Fixes https://github.com/bevyengine/bevy/issues/10974
# Objective
Duplicate the ordering logic of the `Main` schedule into the `FixedMain`
schedule.
---
## Changelog
- `FixedUpdate` is no longer the main schedule ran in
`RunFixedUpdateLoop`, `FixedMain` has replaced this and has a similar
structure to `Main`.
## Migration Guide
- Usage of `RunFixedUpdateLoop` should be renamed to `RunFixedMainLoop`.
# Objective
There is no easy way to discard some amount for `Time<Fixed>`'s
overstep. This can be useful for online games when the client receives
information about a tick (which happens when you get a FPS drop or the
ping changes for example) it has not yet processed, it can discard
overstep equal to the number of ticks it can jump ahead.
Currently the workaround would be to create a new `Time<Fixed>` copy the
old timestep, advance it by the overstep amount that would remain after
subtracting the discarded amount, and using `.context_mut()` to
overwrite the old context with the new one. If you overwrite the whole
`Time<Fixed>` or forget to copy over the timestep you can introduce
undesirable side effects.
## Solution
Introduce a `discard_overstep` method, which discards the provided
amount of overstep. It uses satuarting_sub to avoid errors (negative
`Duration`s do not exist).
---
## Changelog
- Added `discard_overstep` function to `Time<Fixed>`
---------
Co-authored-by: Alice Cecile <alice.i.cecile@gmail.com>
# Objective
This is similar to #10439.
`Time::<Fixed>::overstep_percentage()` and
`Time::<Fixed>::overstep_percentage_f64()` returns values from 0.0 to
1.0, but their names use the word "percentage". These function names
make it easy to misunderstand that they return values from 0.0 to 100.0.
To avoid confusion, these functions should be renamed to
"`overstep_fraction(_f64)`".
## Solution
Rename them.
---
## Changelog
### Changed
- Renamed `Time::<Fixed>::overstep_percentage()` to
`Time::<Fixed>::overstep_fraction()`
- Renamed `Time::<Fixed>::overstep_percentage_f64()` to
`Time::<Fixed>::overstep_fraction_f64()`
## Migration Guide
- `Time::<Fixed>::overstep_percentage()` has been renamed to
`Time::<Fixed>::overstep_fraction()`
- `Time::<Fixed>::overstep_percentage_f64()` has been renamed to
`Time::<Fixed>::overstep_fraction_f64()`
# Objective
Current `FixedTime` and `Time` have several problems. This pull aims to
fix many of them at once.
- If there is a longer pause between app updates, time will jump forward
a lot at once and fixed time will iterate on `FixedUpdate` for a large
number of steps. If the pause is merely seconds, then this will just
mean jerkiness and possible unexpected behaviour in gameplay. If the
pause is hours/days as with OS suspend, the game will appear to freeze
until it has caught up with real time.
- If calculating a fixed step takes longer than specified fixed step
period, the game will enter a death spiral where rendering each frame
takes longer and longer due to more and more fixed step updates being
run per frame and the game appears to freeze.
- There is no way to see current fixed step elapsed time inside fixed
steps. In order to track this, the game designer needs to add a custom
system inside `FixedUpdate` that calculates elapsed or step count in a
resource.
- Access to delta time inside fixed step is `FixedStep::period` rather
than `Time::delta`. This, coupled with the issue that `Time::elapsed`
isn't available at all for fixed steps, makes it that time requiring
systems are either implemented to be run in `FixedUpdate` or `Update`,
but rarely work in both.
- Fixes#8800
- Fixes#8543
- Fixes#7439
- Fixes#5692
## Solution
- Create a generic `Time<T>` clock that has no processing logic but
which can be instantiated for multiple usages. This is also exposed for
users to add custom clocks.
- Create three standard clocks, `Time<Real>`, `Time<Virtual>` and
`Time<Fixed>`, all of which contain their individual logic.
- Create one "default" clock, which is just `Time` (or `Time<()>`),
which will be overwritten from `Time<Virtual>` on each update, and
`Time<Fixed>` inside `FixedUpdate` schedule. This way systems that do
not care specifically which time they track can work both in `Update`
and `FixedUpdate` without changes and the behaviour is intuitive.
- Add `max_delta` to virtual time update, which limits how much can be
added to virtual time by a single update. This fixes both the behaviour
after a long freeze, and also the death spiral by limiting how many
fixed timestep iterations there can be per update. Possible future work
could be adding `max_accumulator` to add a sort of "leaky bucket" time
processing to possibly smooth out jumps in time while keeping frame rate
stable.
- Many minor tweaks and clarifications to the time functions and their
documentation.
## Changelog
- `Time::raw_delta()`, `Time::raw_elapsed()` and related methods are
moved to `Time<Real>::delta()` and `Time<Real>::elapsed()` and now match
`Time` API
- `FixedTime` is now `Time<Fixed>` and matches `Time` API.
- `Time<Fixed>` default timestep is now 64 Hz, or 15625 microseconds.
- `Time` inside `FixedUpdate` now reflects fixed timestep time, making
systems portable between `Update ` and `FixedUpdate`.
- `Time::pause()`, `Time::set_relative_speed()` and related methods must
now be called as `Time<Virtual>::pause()` etc.
- There is a new `max_delta` setting in `Time<Virtual>` that limits how
much the clock can jump by a single update. The default value is 0.25
seconds.
- Removed `on_fixed_timer()` condition as `on_timer()` does the right
thing inside `FixedUpdate` now.
## Migration Guide
- Change all `Res<Time>` instances that access `raw_delta()`,
`raw_elapsed()` and related methods to `Res<Time<Real>>` and `delta()`,
`elapsed()`, etc.
- Change access to `period` from `Res<FixedTime>` to `Res<Time<Fixed>>`
and use `delta()`.
- The default timestep has been changed from 60 Hz to 64 Hz. If you wish
to restore the old behaviour, use
`app.insert_resource(Time::<Fixed>::from_hz(60.0))`.
- Change `app.insert_resource(FixedTime::new(duration))` to
`app.insert_resource(Time::<Fixed>::from_duration(duration))`
- Change `app.insert_resource(FixedTime::new_from_secs(secs))` to
`app.insert_resource(Time::<Fixed>::from_seconds(secs))`
- Change `system.on_fixed_timer(duration)` to
`system.on_timer(duration)`. Timers in systems placed in `FixedUpdate`
schedule automatically use the fixed time clock.
- Change `ResMut<Time>` calls to `pause()`, `is_paused()`,
`set_relative_speed()` and related methods to `ResMut<Time<Virtual>>`
calls. The API is the same, with the exception that `relative_speed()`
will return the actual last ste relative speed, while
`effective_relative_speed()` returns 0.0 if the time is paused and
corresponds to the speed that was set when the update for the current
frame started.
## Todo
- [x] Update pull name and description
- [x] Top level documentation on usage
- [x] Fix examples
- [x] Decide on default `max_delta` value
- [x] Decide naming of the three clocks: is `Real`, `Virtual`, `Fixed`
good?
- [x] Decide if the three clock inner structures should be in prelude
- [x] Decide on best way to configure values at startup: is manually
inserting a new clock instance okay, or should there be config struct
separately?
- [x] Fix links in docs
- [x] Decide what should be public and what not
- [x] Decide how `wrap_period` should be handled when it is changed
- [x] ~~Add toggles to disable setting the clock as default?~~ No,
separate pull if needed.
- [x] Add tests
- [x] Reformat, ensure adheres to conventions etc.
- [x] Build documentation and see that it looks correct
## Contributors
Huge thanks to @alice-i-cecile and @maniwani while building this pull.
It was a shared effort!
---------
Co-authored-by: Alice Cecile <alice.i.cecile@gmail.com>
Co-authored-by: Cameron <51241057+maniwani@users.noreply.github.com>
Co-authored-by: Jerome Humbert <djeedai@gmail.com>
