Allow ordering variable timesteps around fixed timesteps (#14881)

# Objective

- Fixes #14873, see that issue for a whole lot of context

## Solution

- Add a blessed system set for this stuff. See [this Discord
discussion](https://discord.com/channels/691052431525675048/749335865876021248/1276262931327094908).

Note that the gizmo systems,
[LWIM](https://github.com/Leafwing-Studios/leafwing-input-manager/pull/522/files#diff-9b59ee4899ad0a5d008889ea89a124a7291316532e42f9f3d6ae842b906fb095R154)
and now a new plugin I'm working on are all already ordering against
`run_fixed_main_schedule`, so having a dedicated system set should be
more robust and hopefully also more discoverable.

---

## ~~Showcase~~

~~I can add a little video of a smooth camera later if this gets merged
:)~~
Apparently a release note is not needed, so I'll leave it out. See the
changes in the fixed timestep example for usage showcase and the video
in #14873 for a more or less accurate video of the effect (it does not
use the same solution though, so it is not quite the same)

## Migration Guide


[run_fixed_main_schedule](https://docs.rs/bevy/latest/bevy/time/fn.run_fixed_main_schedule.html)
is no longer public. If you used to order against it, use the new
dedicated `RunFixedMainLoopSystem` system set instead. You can replace
your usage of `run_fixed_main_schedule` one for one by
`RunFixedMainLoopSystem::FixedMainLoop`, but it is now more idiomatic to
place your systems in either
`RunFixedMainLoopSystem::BeforeFixedMainLoop` or
`RunFixedMainLoopSystem::AfterFixedMainLoop`

Old:
```rust
app.add_systems(
    RunFixedMainLoop,
    some_system.before(run_fixed_main_schedule)
);
```

New:
```rust
app.add_systems(
    RunFixedMainLoop,
    some_system.in_set(RunFixedMainLoopSystem::BeforeFixedMainLoop)
);
```

---------

Co-authored-by: Tau Gärtli <git@tau.garden>

crates/bevy_app/src/lib.rs

@@ -34,7 +34,8 @@ pub mod prelude {
         app::{App, AppExit},
         main_schedule::{
             First, FixedFirst, FixedLast, FixedPostUpdate, FixedPreUpdate, FixedUpdate, Last, Main,
-            PostStartup, PostUpdate, PreStartup, PreUpdate, SpawnScene, Startup, Update,
+            PostStartup, PostUpdate, PreStartup, PreUpdate, RunFixedMainLoop,
+            RunFixedMainLoopSystem, SpawnScene, Startup, Update,
         },
         sub_app::SubApp,
         Plugin, PluginGroup,

crates/bevy_app/src/main_schedule.rs

@@ -1,6 +1,9 @@
 use crate::{App, Plugin};
 use bevy_ecs::{
-    schedule::{ExecutorKind, InternedScheduleLabel, Schedule, ScheduleLabel},
+    schedule::{
+        ExecutorKind, InternedScheduleLabel, IntoSystemSetConfigs, Schedule, ScheduleLabel,
+        SystemSet,
+    },
     system::{Local, Resource},
     world::{Mut, World},
 };
@@ -75,6 +78,11 @@ pub struct First;
 pub struct PreUpdate;
 
 /// Runs the [`FixedMain`] schedule in a loop according until all relevant elapsed time has been "consumed".
+/// If you need to order your variable timestep systems
+/// before or after the fixed update logic, use the [`RunFixedMainLoopSystem`] system set.
+///
+/// Note that in contrast to most other Bevy schedules, systems added directly to
+/// [`RunFixedMainLoop`] will *not* be parallelized between each other.
 ///
 /// See the [`Main`] schedule for some details about how schedules are run.
 #[derive(ScheduleLabel, Clone, Debug, PartialEq, Eq, Hash)]
@@ -126,8 +134,8 @@ pub struct FixedLast;
 
 /// The schedule that contains systems which only run after a fixed period of time has elapsed.
 ///
-/// The exclusive `run_fixed_main_schedule` system runs this schedule.
-/// This is run by the [`RunFixedMainLoop`] schedule.
+/// This is run by the [`RunFixedMainLoop`] schedule. If you need to order your variable timestep systems
+/// before or after the fixed update logic, use the [`RunFixedMainLoopSystem`] system set.
 ///
 /// Frequency of execution is configured by inserting `Time<Fixed>` resource, 64 Hz by default.
 /// See [this example](https://github.com/bevyengine/bevy/blob/latest/examples/time/time.rs).
@@ -288,7 +296,16 @@ impl Plugin for MainSchedulePlugin {
             .init_resource::<MainScheduleOrder>()
             .init_resource::<FixedMainScheduleOrder>()
             .add_systems(Main, Main::run_main)
-            .add_systems(FixedMain, FixedMain::run_fixed_main);
+            .add_systems(FixedMain, FixedMain::run_fixed_main)
+            .configure_sets(
+                RunFixedMainLoop,
+                (
+                    RunFixedMainLoopSystem::BeforeFixedMainLoop,
+                    RunFixedMainLoopSystem::FixedMainLoop,
+                    RunFixedMainLoopSystem::AfterFixedMainLoop,
+                )
+                    .chain(),
+            );
 
         #[cfg(feature = "bevy_debug_stepping")]
         {
@@ -352,3 +369,96 @@ impl FixedMain {
         });
     }
 }
+
+/// Set enum for the systems that want to run inside [`RunFixedMainLoop`],
+/// but before or after the fixed update logic. Systems in this set
+/// will run exactly once per frame, regardless of the number of fixed updates.
+/// They will also run under a variable timestep.
+///
+/// This is useful for handling things that need to run every frame, but
+/// also need to be read by the fixed update logic. See the individual variants
+/// for examples of what kind of systems should be placed in each.
+///
+/// Note that in contrast to most other Bevy schedules, systems added directly to
+/// [`RunFixedMainLoop`] will *not* be parallelized between each other.
+#[derive(Debug, Hash, PartialEq, Eq, Copy, Clone, SystemSet)]
+pub enum RunFixedMainLoopSystem {
+    /// Runs before the fixed update logic.
+    ///
+    /// A good example of a system that fits here
+    /// is camera movement, which needs to be updated in a variable timestep,
+    /// as you want the camera to move with as much precision and updates as
+    /// the frame rate allows. A physics system that needs to read the camera
+    /// position and orientation, however, should run in the fixed update logic,
+    /// as it needs to be deterministic and run at a fixed rate for better stability.
+    /// Note that we are not placing the camera movement system in `Update`, as that
+    /// would mean that the physics system already ran at that point.
+    ///
+    /// # Example
+    /// ```
+    /// # use bevy_app::prelude::*;
+    /// # use bevy_ecs::prelude::*;
+    /// App::new()
+    ///   .add_systems(
+    ///     RunFixedMainLoop,
+    ///     update_camera_rotation.in_set(RunFixedMainLoopSystem::BeforeFixedMainLoop))
+    ///   .add_systems(FixedUpdate, update_physics);
+    ///
+    /// # fn update_camera_rotation() {}
+    /// # fn update_physics() {}
+    /// ```
+    BeforeFixedMainLoop,
+    /// Contains the fixed update logic.
+    /// Runs [`FixedMain`] zero or more times based on delta of
+    /// [`Time<Virtual>`] and [`Time::overstep`].
+    ///
+    /// Don't place systems here, use [`FixedUpdate`] and friends instead.
+    /// Use this system instead to order your systems to run specifically inbetween the fixed update logic and all
+    /// other systems that run in [`RunFixedMainLoopSystem::BeforeFixedMainLoop`] or [`RunFixedMainLoopSystem::AfterFixedMainLoop`].
+    ///
+    /// [`Time<Virtual>`]: https://docs.rs/bevy/latest/bevy/prelude/struct.Virtual.html
+    /// [`Time::overstep`]: https://docs.rs/bevy/latest/bevy/time/struct.Time.html#method.overstep
+    /// # Example
+    /// ```
+    /// # use bevy_app::prelude::*;
+    /// # use bevy_ecs::prelude::*;
+    /// App::new()
+    ///   .add_systems(FixedUpdate, update_physics)
+    ///   .add_systems(
+    ///     RunFixedMainLoop,
+    ///     (
+    ///       // This system will be called before all interpolation systems
+    ///       // that third-party plugins might add.
+    ///       prepare_for_interpolation
+    ///         .after(RunFixedMainLoopSystem::FixedMainLoop)
+    ///         .before(RunFixedMainLoopSystem::AfterFixedMainLoop),
+    ///     )
+    ///   );
+    ///
+    /// # fn prepare_for_interpolation() {}
+    /// # fn update_physics() {}
+    /// ```
+    FixedMainLoop,
+    /// Runs after the fixed update logic.
+    ///
+    /// A good example of a system that fits here
+    /// is a system that interpolates the transform of an entity between the last and current fixed update.
+    /// See the [fixed timestep example] for more details.
+    ///
+    /// [fixed timestep example]: https://github.com/bevyengine/bevy/blob/main/examples/movement/physics_in_fixed_timestep.rs
+    ///
+    /// # Example
+    /// ```
+    /// # use bevy_app::prelude::*;
+    /// # use bevy_ecs::prelude::*;
+    /// App::new()
+    ///   .add_systems(FixedUpdate, update_physics)
+    ///   .add_systems(
+    ///     RunFixedMainLoop,
+    ///     interpolate_transforms.in_set(RunFixedMainLoopSystem::AfterFixedMainLoop));
+    ///
+    /// # fn interpolate_transforms() {}
+    /// # fn update_physics() {}
+    /// ```
+    AfterFixedMainLoop,
+}

crates/bevy_gizmos/src/lib.rs

@@ -246,13 +246,15 @@ impl AppGizmoBuilder for App {
             .init_resource::<GizmoStorage<Config, Swap<Fixed>>>()
             .add_systems(
                 RunFixedMainLoop,
-                start_gizmo_context::<Config, Fixed>.before(bevy_time::run_fixed_main_schedule),
+                start_gizmo_context::<Config, Fixed>
+                    .in_set(bevy_app::RunFixedMainLoopSystem::BeforeFixedMainLoop),
             )
             .add_systems(FixedFirst, clear_gizmo_context::<Config, Fixed>)
             .add_systems(FixedLast, collect_requested_gizmos::<Config, Fixed>)
             .add_systems(
                 RunFixedMainLoop,
-                end_gizmo_context::<Config, Fixed>.after(bevy_time::run_fixed_main_schedule),
+                end_gizmo_context::<Config, Fixed>
+                    .in_set(bevy_app::RunFixedMainLoopSystem::AfterFixedMainLoop),
             )
             .add_systems(
                 Last,

crates/bevy_time/src/fixed.rs

@@ -233,8 +233,10 @@ impl Default for Fixed {
 }
 
 /// 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) {
+/// [`Time<Virtual>`](Virtual) and [`Time::overstep`].
+/// You can order your systems relative to this by using
+/// [`RunFixedMainLoopSystem`](bevy_app::prelude::RunFixedMainLoopSystem).
+pub(super) fn run_fixed_main_schedule(world: &mut World) {
     let delta = world.resource::<Time<Virtual>>().delta();
     world.resource_mut::<Time<Fixed>>().accumulate(delta);
 

crates/bevy_time/src/lib.rs

@@ -70,7 +70,10 @@ impl Plugin for TimePlugin {
                 .in_set(TimeSystem)
                 .ambiguous_with(event_update_system),
         )
-        .add_systems(RunFixedMainLoop, run_fixed_main_schedule);
+        .add_systems(
+            RunFixedMainLoop,
+            run_fixed_main_schedule.in_set(RunFixedMainLoopSystem::FixedMainLoop),
+        );
 
         // Ensure the events are not dropped until `FixedMain` systems can observe them
         app.add_systems(FixedPostUpdate, signal_event_update_system);

examples/movement/physics_in_fixed_timestep.rs

@@ -53,16 +53,22 @@
 //!
 //! ## Implementation
 //!
+//! - The player's inputs since the last physics update are stored in the `AccumulatedInput` component.
 //! - The player's velocity is stored in a `Velocity` component. This is the speed in units per second.
 //! - The player's current position in the physics simulation is stored in a `PhysicalTranslation` component.
 //! - The player's previous position in the physics simulation is stored in a `PreviousPhysicalTranslation` component.
 //! - The player's visual representation is stored in Bevy's regular `Transform` component.
 //! - Every frame, we go through the following steps:
+//!    - Accumulate the player's input and set the current speed in the `handle_input` system.
+//!        This is run in the `RunFixedMainLoop` schedule, ordered in `RunFixedMainLoopSystem::BeforeFixedMainLoop`,
+//!        which runs before the fixed timestep loop. This is run every frame.
 //!    - Advance the physics simulation by one fixed timestep in the `advance_physics` system.
-//!        This is run in the `FixedUpdate` schedule, which runs before the `Update` schedule.
-//!    - Update the player's visual representation in the `update_rendered_transform` system.
+//!        Accumulated input is consumed here.
+//!        This is run in the `FixedUpdate` schedule, which runs zero or multiple times per frame.
+//!    - Update the player's visual representation in the `interpolate_rendered_transform` system.
 //!        This interpolates between the player's previous and current position in the physics simulation.
-//!    - Update the player's velocity based on the player's input in the `handle_input` system.
+//!        It is run in the `RunFixedMainLoop` schedule, ordered in `RunFixedMainLoopSystem::AfterFixedMainLoop`,
+//!        which runs after the fixed timestep loop. This is run every frame.
 //!
 //!
 //! ## Controls
@@ -80,13 +86,31 @@ fn main() {
     App::new()
         .add_plugins(DefaultPlugins)
         .add_systems(Startup, (spawn_text, spawn_player))
-        // `FixedUpdate` runs before `Update`, so the physics simulation is advanced before the player's visual representation is updated.
+        // Advance the physics simulation using a fixed timestep.
         .add_systems(FixedUpdate, advance_physics)
-        .add_systems(Update, (update_rendered_transform, handle_input).chain())
+        .add_systems(
+            // The `RunFixedMainLoop` schedule allows us to schedule systems to run before and after the fixed timestep loop.
+            RunFixedMainLoop,
+            (
+                // The physics simulation needs to know the player's input, so we run this before the fixed timestep loop.
+                // Note that if we ran it in `Update`, it would be too late, as the physics simulation would already have been advanced.
+                // If we ran this in `FixedUpdate`, it would sometimes not register player input, as that schedule may run zero times per frame.
+                handle_input.in_set(RunFixedMainLoopSystem::BeforeFixedMainLoop),
+                // The player's visual representation needs to be updated after the physics simulation has been advanced.
+                // This could be run in `Update`, but if we run it here instead, the systems in `Update`
+                // will be working with the `Transform` that will actually be shown on screen.
+                interpolate_rendered_transform.in_set(RunFixedMainLoopSystem::AfterFixedMainLoop),
+            ),
+        )
         .run();
 }
 
-/// How many units per second the player should move.
+/// A vector representing the player's input, accumulated over all frames that ran
+/// since the last time the physics simulation was advanced.
+#[derive(Debug, Component, Clone, Copy, PartialEq, Default, Deref, DerefMut)]
+struct AccumulatedInput(Vec2);
+
+/// A vector representing the player's velocity in the physics simulation.
 #[derive(Debug, Component, Clone, Copy, PartialEq, Default, Deref, DerefMut)]
 struct Velocity(Vec3);
 
@@ -100,7 +124,7 @@ struct Velocity(Vec3);
 struct PhysicalTranslation(Vec3);
 
 /// The value [`PhysicalTranslation`] had in the last fixed timestep.
-/// Used for interpolation in the `update_rendered_transform` system.
+/// Used for interpolation in the `interpolate_rendered_transform` system.
 #[derive(Debug, Component, Clone, Copy, PartialEq, Default, Deref, DerefMut)]
 struct PreviousPhysicalTranslation(Vec3);
 
@@ -114,6 +138,7 @@ fn spawn_player(mut commands: Commands, asset_server: Res<AssetServer>) {
             transform: Transform::from_scale(Vec3::splat(0.3)),
             ..default()
         },
+        AccumulatedInput::default(),
         Velocity::default(),
         PhysicalTranslation::default(),
         PreviousPhysicalTranslation::default(),
@@ -143,31 +168,35 @@ fn spawn_text(mut commands: Commands) {
         });
 }
 
-/// Handle keyboard input to move the player.
-fn handle_input(keyboard_input: Res<ButtonInput<KeyCode>>, mut query: Query<&mut Velocity>) {
+/// Handle keyboard input and accumulate it in the `AccumulatedInput` component.
+/// There are many strategies for how to handle all the input that happened since the last fixed timestep.
+/// This is a very simple one: we just accumulate the input and average it out by normalizing it.
+fn handle_input(
+    keyboard_input: Res<ButtonInput<KeyCode>>,
+    mut query: Query<(&mut AccumulatedInput, &mut Velocity)>,
+) {
     /// Since Bevy's default 2D camera setup is scaled such that
     /// one unit is one pixel, you can think of this as
     /// "How many pixels per second should the player move?"
     const SPEED: f32 = 210.0;
-    for mut velocity in query.iter_mut() {
-        velocity.0 = Vec3::ZERO;
-
+    for (mut input, mut velocity) in query.iter_mut() {
         if keyboard_input.pressed(KeyCode::KeyW) {
-            velocity.y += 1.0;
+            input.y += 1.0;
         }
         if keyboard_input.pressed(KeyCode::KeyS) {
-            velocity.y -= 1.0;
+            input.y -= 1.0;
         }
         if keyboard_input.pressed(KeyCode::KeyA) {
-            velocity.x -= 1.0;
+            input.x -= 1.0;
         }
         if keyboard_input.pressed(KeyCode::KeyD) {
-            velocity.x += 1.0;
+            input.x += 1.0;
         }
 
         // Need to normalize and scale because otherwise
         // diagonal movement would be faster than horizontal or vertical movement.
-        velocity.0 = velocity.normalize_or_zero() * SPEED;
+        // This effectively averages the accumulated input.
+        velocity.0 = input.extend(0.0).normalize_or_zero() * SPEED;
     }
 }
 
@@ -180,18 +209,26 @@ fn advance_physics(
     mut query: Query<(
         &mut PhysicalTranslation,
         &mut PreviousPhysicalTranslation,
+        &mut AccumulatedInput,
         &Velocity,
     )>,
 ) {
-    for (mut current_physical_translation, mut previous_physical_translation, velocity) in
-        query.iter_mut()
+    for (
+        mut current_physical_translation,
+        mut previous_physical_translation,
+        mut input,
+        velocity,
+    ) in query.iter_mut()
     {
         previous_physical_translation.0 = current_physical_translation.0;
         current_physical_translation.0 += velocity.0 * fixed_time.delta_seconds();
+
+        // Reset the input accumulator, as we are currently consuming all input that happened since the last fixed timestep.
+        input.0 = Vec2::ZERO;
     }
 }
 
-fn update_rendered_transform(
+fn interpolate_rendered_transform(
     fixed_time: Res<Time<Fixed>>,
     mut query: Query<(
         &mut Transform,