bevy_time/fixed.rs
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use bevy_app::FixedMain;
use bevy_ecs::world::World;
#[cfg(feature = "bevy_reflect")]
use bevy_reflect::Reflect;
use bevy_utils::Duration;
use crate::{time::Time, virt::Virtual};
/// The fixed timestep game clock following virtual time.
///
/// A specialization of the [`Time`] structure. **For method documentation, see
/// [`Time<Fixed>#impl-Time<Fixed>`].**
///
/// It is automatically inserted as a resource by
/// [`TimePlugin`](crate::TimePlugin) and updated based on
/// [`Time<Virtual>`](Virtual). The fixed clock is automatically set as the
/// generic [`Time`] resource during [`FixedUpdate`](bevy_app::FixedUpdate)
/// schedule processing.
///
/// The fixed timestep clock advances in fixed-size increments, which is
/// extremely useful for writing logic (like physics) that should have
/// consistent behavior, regardless of framerate.
///
/// The default [`timestep()`](Time::timestep) is 64 hertz, or 15625
/// microseconds. This value was chosen because using 60 hertz has the potential
/// for a pathological interaction with the monitor refresh rate where the game
/// alternates between running two fixed timesteps and zero fixed timesteps per
/// frame (for example when running two fixed timesteps takes longer than a
/// frame). Additionally, the value is a power of two which losslessly converts
/// into [`f32`] and [`f64`].
///
/// To run a system on a fixed timestep, add it to one of the [`FixedMain`]
/// schedules, most commonly [`FixedUpdate`](bevy_app::FixedUpdate).
///
/// This schedule is run a number of times between
/// [`PreUpdate`](bevy_app::PreUpdate) and [`Update`](bevy_app::Update)
/// according to the accumulated [`overstep()`](Time::overstep) time divided by
/// the [`timestep()`](Time::timestep). This means the schedule may run 0, 1 or
/// more times during a single update (which typically corresponds to a rendered
/// frame).
///
/// `Time<Fixed>` and the generic [`Time`] resource will report a
/// [`delta()`](Time::delta) equal to [`timestep()`](Time::timestep) and always
/// grow [`elapsed()`](Time::elapsed) by one [`timestep()`](Time::timestep) per
/// iteration.
///
/// The fixed timestep clock follows the [`Time<Virtual>`](Virtual) clock, which
/// means it is affected by [`pause()`](Time::pause),
/// [`set_relative_speed()`](Time::set_relative_speed) and
/// [`set_max_delta()`](Time::set_max_delta) from virtual time. If the virtual
/// clock is paused, the [`FixedUpdate`](bevy_app::FixedUpdate) schedule will
/// not run. It is guaranteed that the [`elapsed()`](Time::elapsed) time in
/// `Time<Fixed>` is always between the previous `elapsed()` and the current
/// `elapsed()` value in `Time<Virtual>`, so the values are compatible.
///
/// Changing the timestep size while the game is running should not normally be
/// done, as having a regular interval is the point of this schedule, but it may
/// be necessary for effects like "bullet-time" if the normal granularity of the
/// fixed timestep is too big for the slowed down time. In this case,
/// [`set_timestep()`](Time::set_timestep) and be called to set a new value. The
/// new value will be used immediately for the next run of the
/// [`FixedUpdate`](bevy_app::FixedUpdate) schedule, meaning that it will affect
/// the [`delta()`](Time::delta) value for the very next
/// [`FixedUpdate`](bevy_app::FixedUpdate), even if it is still during the same
/// frame. Any [`overstep()`](Time::overstep) present in the accumulator will be
/// processed according to the new [`timestep()`](Time::timestep) value.
#[derive(Debug, Copy, Clone)]
#[cfg_attr(feature = "bevy_reflect", derive(Reflect))]
pub struct Fixed {
timestep: Duration,
overstep: Duration,
}
impl Time<Fixed> {
/// Corresponds to 64 Hz.
const DEFAULT_TIMESTEP: Duration = Duration::from_micros(15625);
/// Return new fixed time clock with given timestep as [`Duration`]
///
/// # Panics
///
/// Panics if `timestep` is zero.
pub fn from_duration(timestep: Duration) -> Self {
let mut ret = Self::default();
ret.set_timestep(timestep);
ret
}
/// Return new fixed time clock with given timestep seconds as `f64`
///
/// # Panics
///
/// Panics if `seconds` is zero, negative or not finite.
pub fn from_seconds(seconds: f64) -> Self {
let mut ret = Self::default();
ret.set_timestep_seconds(seconds);
ret
}
/// Return new fixed time clock with given timestep frequency in Hertz (1/seconds)
///
/// # Panics
///
/// Panics if `hz` is zero, negative or not finite.
pub fn from_hz(hz: f64) -> Self {
let mut ret = Self::default();
ret.set_timestep_hz(hz);
ret
}
/// Returns the amount of virtual time that must pass before the fixed
/// timestep schedule is run again.
#[inline]
pub fn timestep(&self) -> Duration {
self.context().timestep
}
/// Sets the amount of virtual time that must pass before the fixed timestep
/// schedule is run again, as [`Duration`].
///
/// Takes effect immediately on the next run of the schedule, respecting
/// what is currently in [`Self::overstep`].
///
/// # Panics
///
/// Panics if `timestep` is zero.
#[inline]
pub fn set_timestep(&mut self, timestep: Duration) {
assert_ne!(
timestep,
Duration::ZERO,
"attempted to set fixed timestep to zero"
);
self.context_mut().timestep = timestep;
}
/// Sets the amount of virtual time that must pass before the fixed timestep
/// schedule is run again, as seconds.
///
/// Timestep is stored as a [`Duration`], which has fixed nanosecond
/// resolution and will be converted from the floating point number.
///
/// Takes effect immediately on the next run of the schedule, respecting
/// what is currently in [`Self::overstep`].
///
/// # Panics
///
/// Panics if `seconds` is zero, negative or not finite.
#[inline]
pub fn set_timestep_seconds(&mut self, seconds: f64) {
assert!(
seconds.is_sign_positive(),
"seconds less than or equal to zero"
);
assert!(seconds.is_finite(), "seconds is infinite");
self.set_timestep(Duration::from_secs_f64(seconds));
}
/// Sets the amount of virtual time that must pass before the fixed timestep
/// schedule is run again, as frequency.
///
/// The timestep value is set to `1 / hz`, converted to a [`Duration`] which
/// has fixed nanosecond resolution.
///
/// Takes effect immediately on the next run of the schedule, respecting
/// what is currently in [`Self::overstep`].
///
/// # Panics
///
/// Panics if `hz` is zero, negative or not finite.
#[inline]
pub fn set_timestep_hz(&mut self, hz: f64) {
assert!(hz.is_sign_positive(), "Hz less than or equal to zero");
assert!(hz.is_finite(), "Hz is infinite");
self.set_timestep_seconds(1.0 / hz);
}
/// Returns the amount of overstep time accumulated toward new steps, as
/// [`Duration`].
#[inline]
pub fn overstep(&self) -> Duration {
self.context().overstep
}
/// Discard a part of the overstep amount.
///
/// If `discard` is higher than overstep, the overstep becomes zero.
#[inline]
pub fn discard_overstep(&mut self, discard: Duration) {
let context = self.context_mut();
context.overstep = context.overstep.saturating_sub(discard);
}
/// Returns the amount of overstep time accumulated toward new steps, as an
/// [`f32`] fraction of the timestep.
#[inline]
pub fn overstep_fraction(&self) -> f32 {
self.context().overstep.as_secs_f32() / self.context().timestep.as_secs_f32()
}
/// Returns the amount of overstep time accumulated toward new steps, as an
/// [`f64`] fraction of the timestep.
#[inline]
pub fn overstep_fraction_f64(&self) -> f64 {
self.context().overstep.as_secs_f64() / self.context().timestep.as_secs_f64()
}
fn accumulate(&mut self, delta: Duration) {
self.context_mut().overstep += delta;
}
fn expend(&mut self) -> bool {
let timestep = self.timestep();
if let Some(new_value) = self.context_mut().overstep.checked_sub(timestep) {
// reduce accumulated and increase elapsed by period
self.context_mut().overstep = new_value;
self.advance_by(timestep);
true
} else {
// no more periods left in accumulated
false
}
}
}
impl Default for Fixed {
fn default() -> Self {
Self {
timestep: Time::<Fixed>::DEFAULT_TIMESTEP,
overstep: Duration::ZERO,
}
}
}
/// Runs [`FixedMain`] zero or more times based on delta of
/// [`Time<Virtual>`](Virtual) and [`Time::overstep`]
pub fn run_fixed_main_schedule(world: &mut World) {
let delta = world.resource::<Time<Virtual>>().delta();
world.resource_mut::<Time<Fixed>>().accumulate(delta);
// Run the schedule until we run out of accumulated time
let _ = world.try_schedule_scope(FixedMain, |world, schedule| {
while world.resource_mut::<Time<Fixed>>().expend() {
*world.resource_mut::<Time>() = world.resource::<Time<Fixed>>().as_generic();
schedule.run(world);
}
});
*world.resource_mut::<Time>() = world.resource::<Time<Virtual>>().as_generic();
}
#[cfg(test)]
mod test {
use super::*;
#[test]
fn test_set_timestep() {
let mut time = Time::<Fixed>::default();
assert_eq!(time.timestep(), Time::<Fixed>::DEFAULT_TIMESTEP);
time.set_timestep(Duration::from_millis(500));
assert_eq!(time.timestep(), Duration::from_millis(500));
time.set_timestep_seconds(0.25);
assert_eq!(time.timestep(), Duration::from_millis(250));
time.set_timestep_hz(8.0);
assert_eq!(time.timestep(), Duration::from_millis(125));
}
#[test]
fn test_expend() {
let mut time = Time::<Fixed>::from_seconds(2.0);
assert_eq!(time.delta(), Duration::ZERO);
assert_eq!(time.elapsed(), Duration::ZERO);
time.accumulate(Duration::from_secs(1));
assert_eq!(time.delta(), Duration::ZERO);
assert_eq!(time.elapsed(), Duration::ZERO);
assert_eq!(time.overstep(), Duration::from_secs(1));
assert_eq!(time.overstep_fraction(), 0.5);
assert_eq!(time.overstep_fraction_f64(), 0.5);
assert!(!time.expend()); // false
assert_eq!(time.delta(), Duration::ZERO);
assert_eq!(time.elapsed(), Duration::ZERO);
assert_eq!(time.overstep(), Duration::from_secs(1));
assert_eq!(time.overstep_fraction(), 0.5);
assert_eq!(time.overstep_fraction_f64(), 0.5);
time.accumulate(Duration::from_secs(1));
assert_eq!(time.delta(), Duration::ZERO);
assert_eq!(time.elapsed(), Duration::ZERO);
assert_eq!(time.overstep(), Duration::from_secs(2));
assert_eq!(time.overstep_fraction(), 1.0);
assert_eq!(time.overstep_fraction_f64(), 1.0);
assert!(time.expend()); // true
assert_eq!(time.delta(), Duration::from_secs(2));
assert_eq!(time.elapsed(), Duration::from_secs(2));
assert_eq!(time.overstep(), Duration::ZERO);
assert_eq!(time.overstep_fraction(), 0.0);
assert_eq!(time.overstep_fraction_f64(), 0.0);
assert!(!time.expend()); // false
assert_eq!(time.delta(), Duration::from_secs(2));
assert_eq!(time.elapsed(), Duration::from_secs(2));
assert_eq!(time.overstep(), Duration::ZERO);
assert_eq!(time.overstep_fraction(), 0.0);
assert_eq!(time.overstep_fraction_f64(), 0.0);
time.accumulate(Duration::from_secs(1));
assert_eq!(time.delta(), Duration::from_secs(2));
assert_eq!(time.elapsed(), Duration::from_secs(2));
assert_eq!(time.overstep(), Duration::from_secs(1));
assert_eq!(time.overstep_fraction(), 0.5);
assert_eq!(time.overstep_fraction_f64(), 0.5);
assert!(!time.expend()); // false
assert_eq!(time.delta(), Duration::from_secs(2));
assert_eq!(time.elapsed(), Duration::from_secs(2));
assert_eq!(time.overstep(), Duration::from_secs(1));
assert_eq!(time.overstep_fraction(), 0.5);
assert_eq!(time.overstep_fraction_f64(), 0.5);
}
#[test]
fn test_expend_multiple() {
let mut time = Time::<Fixed>::from_seconds(2.0);
time.accumulate(Duration::from_secs(7));
assert_eq!(time.overstep(), Duration::from_secs(7));
assert!(time.expend()); // true
assert_eq!(time.elapsed(), Duration::from_secs(2));
assert_eq!(time.overstep(), Duration::from_secs(5));
assert!(time.expend()); // true
assert_eq!(time.elapsed(), Duration::from_secs(4));
assert_eq!(time.overstep(), Duration::from_secs(3));
assert!(time.expend()); // true
assert_eq!(time.elapsed(), Duration::from_secs(6));
assert_eq!(time.overstep(), Duration::from_secs(1));
assert!(!time.expend()); // false
assert_eq!(time.elapsed(), Duration::from_secs(6));
assert_eq!(time.overstep(), Duration::from_secs(1));
}
}