mirage_engine/

context.rs

use core::time::Duration;
use std::collections::HashMap;
#[cfg(feature = "ui")]
use std::sync::Arc;

#[cfg(feature = "ui")]
use crate::Error;
use crate::animation::{AnimationStates, Animator};
use crate::assets::Assets;
#[cfg(feature = "ui")]
use crate::assets::font;
use crate::input::{
    Axis2Binding, AxisBinding, ButtonBinding, Cursor, Input, InputAction, InputAxis2Action,
    InputAxisAction, InputButtonAction, Ticking,
};
use crate::math::{UVec2, Vec2};
use crate::mesh::{Instance, Mesh, Part};
use crate::post_effect::{PostEffect, PostEffectId, PostEffects};
use crate::renderer::Renderer;
use crate::renderer::mesh_cache::MeshCache;
use crate::save::{SaveKey, Saved};
use crate::sound::{Audio, SoundCue};
use crate::surface_style::{SurfaceStyle, SurfaceStyleId, SurfaceStyles};
use crate::time::{FrameTime, TickInterval};
use crate::ui::{Claims, Layer, Overlay};
use crate::{Camera, Config, Game, Holds, Light, View};

/// The work the startup closure [`run`](crate::run) takes may do.
pub struct InitContext<'a, G: Game> {
    startup: Startup<'a>,
    renderer: &'a mut Renderer<G::Meshes, G::Skyboxes>,
    audio: &'a mut Audio<G::Sounds>,
}

impl<'a, G: Game> InitContext<'a, G> {
    pub(crate) fn new(
        engine: Engine<'a>,
        renderer: &'a mut Renderer<G::Meshes, G::Skyboxes>,
        audio: &'a mut Audio<G::Sounds>,
        saves: &'a Saved,
        assets: &'a Assets,
        overlay: &'a Overlay,
    ) -> Self {
        Self {
            startup: Startup::new(engine, saves, assets, overlay),
            renderer,
            audio,
        }
    }

    /// The startup work that is typed by no vocabulary of the game's own.
    ///
    /// Required if you want to start two games the same way: `Startup` is
    /// one type whatever the game, so code shared between them takes
    /// `&mut Startup<'_>`.
    pub fn startup(&mut self) -> &mut Startup<'a> {
        &mut self.startup
    }

    /// How long `sound` plays for at its own pitch: the whole clip, before
    /// any trim.
    ///
    /// Required if you want to time a game against a sound. Every value the
    /// vocabulary catalogs is built before this runs, and a streamed one
    /// reports what the decode at startup measured.
    pub fn duration(&mut self, sound: G::Sounds) -> Duration {
        self.audio.duration(&sound)
    }

    /// How long each value that the vocabulary catalogs plays for, keyed by the
    /// value.
    ///
    /// Required if you want to time a game against a whole vocabulary. A
    /// value the catalog leaves out is absent from the map; read that one
    /// through [`duration`](Self::duration).
    pub fn durations(&mut self) -> HashMap<G::Sounds, Duration> {
        self.audio.durations()
    }

    /// Builds and uploads `mesh` now, instead of on its first draw; the
    /// copy is then held like any drawn mesh's under
    /// [`Config::with_mesh_memory`], and no longer than that.
    ///
    /// Takes a mesh of [`Game::Meshes`](crate::Game::Meshes) and no other.
    pub fn prepare<M>(&mut self, mesh: M)
    where
        G::Meshes: Holds<M> + From<M>,
    {
        let engine = &self.startup.engine;
        self.renderer
            .prepare(engine.device, engine.queue, mesh.into());
    }
}

/// The part of [`InitContext`] that is typed by no vocabulary of the game's
/// own, which [`InitContext::startup`] returns.
pub struct Startup<'a> {
    engine: Engine<'a>,
    saves: &'a Saved,
    #[cfg(feature = "ui")]
    assets: &'a Assets,
    #[cfg(feature = "ui")]
    overlay: &'a Overlay,
}

impl<'a> Startup<'a> {
    /// What a startup reads: the values a run saved, the store its sources
    /// loaded into, and the UI.
    #[cfg(feature = "ui")]
    pub(crate) fn new(
        engine: Engine<'a>,
        saves: &'a Saved,
        assets: &'a Assets,
        overlay: &'a Overlay,
    ) -> Self {
        Self {
            engine,
            saves,
            assets,
            overlay,
        }
    }

    /// The same without the UI, which is the one reader of a font.
    #[cfg(not(feature = "ui"))]
    pub(crate) fn new(
        engine: Engine<'a>,
        saves: &'a Saved,
        _assets: &'a Assets,
        _overlay: &'a Overlay,
    ) -> Self {
        Self { engine, saves }
    }

    /// The value the last run to save `key` kept, or its fallback where
    /// none did, or where what was kept no longer reads as the key's own
    /// value, with a debug log.
    pub fn saved<K: SaveKey>(&self, key: K) -> K::Value {
        self.saves.read(key)
    }

    /// The window's drawing area, in physical pixels; zero while minimized.
    pub fn window_size(&self) -> UVec2 {
        self.engine.window_size
    }

    /// The configuration the engine started with.
    pub fn config(&self) -> &Config {
        self.engine.config
    }

    /// The font a source loaded under `name`, to name in an
    /// `egui::FontDefinitions`.
    ///
    /// Required if you want the UI to draw in a game's own font: a source is
    /// a `.ttf` or an `.otf` [`Config::with_assets`] loads, named by its
    /// stem. A name no source loaded a font under, or one two sources share,
    /// is an error. The `ui` feature's own.
    #[cfg(feature = "ui")]
    pub fn font(&self, name: &str) -> Result<egui::FontData, Error> {
        let bytes = self.assets.font(name)?;

        Ok(egui::FontData::from_owned(bytes.to_vec()))
    }

    /// Draws every frame's UI in `fonts`, from the first frame on.
    ///
    /// Required if you want the UI to draw in a game's own font. Fails,
    /// naming the font, where an entry of `font_data` does not decode at its
    /// own index, and where a family lists a name `font_data` does not hold.
    /// egui's own fonts stay wherever `fonts` keeps them, and the last call
    /// is what draws. The `ui` feature's own.
    #[cfg(feature = "ui")]
    pub fn set_fonts(&mut self, fonts: egui::FontDefinitions) -> Result<(), Error> {
        for (name, data) in &fonts.font_data {
            font::decode_at(&data.font, data.index)
                .map_err(|error| Error::msg(format!("the font `{name}` {error}")))?;
        }
        for (family, names) in &fonts.families {
            for name in names {
                if !fonts.font_data.contains_key(name) {
                    return Err(Error::msg(format!(
                        "the font family `{family}` lists `{name}`, which `font_data` does \
                         not hold"
                    )));
                }
            }
        }
        self.overlay.set_fonts(fonts);

        Ok(())
    }
}

/// The work [`Game::tick`](crate::Game::tick) may do: simulate, never draw.
pub struct TickContext<'a, G: Game> {
    engine: Engine<'a>,
    meshes: &'a mut MeshCache<G::Meshes>,
    audio: &'a mut Audio<G::Sounds>,
    ticking: &'a Ticking<'a>,
    saves: &'a mut Saved,
    run: &'a mut Run,
    dt: Duration,
    /// The run clock at the frame these ticks belong to, which every draw
    /// of that frame is posed at too.
    elapsed: Duration,
    claims: Claims,
}

impl<'a, G: Game> TickContext<'a, G> {
    /// Whether `action` is held right now.
    ///
    /// Every tick of one drawn frame reads the same controls.
    pub fn down<A: InputButtonAction>(&self, action: A) -> bool
    where
        G::InputActions: Holds<A, A::Binding>,
    {
        self.ticking.down(action)
    }

    /// Whether `action` went down since the last frame whose ticks ran.
    ///
    /// A frame that runs no ticks holds its edges for the ticks that follow,
    /// so every press is read by the ticks of exactly one frame.
    pub fn pressed<A: InputButtonAction>(&self, action: A) -> bool
    where
        G::InputActions: Holds<A, A::Binding>,
    {
        self.ticking.pressed(action)
    }

    /// Whether `action` came up since the last frame whose ticks ran.
    ///
    /// A release is held for the ticks that follow, the same as a press.
    pub fn released<A: InputButtonAction>(&self, action: A) -> bool
    where
        G::InputActions: Holds<A, A::Binding>,
    {
        self.ticking.released(action)
    }

    /// Presses of `action` in a row, counting the one these ticks read:
    /// `1` for a single click, `2` for a double, `0` where they read no
    /// press; see [`FrameContext::clicks`].
    ///
    /// The ticks of one frame read one press however many landed in them,
    /// so a control pressed twice inside one frame reads one press of two
    /// clicks.
    pub fn clicks<A: InputButtonAction>(&self, action: A) -> u32
    where
        G::InputActions: Holds<A, A::Binding>,
    {
        self.ticking.clicks(action)
    }

    /// Analog reading of `action`: a fraction in `-1..=1` from a pad axis,
    /// a joystick axis or a button composite, of which a trigger reads
    /// `0..=1`, and the scaled distance, which nothing clamps, from a
    /// [`PointerDelta`](crate::PointerDelta) or
    /// [`WheelDelta`](crate::WheelDelta) lane.
    ///
    /// Read those two in [`Game::frame`](crate::Game::frame): the ticks of
    /// one frame each read the whole distance that frame moved.
    pub fn axis<A: InputAxisAction>(&self, action: A) -> f32
    where
        G::InputActions: Holds<A, A::Binding>,
    {
        self.ticking.axis(action)
    }

    /// `action`'s reading: a vector no longer than `1` from a stick or a
    /// button composite, and the scaled distance, which nothing clamps,
    /// from [`Axis2Binding::pointer`].
    ///
    /// Read the pointer in [`Game::frame`](crate::Game::frame): the ticks
    /// of one frame each read the whole distance that frame moved.
    pub fn axis2<A: InputAxis2Action>(&self, action: A) -> Vec2
    where
        G::InputActions: Holds<A, A::Binding>,
    {
        self.ticking.axis2(action)
    }

    /// Pointer position, in physical pixels from the drawing area's top
    /// left; the origin until it is first seen.
    ///
    /// The mouse and the first touch share it, and
    /// [`window_size`](Self::window_size) is in the same pixels, so
    /// [`Camera::ray_through`] takes it as it is.
    pub fn pointer(&self) -> Vec2 {
        self.ticking.pointer()
    }

    /// Plays `sound` once, keeping wherever it is placed as of this tick.
    ///
    /// Every call is a voice of its own, so the same sound twice over is
    /// heard twice; the ticks of one frame are played together at the end
    /// of it.
    pub fn play(&mut self, sound: impl Into<SoundCue<G::Sounds>>) {
        self.audio.play(sound.into());
    }

    /// The value the last run to save `key` kept, or its fallback where
    /// none did, or where what was kept no longer reads as the key's own
    /// value, with a debug log.
    pub fn saved<K: SaveKey>(&self, key: K) -> K::Value {
        self.saves.read(key)
    }

    /// Keeps `value` under `key`, for the rest of this run and the runs
    /// after it.
    ///
    /// The store is written once the frame these ticks belong to is drawn,
    /// and only where a value changed, so saving every tick costs nothing.
    pub fn save<K: SaveKey>(&mut self, key: K, value: K::Value) {
        self.saves.write(key, value);
    }

    /// Whether the platform allows sound to start right now; see
    /// [`FrameContext::sound_unlocked`].
    pub fn sound_unlocked(&self) -> bool {
        self.audio.unlocked()
    }

    /// Runs `animator` up to this tick against `input`, over the clips
    /// `mesh` holds: where its state goes from here, and what the state it
    /// lands in plays.
    ///
    /// Takes a mesh of [`Game::Meshes`](crate::Game::Meshes) and a machine
    /// typed by that mesh, so a machine runs on the clips of the value the
    /// game draws and no other. Every tick of one drawn
    /// frame runs at one instant, as every one of them reads one snapshot
    /// of the controls, and the frame draws at that same instant.
    pub fn animate<M, P, S>(&mut self, mesh: M, animator: &mut Animator<M, S>, input: &S::Input)
    where
        G::Meshes: Holds<M> + From<M>,
        M: Mesh<P, S::Clip>,
        P: Part,
        S: AnimationStates,
    {
        let now = self.elapsed;
        animator.animate(input, now, self.meshes.clips(mesh));
    }

    /// This tick's fixed time step: [`Config::tick_interval`] until
    /// [`set_tick_interval`](Self::set_tick_interval) changes it.
    pub fn dt(&self) -> Duration {
        self.dt
    }

    /// Sets the simulated time every later tick covers; see
    /// [`FrameContext::set_tick_interval`].
    pub fn set_tick_interval(&mut self, interval: Duration) {
        self.run.tick_interval.set(interval);
    }

    /// Ends the run once the frame these ticks belong to is drawn; see
    /// [`FrameContext::close`].
    pub fn close(&mut self) {
        self.run.closing = true;
    }

    /// Whether the UI took the pointer last frame. Always false without the
    /// `ui` feature.
    pub fn ui_wants_pointer(&self) -> bool {
        self.claims.pointer
    }

    /// Whether the UI took the keyboard last frame. Always false without the
    /// `ui` feature.
    pub fn ui_wants_keyboard(&self) -> bool {
        self.claims.keyboard
    }

    /// The window's drawing area, in physical pixels; zero while minimized.
    pub fn window_size(&self) -> UVec2 {
        self.engine.window_size
    }

    /// The camera the last drawn frame was viewed from; [`Camera::default`]
    /// before the first frame.
    ///
    /// Required if you want a ray through a pixel: the player points at what
    /// was last drawn.
    pub fn last_camera(&self) -> Camera {
        self.engine.last_camera
    }

    /// The configuration the engine started with.
    pub fn config(&self) -> &Config {
        self.engine.config
    }
}

/// The work [`Game::frame`](crate::Game::frame) may do.
pub struct FrameContext<'a, G: Game> {
    engine: Engine<'a>,
    renderer: &'a mut Renderer<G::Meshes, G::Skyboxes>,
    audio: &'a mut Audio<G::Sounds>,
    input: &'a mut Input,
    saves: &'a mut Saved,
    run: &'a mut Run,
    time: FrameTime,
    layer: Layer<'a>,
}

impl<'a, G: Game> FrameContext<'a, G> {
    /// Whether `action` is held right now.
    pub fn down<A: InputButtonAction>(&self, action: A) -> bool
    where
        G::InputActions: Holds<A, A::Binding>,
    {
        self.input.down(action)
    }

    /// Whether `action` went down during this frame.
    pub fn pressed<A: InputButtonAction>(&self, action: A) -> bool
    where
        G::InputActions: Holds<A, A::Binding>,
    {
        self.input.pressed(action)
    }

    /// Whether `action` came up during this frame.
    pub fn released<A: InputButtonAction>(&self, action: A) -> bool
    where
        G::InputActions: Holds<A, A::Binding>,
    {
        self.input.released(action)
    }

    /// Presses of `action` in a row, counting this frame's: `1` for a
    /// single click, `2` for a double, and `0` on a frame where `action`
    /// was not pressed.
    ///
    /// A press counts with the one before it where the same control took
    /// both, no later than the double click interval after it — the
    /// platform's own until
    /// [`Config::with_double_click_interval`](crate::Config::with_double_click_interval)
    /// sets one. The engine counts one control at a time, so an action
    /// whose press lands in the same frame as another control's reads `0`
    /// where the count is on that other one. A headless session counts
    /// against the clock its caller drives, so presses at one instant of it
    /// count together.
    pub fn clicks<A: InputButtonAction>(&self, action: A) -> u32
    where
        G::InputActions: Holds<A, A::Binding>,
    {
        self.input.clicks(action)
    }

    /// Analog reading of `action`: a fraction in `-1..=1` from a pad axis,
    /// a joystick axis or a button composite, of which a trigger reads
    /// `0..=1`, and the scaled distance, which nothing clamps, from a
    /// [`PointerDelta`](crate::PointerDelta) or
    /// [`WheelDelta`](crate::WheelDelta) lane.
    pub fn axis<A: InputAxisAction>(&self, action: A) -> f32
    where
        G::InputActions: Holds<A, A::Binding>,
    {
        self.input.axis(action)
    }

    /// `action`'s reading: a vector no longer than `1` from a stick or a
    /// button composite, and the scaled distance, which nothing clamps,
    /// from [`Axis2Binding::pointer`].
    pub fn axis2<A: InputAxis2Action>(&self, action: A) -> Vec2
    where
        G::InputActions: Holds<A, A::Binding>,
    {
        self.input.axis2(action)
    }

    /// Pointer position, in physical pixels from the drawing area's top
    /// left; the origin until it is first seen.
    ///
    /// The mouse and the first touch share it, and
    /// [`window_size`](Self::window_size) is in the same pixels, so
    /// [`Camera::ray_through`] takes it as it is.
    pub fn pointer(&self) -> Vec2 {
        self.input.pointer()
    }

    /// Draws the pointer as `cursor` this frame; the last call in a frame
    /// is the one it draws.
    ///
    /// A frame that never calls this draws it as [`Cursor::Arrow`]. Where
    /// the UI sets a cursor of its own, the UI's is drawn instead.
    ///
    /// [`Cursor::Held`] holds the pointer in place, and the UI's own
    /// cursor does not take `Held` over: [`pointer`](Self::pointer) reads
    /// the place it was held at, a [`PointerDelta`](crate::PointerDelta)
    /// binding keeps reading how far it moves, and the first frame to set
    /// another cursor releases it. A browser takes the pointer lock only from
    /// inside a gesture of the player's, so there the hold is taken on the
    /// player's next press or touch. A window loses the hold as it loses
    /// focus, and takes it again on the first frame to set `Held` after the
    /// focus returns.
    pub fn set_cursor(&mut self, cursor: Cursor) {
        *self.layer.cursor = cursor;
    }

    /// Binds `action` to `bindings` for the rest of the run, and keeps it
    /// for the runs after that — written when the frame ends, and only
    /// when these bindings are not already what `action` is bound to.
    ///
    /// The kind of binding follows the action, so a stick cannot be bound to
    /// a button.
    pub fn rebind<A: InputAction>(&mut self, action: A, bindings: Vec<A::Binding>)
    where
        G::InputActions: Holds<A, A::Binding>,
    {
        self.input.rebind(action, bindings);
    }

    /// The bindings `action` reads through right now, which a controls menu
    /// shows through each binding's text.
    pub fn bindings<A: InputAction>(&self, action: A) -> Vec<A::Binding>
    where
        G::InputActions: Holds<A, A::Binding>,
    {
        self.input.bindings(action)
    }

    /// The button control the player pressed this frame, for a controls menu
    /// listening for one to bind.
    ///
    /// Nothing where the player pressed nothing new; polling is the whole
    /// mechanism, so a menu that stops calling this stops listening.
    pub fn actuated_button(&self) -> Option<ButtonBinding> {
        self.input.actuated_button()
    }

    /// The analog control the player pushed this frame, past the deadzone a
    /// binding starts with.
    ///
    /// The pointer and the wheel are never returned: they would take the
    /// smallest nudge for a choice.
    pub fn actuated_axis(&self) -> Option<AxisBinding> {
        self.input.actuated_axis()
    }

    /// The stick the player pushed this frame, past the deadzone a binding
    /// starts with.
    pub fn actuated_axis2(&self) -> Option<Axis2Binding> {
        self.input.actuated_axis2()
    }

    /// Duration of the previous frame: this frame's own variable time
    /// step, distinct from the fixed one `tick` runs at.
    pub fn dt(&self) -> Duration {
        self.time.dt
    }

    /// Duration the game has been running.
    pub fn elapsed(&self) -> Duration {
        self.time.elapsed
    }

    /// This frame's position into the next simulation step, a fraction in
    /// `0.0..1.0`; used to draw between two tick states.
    pub fn alpha(&self) -> f32 {
        self.time.alpha
    }

    /// Sets the simulated time every later tick covers, from the next frame
    /// on; held to at least `Duration::from_micros(1)`.
    ///
    /// Required if you want to pace the simulation against something outside
    /// the engine, such as a program on another machine. The ticks a frame
    /// already runs keep the step they started with.
    pub fn set_tick_interval(&mut self, interval: Duration) {
        self.run.tick_interval.set(interval);
    }

    /// Ends the run once this frame is drawn: what the frame saved is
    /// written, and no tick or frame runs after it.
    ///
    /// On the desktop the window closes and [`run`](crate::run) returns. In
    /// the browser there is no program to end: the loop stops, and the
    /// canvas the engine created is dropped from the page, while one the
    /// game named through [`Config::with_canvas_id`] stays as the page left
    /// it.
    pub fn close(&mut self) {
        self.run.closing = true;
    }

    /// Views the rest of the frame from `camera`. A later call in the same
    /// frame replaces this one; without any, [`Camera::default`] is used.
    pub fn set_camera(&mut self, camera: Camera) {
        self.renderer.set_camera(camera);
    }

    /// Draws `instance` this frame. The engine decides the order and which
    /// draws share GPU work.
    ///
    /// Takes a draw of a mesh of [`Game::Meshes`](crate::Game::Meshes) and
    /// no other.
    pub fn draw<M>(&mut self, instance: Instance<M, G::SurfaceStyles>)
    where
        G::Meshes: Holds<M> + From<M>,
    {
        self.renderer.draw(instance.record().into_set());
    }

    /// Passes the style `T` the values its WGSL reads this frame; the last
    /// call for a style is the one it reads.
    ///
    /// A frame that never calls this for a style leaves it reading the
    /// default value of every field. Takes a style of
    /// [`Game::SurfaceStyles`](crate::Game::SurfaceStyles) and no other.
    pub fn set_surface_style<T: SurfaceStyle>(&mut self, style: T)
    where
        G::SurfaceStyles: Holds<T> + From<T>,
    {
        let style = G::SurfaceStyles::from(style);
        self.renderer
            .set_surface_style(SurfaceStyleId(style.seat()), &style);
    }

    /// Runs the post effect `T` over this frame with the values its WGSL
    /// reads; the last call for an effect is the one it runs with.
    ///
    /// A frame that never calls this for an effect runs no pass for it.
    /// Takes an effect of
    /// [`Game::PostEffects`](crate::Game::PostEffects) and no other.
    pub fn set_post_effect<T: PostEffect>(&mut self, effect: T)
    where
        G::PostEffects: Holds<T> + From<T>,
    {
        let effect = G::PostEffects::from(effect);
        self.renderer
            .set_post_effect(PostEffectId(effect.seat()), &effect);
    }

    /// Draws and lights this frame by `sky`: what it draws where nothing
    /// else was drawn, and the ambient — the light every surface takes from
    /// every direction.
    ///
    /// Takes a value of [`Game::Skyboxes`](crate::Game::Skyboxes) and no
    /// other; startup built every one of them. The last call in a frame is
    /// the one it draws, and a frame that never calls this draws and is lit
    /// by the default sky.
    pub fn set_skybox(&mut self, sky: G::Skyboxes) {
        self.renderer
            .set_skybox(self.engine.device, self.engine.queue, &sky);
    }

    /// Lights the frame with `light`, in addition to any already submitted.
    ///
    /// The first call replaces the default environment's light; past
    /// [`MAX_LIGHTS`](crate::MAX_LIGHTS), excess is ignored — warned the
    /// first frame, a debug log after.
    pub fn light(&mut self, light: Light) {
        self.renderer.light(light);
    }

    /// Scales the frame's light before the curve, as a fraction of it; a
    /// value under `0.0` is clamped to it, and `1.0` is used where a frame
    /// never calls this.
    ///
    /// The last call in a frame replaces the rest.
    pub fn set_exposure(&mut self, exposure: f32) {
        self.renderer.set_exposure(exposure);
    }

    /// Spreads the frame's brightest light over what is around it; the
    /// value is clamped into `0.0..=1.0`, and `0.0` is used where a frame
    /// never calls this.
    ///
    /// The value is the fraction of the frame the spread replaces, and `0.0`
    /// is no work at all. The last call in a frame replaces the rest.
    pub fn set_bloom(&mut self, amount: f32) {
        self.renderer.set_bloom(amount);
    }

    /// Plays `sound` once, keeping wherever it is placed as of this frame.
    ///
    /// Every call is a voice of its own, so the same sound twice over is
    /// heard twice.
    pub fn play(&mut self, sound: impl Into<SoundCue<G::Sounds>>) {
        self.audio.play(sound.into());
    }

    /// Keeps `sound` playing while frames go on declaring it, and fades it
    /// out over its fade once one does not.
    ///
    /// What a frame declares is the whole of what it wants sounding. One
    /// voice per value, whose knobs follow what each frame declares; the
    /// last call for a value in a frame is the one that counts. A value a
    /// frame stops declaring is dropped once it has faded, so declaring it
    /// again after that starts it at its window start.
    pub fn sustain(&mut self, sound: impl Into<SoundCue<G::Sounds>>) {
        self.audio.sustain(sound.into());
    }

    /// Hears the frame from `listener`, in place of the frame's camera.
    pub fn set_listener(&mut self, listener: View) {
        self.audio.set_listener(listener);
    }

    /// Plays everything this frame at `volume`, the fraction of each
    /// sound's own gain it multiplies; `1.0` where a frame never calls this.
    ///
    /// The mix slides to it over [`SoundCue::DEFAULT_GLIDE`], so a slider a
    /// player moves never steps the sound.
    pub fn set_volume(&mut self, volume: f32) {
        self.audio.set_volume(volume);
    }

    /// Whether the platform allows sound to start right now.
    ///
    /// True from the first frame on the desktop, with or without an audio
    /// device: what holds it false is the browser, which plays nothing until
    /// the player has done something. A one-shot played while it is false is
    /// dropped; a sustain declared then starts when it turns true.
    pub fn sound_unlocked(&self) -> bool {
        self.audio.unlocked()
    }

    /// The value the last run to save `key` kept, or its fallback where
    /// none did, or where what was kept no longer reads as the key's own
    /// value, with a debug log.
    pub fn saved<K: SaveKey>(&self, key: K) -> K::Value {
        self.saves.read(key)
    }

    /// Keeps `value` under `key`, for the rest of this run and the runs
    /// after it.
    ///
    /// The store is written once the frame is drawn, and only where a value
    /// changed, so saving every frame costs nothing.
    pub fn save<K: SaveKey>(&mut self, key: K, value: K::Value) {
        self.saves.write(key, value);
    }

    /// Builds this frame's UI, drawn over the scene.
    ///
    /// Calls append to one layer, which is placed `8` points clear of the
    /// window's edges; floating windows go through `ui.ctx()`, and
    /// `egui::Panel::left` and the three beside it hold a panel against one
    /// side of that layer, shown in the `ui` this call takes.
    #[cfg(feature = "ui")]
    pub fn ui(&mut self, build: impl FnOnce(&mut egui::Ui)) {
        build(&mut *self.layer.ui);
    }

    /// Physical pixels per logical point of the UI this frame, a fraction
    /// over `1.0` on a dense screen: what a pixel [`Camera::pixel_of`]
    /// returns is divided by before the UI draws at it.
    #[cfg(feature = "ui")]
    pub fn pixels_per_point(&self) -> f32 {
        self.layer.pixels_per_point
    }

    /// `text` laid out in `font` at no width, so a row ends only where a
    /// `\n` starts the next one.
    ///
    /// Required if you want to size or place what you draw against text: a
    /// box a name has to fit in, a line as wide as the word over it. Its
    /// `size()` is the width and the height the text takes, in logical
    /// points. One kept past a change of
    /// [`pixels_per_point`](Self::pixels_per_point) still reports the size
    /// the frame that laid it out measured. The `ui` feature's own.
    #[cfg(feature = "ui")]
    pub fn text_layout(&self, text: &str, font: egui::FontId) -> Arc<egui::Galley> {
        self.layer.ui.ctx().fonts_mut(|fonts| {
            fonts.layout_no_wrap(text.to_owned(), font, egui::Color32::PLACEHOLDER)
        })
    }

    /// Whether the UI took the pointer last frame. Always false without the
    /// `ui` feature.
    pub fn ui_wants_pointer(&self) -> bool {
        self.layer.claims.pointer
    }

    /// Whether the UI took the keyboard last frame. Always false without the
    /// `ui` feature.
    pub fn ui_wants_keyboard(&self) -> bool {
        self.layer.claims.keyboard
    }

    /// The window's drawing area, in physical pixels; zero while minimized.
    pub fn window_size(&self) -> UVec2 {
        self.engine.window_size
    }

    /// The camera the last drawn frame was viewed from; [`Camera::default`]
    /// before the first frame.
    ///
    /// Required if you want a ray through a pixel: the player points at what
    /// was last drawn, not at what this frame has set since.
    pub fn last_camera(&self) -> Camera {
        self.engine.last_camera
    }

    /// The configuration the engine started with.
    pub fn config(&self) -> &Config {
        self.engine.config
    }

    /// Builds and uploads `mesh` now, instead of on its first draw; the
    /// copy is then held like any drawn mesh's under
    /// [`Config::with_mesh_memory`], and no longer than that.
    ///
    /// Takes a mesh of [`Game::Meshes`](crate::Game::Meshes) and no other.
    pub fn prepare<M>(&mut self, mesh: M)
    where
        G::Meshes: Holds<M> + From<M>,
    {
        self.renderer
            .prepare(self.engine.device, self.engine.queue, mesh.into());
    }
}

/// Everything a tick is recorded against: what it plays through, keeps
/// in, and the run it paces and ends.
pub(crate) struct Simulating<'a, G: Game> {
    pub(crate) audio: &'a mut Audio<G::Sounds>,
    pub(crate) saves: &'a mut Saved,
    pub(crate) run: &'a mut Run,
}

impl<'a, G: Game> Simulating<'a, G> {
    /// The context one tick of `dt` runs with, reading `ticking`'s controls
    /// and running every machine it is passed at `elapsed` on the run
    /// clock.
    pub(crate) fn tick_context(
        self,
        engine: Engine<'a>,
        meshes: &'a mut MeshCache<G::Meshes>,
        ticking: &'a Ticking<'a>,
        dt: Duration,
        elapsed: Duration,
        claims: Claims,
    ) -> TickContext<'a, G> {
        let Self { audio, saves, run } = self;
        TickContext {
            engine,
            meshes,
            audio,
            ticking,
            saves,
            run,
            dt,
            elapsed,
            claims,
        }
    }
}

/// Everything a frame is recorded against: what a tick is, and what it
/// draws through and reads its controls from.
pub(crate) struct Recording<'a, G: Game> {
    pub(crate) renderer: &'a mut Renderer<G::Meshes, G::Skyboxes>,
    pub(crate) input: &'a mut Input,
    pub(crate) simulating: Simulating<'a, G>,
}

impl<'a, G: Game> Recording<'a, G> {
    /// The context one frame at `time` runs with, building its UI in
    /// `layer`.
    ///
    /// The frame starts at the instant `time` reads, so every draw it
    /// records is posed at that one instant.
    pub(crate) fn frame_context(
        self,
        engine: Engine<'a>,
        time: FrameTime,
        layer: Layer<'a>,
    ) -> FrameContext<'a, G> {
        let Self {
            renderer,
            input,
            simulating: Simulating { audio, saves, run },
        } = self;
        renderer.start_frame(time.elapsed);

        FrameContext {
            engine,
            renderer,
            audio,
            input,
            saves,
            run,
            time,
            layer,
        }
    }
}

/// What a game changes about the run itself: how long its ticks are, and
/// whether it goes on.
pub(crate) struct Run {
    tick_interval: TickInterval,
    closing: bool,
}

impl Run {
    /// A run that ticks every `tick_interval` until the game closes it.
    pub(crate) fn new(tick_interval: Duration) -> Self {
        Self {
            tick_interval: TickInterval::new(tick_interval),
            closing: false,
        }
    }

    /// Starts a batch of ticks, and returns the step each one takes.
    pub(crate) fn start_ticks(&mut self) -> Duration {
        self.tick_interval.advance();
        self.tick_interval.current()
    }

    /// The step the ticks running now take.
    pub(crate) fn tick_interval(&self) -> Duration {
        self.tick_interval.current()
    }

    /// Whether the game has requested that the run end.
    pub(crate) fn closing(&self) -> bool {
        self.closing
    }
}

/// Everything every context uses: the GPU meshes are built on, the area
/// being drawn to, the settings the engine started with, and the camera the
/// last frame was drawn from.
pub(crate) struct Engine<'a> {
    device: &'a wgpu::Device,
    queue: &'a wgpu::Queue,
    window_size: UVec2,
    config: &'a Config,
    last_camera: Camera,
}

impl<'a> Engine<'a> {
    pub(crate) fn new(
        device: &'a wgpu::Device,
        queue: &'a wgpu::Queue,
        window_size: UVec2,
        config: &'a Config,
        last_camera: Camera,
    ) -> Self {
        Self {
            device,
            queue,
            window_size,
            config,
            last_camera,
        }
    }
}