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//! This example illustrates the use of [`ComputedStates`] for more complex state handling patterns.
//!
//! In this case, we'll be implementing the following pattern:
//! - The game will start in a `Menu` state, which we can return to with `Esc`
//! - From there, we can enter the game - where our bevy symbol moves around and changes color
//! - While in game, we can pause and unpause the game using `Space`
//! - We can also toggle "Turbo Mode" with the `T` key - where the movement and color changes are all faster. This
//! is retained between pauses, but not if we exit to the main menu.
//!
//! In addition, we want to enable a "tutorial" mode, which will involve it's own state that is toggled in the main menu.
//! This will display instructions about movement and turbo mode when in game and unpaused, and instructions on how to unpause when paused.
//!
//! To implement this, we will create 2 root-level states: [`AppState`] and [`TutorialState`].
//! We will then create some computed states that derive from [`AppState`]: [`InGame`] and [`TurboMode`] are marker states implemented
//! as Zero-Sized Structs (ZSTs), while [`IsPaused`] is an enum with 2 distinct states.
//! And lastly, we'll add [`Tutorial`], a computed state deriving from [`TutorialState`], [`InGame`] and [`IsPaused`], with 2 distinct
//! states to display the 2 tutorial texts.
use bevy::prelude::*;
use ui::*;
// To begin, we want to define our state objects.
#[derive(Debug, Clone, Copy, Default, Eq, PartialEq, Hash, States)]
enum AppState {
#[default]
Menu,
// Unlike in the `states` example, we're adding more data in this
// version of our AppState. In this case, we actually have
// 4 distinct "InGame" states - unpaused and no turbo, paused and no
// turbo, unpaused and turbo and paused and turbo.
InGame {
paused: bool,
turbo: bool,
},
}
// The tutorial state object, on the other hand, is a fairly simple enum.
#[derive(Debug, Clone, Copy, Default, Eq, PartialEq, Hash, States)]
enum TutorialState {
#[default]
Active,
Inactive,
}
// Because we have 4 distinct values of `AppState` that mean we're "InGame", we're going to define
// a separate "InGame" type and implement `ComputedStates` for it.
// This allows us to only need to check against one type
// when otherwise we'd need to check against multiple.
#[derive(Debug, Clone, Copy, Eq, PartialEq, Hash)]
struct InGame;
impl ComputedStates for InGame {
// Our computed state depends on `AppState`, so we need to specify it as the SourceStates type.
type SourceStates = AppState;
// The compute function takes in the `SourceStates`
fn compute(sources: AppState) -> Option<Self> {
// You might notice that InGame has no values - instead, in this case, the `State<InGame>` resource only exists
// if the `compute` function would return `Some` - so only when we are in game.
match sources {
// No matter what the value of `paused` or `turbo` is, we're still in the game rather than a menu
AppState::InGame { .. } => Some(Self),
_ => None,
}
}
}
// Similarly, we want to have the TurboMode state - so we'll define that now.
//
// Having it separate from [`InGame`] and [`AppState`] like this allows us to check each of them separately, rather than
// needing to compare against every version of the AppState that could involve them.
//
// In addition, it allows us to still maintain a strict type representation - you can't Turbo
// if you aren't in game, for example - while still having the
// flexibility to check for the states as if they were completely unrelated.
#[derive(Debug, Clone, Copy, Eq, PartialEq, Hash)]
struct TurboMode;
impl ComputedStates for TurboMode {
type SourceStates = AppState;
fn compute(sources: AppState) -> Option<Self> {
match sources {
AppState::InGame { turbo: true, .. } => Some(Self),
_ => None,
}
}
}
// For the [`IsPaused`] state, we'll actually use an `enum` - because the difference between `Paused` and `NotPaused`
// involve activating different systems.
//
// To clarify the difference, `InGame` and `TurboMode` both activate systems if they exist, and there is
// no variation within them. So we defined them as Zero-Sized Structs.
//
// In contrast, pausing actually involve 3 distinct potential situations:
// - it doesn't exist - this is when being paused is meaningless, like in the menu.
// - it is `NotPaused` - in which elements like the movement system are active.
// - it is `Paused` - in which those game systems are inactive, and a pause screen is shown.
#[derive(Debug, Clone, Copy, Eq, PartialEq, Hash)]
enum IsPaused {
NotPaused,
Paused,
}
impl ComputedStates for IsPaused {
type SourceStates = AppState;
fn compute(sources: AppState) -> Option<Self> {
// Here we convert from our [`AppState`] to all potential [`IsPaused`] versions.
match sources {
AppState::InGame { paused: true, .. } => Some(Self::Paused),
AppState::InGame { paused: false, .. } => Some(Self::NotPaused),
// If `AppState` is not `InGame`, pausing is meaningless, and so we set it to `None`.
_ => None,
}
}
}
// Lastly, we have our tutorial, which actually has a more complex derivation.
//
// Like `IsPaused`, the tutorial has a few fully distinct possible states, so we want to represent them
// as an Enum. However - in this case they are all dependant on multiple states: the root [`TutorialState`],
// and both [`InGame`] and [`IsPaused`] - which are in turn derived from [`AppState`].
#[derive(Debug, Clone, Copy, Eq, PartialEq, Hash)]
enum Tutorial {
MovementInstructions,
PauseInstructions,
}
impl ComputedStates for Tutorial {
// We can also use tuples of types that implement [`States`] as our [`SourceStates`].
// That includes other [`ComputedStates`] - though circular dependencies are not supported
// and will produce a compile error.
//
// We could define this as relying on [`TutorialState`] and [`AppState`] instead, but
// then we would need to duplicate the derivation logic for [`InGame`] and [`IsPaused`].
// In this example that is not a significant undertaking, but as a rule it is likely more
// effective to rely on the already derived states to avoid the logic drifting apart.
//
// Notice that you can wrap any of the [`States`] here in [`Option`]s. If you do so,
// the the computation will get called even if the state does not exist.
type SourceStates = (TutorialState, InGame, Option<IsPaused>);
// Notice that we aren't using InGame - we're just using it as a source state to
// prevent the computation from executing if we're not in game. Instead - this
// ComputedState will just not exist in that situation.
fn compute(
(tutorial_state, _in_game, is_paused): (TutorialState, InGame, Option<IsPaused>),
) -> Option<Self> {
// If the tutorial is inactive we don't need to worry about it.
if !matches!(tutorial_state, TutorialState::Active) {
return None;
}
// If we're paused, we're in the PauseInstructions tutorial
// Otherwise, we're in the MovementInstructions tutorial
match is_paused? {
IsPaused::NotPaused => Some(Tutorial::MovementInstructions),
IsPaused::Paused => Some(Tutorial::PauseInstructions),
}
}
}
fn main() {
// We start the setup like we did in the states example.
App::new()
.add_plugins(DefaultPlugins)
.init_state::<AppState>()
.init_state::<TutorialState>()
// After initializing the normal states, we'll use `.add_computed_state::<CS>()` to initialize our `ComputedStates`
.add_computed_state::<InGame>()
.add_computed_state::<IsPaused>()
.add_computed_state::<TurboMode>()
.add_computed_state::<Tutorial>()
// we can then resume adding systems just like we would in any other case,
// using our states as normal.
.add_systems(Startup, setup)
.add_systems(OnEnter(AppState::Menu), setup_menu)
.add_systems(Update, menu.run_if(in_state(AppState::Menu)))
.add_systems(OnExit(AppState::Menu), cleanup_menu)
// We only want to run the [`setup_game`] function when we enter the [`AppState::InGame`] state, regardless
// of whether the game is paused or not.
.add_systems(OnEnter(InGame), setup_game)
// And we only want to run the [`clear_game`] function when we leave the [`AppState::InGame`] state, regardless
// of whether we're paused.
.add_systems(OnExit(InGame), clear_state_bound_entities(InGame))
// We want the color change, toggle_pause and quit_to_menu systems to ignore the paused condition, so we can use the [`InGame`] derived
// state here as well.
.add_systems(
Update,
(toggle_pause, change_color, quit_to_menu).run_if(in_state(InGame)),
)
// However, we only want to move or toggle turbo mode if we are not in a paused state.
.add_systems(
Update,
(toggle_turbo, movement).run_if(in_state(IsPaused::NotPaused)),
)
// We can continue setting things up, following all the same patterns used above and in the `states` example.
.add_systems(OnEnter(IsPaused::Paused), setup_paused_screen)
.add_systems(
OnExit(IsPaused::Paused),
clear_state_bound_entities(IsPaused::Paused),
)
.add_systems(OnEnter(TurboMode), setup_turbo_text)
.add_systems(OnExit(TurboMode), clear_state_bound_entities(TurboMode))
.add_systems(
OnEnter(Tutorial::MovementInstructions),
movement_instructions,
)
.add_systems(OnEnter(Tutorial::PauseInstructions), pause_instructions)
.add_systems(
OnExit(Tutorial::MovementInstructions),
clear_state_bound_entities(Tutorial::MovementInstructions),
)
.add_systems(
OnExit(Tutorial::PauseInstructions),
clear_state_bound_entities(Tutorial::PauseInstructions),
)
.add_systems(Update, log_transitions)
.run();
}
fn menu(
mut next_state: ResMut<NextState<AppState>>,
tutorial_state: Res<State<TutorialState>>,
mut next_tutorial: ResMut<NextState<TutorialState>>,
mut interaction_query: Query<
(&Interaction, &mut UiImage, &MenuButton),
(Changed<Interaction>, With<Button>),
>,
) {
for (interaction, mut image, menu_button) in &mut interaction_query {
let color = &mut image.color;
match *interaction {
Interaction::Pressed => {
*color = if menu_button == &MenuButton::Tutorial
&& tutorial_state.get() == &TutorialState::Active
{
PRESSED_ACTIVE_BUTTON
} else {
PRESSED_BUTTON
};
match menu_button {
MenuButton::Play => next_state.set(AppState::InGame {
paused: false,
turbo: false,
}),
MenuButton::Tutorial => next_tutorial.set(match tutorial_state.get() {
TutorialState::Active => TutorialState::Inactive,
TutorialState::Inactive => TutorialState::Active,
}),
};
}
Interaction::Hovered => {
if menu_button == &MenuButton::Tutorial
&& tutorial_state.get() == &TutorialState::Active
{
*color = HOVERED_ACTIVE_BUTTON;
} else {
*color = HOVERED_BUTTON;
}
}
Interaction::None => {
if menu_button == &MenuButton::Tutorial
&& tutorial_state.get() == &TutorialState::Active
{
*color = ACTIVE_BUTTON;
} else {
*color = NORMAL_BUTTON;
}
}
}
}
}
#[derive(Component)]
struct StateBound<S: States>(S);
fn clear_state_bound_entities<S: States>(
state: S,
) -> impl Fn(Commands, Query<(Entity, &StateBound<S>)>) {
info!("Clearing entities for {state:?}");
move |mut commands, query| {
for (entity, bound) in &query {
if bound.0 == state {
commands.entity(entity).despawn_recursive();
}
}
}
}
fn toggle_pause(
input: Res<ButtonInput<KeyCode>>,
current_state: Res<State<AppState>>,
mut next_state: ResMut<NextState<AppState>>,
) {
if input.just_pressed(KeyCode::Space) {
if let AppState::InGame { paused, turbo } = current_state.get() {
next_state.set(AppState::InGame {
paused: !*paused,
turbo: *turbo,
});
}
}
}
fn toggle_turbo(
input: Res<ButtonInput<KeyCode>>,
current_state: Res<State<AppState>>,
mut next_state: ResMut<NextState<AppState>>,
) {
if input.just_pressed(KeyCode::KeyT) {
if let AppState::InGame { paused, turbo } = current_state.get() {
next_state.set(AppState::InGame {
paused: *paused,
turbo: !*turbo,
});
}
}
}
fn quit_to_menu(input: Res<ButtonInput<KeyCode>>, mut next_state: ResMut<NextState<AppState>>) {
if input.just_pressed(KeyCode::Escape) {
next_state.set(AppState::Menu);
}
}
/// print when either an `AppState` transition or a `TutorialState` transition happens
fn log_transitions(
mut transitions: EventReader<StateTransitionEvent<AppState>>,
mut tutorial_transitions: EventReader<StateTransitionEvent<TutorialState>>,
) {
for transition in transitions.read() {
info!(
"transition: {:?} => {:?}",
transition.before, transition.after
);
}
for transition in tutorial_transitions.read() {
info!(
"tutorial transition: {:?} => {:?}",
transition.before, transition.after
);
}
}
mod ui {
use crate::*;
#[derive(Resource)]
pub struct MenuData {
pub root_entity: Entity,
}
#[derive(Component, PartialEq, Eq)]
pub enum MenuButton {
Play,
Tutorial,
}
pub const NORMAL_BUTTON: Color = Color::srgb(0.15, 0.15, 0.15);
pub const HOVERED_BUTTON: Color = Color::srgb(0.25, 0.25, 0.25);
pub const PRESSED_BUTTON: Color = Color::srgb(0.35, 0.75, 0.35);
pub const ACTIVE_BUTTON: Color = Color::srgb(0.15, 0.85, 0.15);
pub const HOVERED_ACTIVE_BUTTON: Color = Color::srgb(0.25, 0.55, 0.25);
pub const PRESSED_ACTIVE_BUTTON: Color = Color::srgb(0.35, 0.95, 0.35);
pub fn setup(mut commands: Commands) {
commands.spawn(Camera2dBundle::default());
}
pub fn setup_menu(mut commands: Commands, tutorial_state: Res<State<TutorialState>>) {
let button_entity = commands
.spawn(NodeBundle {
style: Style {
// center button
width: Val::Percent(100.),
height: Val::Percent(100.),
justify_content: JustifyContent::Center,
align_items: AlignItems::Center,
flex_direction: FlexDirection::Column,
row_gap: Val::Px(10.),
..default()
},
..default()
})
.with_children(|parent| {
parent
.spawn((
ButtonBundle {
style: Style {
width: Val::Px(200.),
height: Val::Px(65.),
// horizontally center child text
justify_content: JustifyContent::Center,
// vertically center child text
align_items: AlignItems::Center,
..default()
},
image: UiImage::default().with_color(NORMAL_BUTTON),
..default()
},
MenuButton::Play,
))
.with_children(|parent| {
parent.spawn(TextBundle::from_section(
"Play",
TextStyle {
font_size: 40.0,
color: Color::srgb(0.9, 0.9, 0.9),
..default()
},
));
});
parent
.spawn((
ButtonBundle {
style: Style {
width: Val::Px(200.),
height: Val::Px(65.),
// horizontally center child text
justify_content: JustifyContent::Center,
// vertically center child text
align_items: AlignItems::Center,
..default()
},
image: UiImage::default().with_color(match tutorial_state.get() {
TutorialState::Active => ACTIVE_BUTTON,
TutorialState::Inactive => NORMAL_BUTTON,
}),
..default()
},
MenuButton::Tutorial,
))
.with_children(|parent| {
parent.spawn(TextBundle::from_section(
"Tutorial",
TextStyle {
font_size: 40.0,
color: Color::srgb(0.9, 0.9, 0.9),
..default()
},
));
});
})
.id();
commands.insert_resource(MenuData {
root_entity: button_entity,
});
}
pub fn cleanup_menu(mut commands: Commands, menu_data: Res<MenuData>) {
commands.entity(menu_data.root_entity).despawn_recursive();
}
pub fn setup_game(mut commands: Commands, asset_server: Res<AssetServer>) {
commands.spawn((
StateBound(InGame),
SpriteBundle {
texture: asset_server.load("branding/icon.png"),
..default()
},
));
}
const SPEED: f32 = 100.0;
const TURBO_SPEED: f32 = 300.0;
pub fn movement(
time: Res<Time>,
input: Res<ButtonInput<KeyCode>>,
turbo: Option<Res<State<TurboMode>>>,
mut query: Query<&mut Transform, With<Sprite>>,
) {
for mut transform in &mut query {
let mut direction = Vec3::ZERO;
if input.pressed(KeyCode::ArrowLeft) {
direction.x -= 1.0;
}
if input.pressed(KeyCode::ArrowRight) {
direction.x += 1.0;
}
if input.pressed(KeyCode::ArrowUp) {
direction.y += 1.0;
}
if input.pressed(KeyCode::ArrowDown) {
direction.y -= 1.0;
}
if direction != Vec3::ZERO {
transform.translation += direction.normalize()
* if turbo.is_some() { TURBO_SPEED } else { SPEED }
* time.delta_seconds();
}
}
}
pub fn setup_paused_screen(mut commands: Commands) {
info!("Printing Pause");
commands
.spawn((
StateBound(IsPaused::Paused),
NodeBundle {
style: Style {
// center button
width: Val::Percent(100.),
height: Val::Percent(100.),
justify_content: JustifyContent::Center,
align_items: AlignItems::Center,
flex_direction: FlexDirection::Column,
row_gap: Val::Px(10.),
position_type: PositionType::Absolute,
..default()
},
..default()
},
))
.with_children(|parent| {
parent
.spawn((
NodeBundle {
style: Style {
width: Val::Px(400.),
height: Val::Px(400.),
// horizontally center child text
justify_content: JustifyContent::Center,
// vertically center child text
align_items: AlignItems::Center,
..default()
},
background_color: NORMAL_BUTTON.into(),
..default()
},
MenuButton::Play,
))
.with_children(|parent| {
parent.spawn(TextBundle::from_section(
"Paused",
TextStyle {
font_size: 40.0,
color: Color::srgb(0.9, 0.9, 0.9),
..default()
},
));
});
});
}
pub fn setup_turbo_text(mut commands: Commands) {
commands
.spawn((
StateBound(TurboMode),
NodeBundle {
style: Style {
// center button
width: Val::Percent(100.),
height: Val::Percent(100.),
justify_content: JustifyContent::Start,
align_items: AlignItems::Center,
flex_direction: FlexDirection::Column,
row_gap: Val::Px(10.),
position_type: PositionType::Absolute,
..default()
},
..default()
},
))
.with_children(|parent| {
parent.spawn(TextBundle::from_section(
"TURBO MODE",
TextStyle {
font_size: 40.0,
color: Color::srgb(0.9, 0.3, 0.1),
..default()
},
));
});
}
pub fn change_color(time: Res<Time>, mut query: Query<&mut Sprite>) {
for mut sprite in &mut query {
let new_color = LinearRgba {
blue: (time.elapsed_seconds() * 0.5).sin() + 2.0,
..LinearRgba::from(sprite.color)
};
sprite.color = new_color.into();
}
}
pub fn movement_instructions(mut commands: Commands) {
commands
.spawn((
StateBound(Tutorial::MovementInstructions),
NodeBundle {
style: Style {
// center button
width: Val::Percent(100.),
height: Val::Percent(100.),
justify_content: JustifyContent::End,
align_items: AlignItems::Center,
flex_direction: FlexDirection::Column,
row_gap: Val::Px(10.),
position_type: PositionType::Absolute,
..default()
},
..default()
},
))
.with_children(|parent| {
parent.spawn(TextBundle::from_section(
"Move the bevy logo with the arrow keys",
TextStyle {
font_size: 40.0,
color: Color::srgb(0.3, 0.3, 0.7),
..default()
},
));
parent.spawn(TextBundle::from_section(
"Press T to enter TURBO MODE",
TextStyle {
font_size: 40.0,
color: Color::srgb(0.3, 0.3, 0.7),
..default()
},
));
parent.spawn(TextBundle::from_section(
"Press SPACE to pause",
TextStyle {
font_size: 40.0,
color: Color::srgb(0.3, 0.3, 0.7),
..default()
},
));
parent.spawn(TextBundle::from_section(
"Press ESCAPE to return to the menu",
TextStyle {
font_size: 40.0,
color: Color::srgb(0.3, 0.3, 0.7),
..default()
},
));
});
}
pub fn pause_instructions(mut commands: Commands) {
commands
.spawn((
StateBound(Tutorial::PauseInstructions),
NodeBundle {
style: Style {
// center button
width: Val::Percent(100.),
height: Val::Percent(100.),
justify_content: JustifyContent::End,
align_items: AlignItems::Center,
flex_direction: FlexDirection::Column,
row_gap: Val::Px(10.),
position_type: PositionType::Absolute,
..default()
},
..default()
},
))
.with_children(|parent| {
parent.spawn(TextBundle::from_section(
"Press SPACE to resume",
TextStyle {
font_size: 40.0,
color: Color::srgb(0.3, 0.3, 0.7),
..default()
},
));
parent.spawn(TextBundle::from_section(
"Press ESCAPE to return to the menu",
TextStyle {
font_size: 40.0,
color: Color::srgb(0.3, 0.3, 0.7),
..default()
},
));
});
}
}