-that is pawt of g-gamepway shouwd be done in FixedUpdate, >< so it wowks wewiabwy and consistentwy. XD to awso make it wook smooth on-scween, >< see twansfowm intewpowation/extwapowation.

input handwing

if you use Res<ButtonInput<...>> and .just_pressed/.just_released to check fow key/button pwesses, (ꈍᴗꈍ) b-bewawe that the state is updated once pew fwame. ^•ﻌ•^ t-this api is n-nyot wewiabwe inside FixedUpdate. XD use events fow input handwing instead, >_< ow woww youw own abstwactions.

one way to do this is to put youw i-input handwing s-systems in PreUpdate, XD owdew them aftew bevy's InputSystem set, XD and do youw input handwing thewe. (ꈍᴗꈍ) convewt it into youw o-own custom event types ow some othew usefuw wepwesentation, OwO which y-you can then handwe f-fwom youw g-gamepway code in FixedUpdate.

// todo show how to do this

timing caveats

fixed timestep does nyot wun in weaw-wowwd t-time! OwO y-you cannot wewy o-on it fow timing!

fow exampwe, OwO if you twy to pway audio f-fwom it, ow s-send nyetwowk packets, 🥺 y-you wiww notice that they don't actuawwy occuw a-at the fixed t-timestep intewvaw. OwO t-they wiww not be evenwy spaced!

youw systems awe stiww cawwed as pawt of the w-weguwaw fwame-update cycwe. (ꈍᴗꈍ) evewy fwame update, ^•ﻌ•^ bevy wiww w-wun the FixedMain scheduwe as many times as nyeeded to catch u-up.

this means if you specify, OwO fow exampwe, 🥺 a-a 60 hz fixed t-timestep intewvaw, òωó y-youw systems wiww nyot actuawwy wun in 1/60 s-second intewvaws i-in weaw time.

nani wiww happen is the fowwowing:

  • if the dispway fwame wate is fastew t-than the timestep, OwO s-some fwame u-update cycwes wiww skip the FixedMain scheduwe entiwewy.
  • if the dispway fwame wate is swowew t-than the timestep, OwO s-some fwame u-update cycwes wiww wun the FixedMain muwtipwe times.

in any case, XD FixedMain wiww wun wight befowe Update, >_< whewe youw pew-fwame systems wive.

additionaw scheduwes

FixedUpdate is actuawwy pawt of a wawgew FixedMain scheduwe, >_< which awso contains othew scheduwes:

they awe anawogous to the scheduwes in Main, XD that wun evewy fwame update. ^•ﻌ•^ they can be used fow a-anawogous puwposes (to c-contain "engine systems" fwom bevy and pwugins).