Bevy Minesweeper: Generic States

Check the repository

Our board plugin is customizable through BoardOptions, but pour app can't interact with it.
We need to make our BoardPlugin generic, to allow control through states.

Plugin

Lets edit our plugin structure:

// lib.rs
use bevy::ecs::schedule::StateData;

pub struct BoardPlugin<T> {
    pub running_state: T,
}

impl<T: StateData> Plugin for BoardPlugin<T> {
    fn build(&self, app: &mut App) {
        // ..
    }
}

impl<T> BoardPlugin<T> {
    // ..
}

Our plugin cannot know what state the app using it has defined, it needs to be generic.

We can now change our systems structure to take in account this running_state:

// lib.rs
// ..
fn build(&self, app: &mut App) {
    // When the running states comes into the stack we load a board
        app.add_system_set(
            SystemSet::on_enter(self.running_state.clone()).with_system(Self::create_board),
        )
        // We handle input and trigger events only if the state is active
        .add_system_set(
            SystemSet::on_update(self.running_state.clone())
                .with_system(systems::input::input_handling)
                .with_system(systems::uncover::trigger_event_handler),
        )
        // We handle uncovering even if the state is inactive
        .add_system_set(
            SystemSet::on_in_stack_update(self.running_state.clone())
                .with_system(systems::uncover::uncover_tiles),
        )
        .add_event::<TileTriggerEvent>();
}

Bevy's states are in a stack:

if a state is at the top of the stack it is considered active
if a state is in the stack but not at the top it is considered inactive
if a state leaves the stack it is considered exited
if a state enters the stack it is considered entered

So what did we do here:

Since we now constrain systems with state conditions, everything is a SystemSet
Instead of a startup_system we call our setup_board system when our state enters the stack
We handle our input, and the trigger events only if the state is active (we allow for paused states)
The uncovering system should not be paused, so we run it if the state is in the stack, active or not.

With this configuration the app using the plugin can have menus or other stuff and trigger our board generation with the running_state.
But we need to be able to clean up the board in case the states exits the stack.

For that the Board resource should have a reference to its own Entity to despawn it with all its children:

// board.rs
// ..
#[derive(Debug)]
pub struct Board {
    pub tile_map: TileMap,
    pub bounds: Bounds2,
    pub tile_size: f32,
    pub covered_tiles: HashMap<Coordinates, Entity>,
+    pub entity: Entity,
}
// ..

Let's edit our create_board system to retrieve the entity:

// lib.rs
fn create_board(
    // ..
) {
    // .. 
    let board_entity = commands
    //        .spawn()
    //        .insert(Name::new("Board"))
    // ..
    .id();
    // ..
    commands.insert_resource(Board {
            // ..
            entity: board_entity,
        })

}

Now we can register a cleaning system for our plugin:

// lib.rs

// ..
fn build(&self, app: &mut App) {
    //..
    .add_system_set(
        SystemSet::on_exit(self.running_state.clone())
            .with_system(Self::cleanup_board),
    )
    // ..
}

impl<T> BoardPlugin<T> {
    // ..
    fn cleanup_board(board: Res<Board>, mut commands: Commands) {
        commands.entity(board.entity).despawn_recursive();
        commands.remove_resource::<Board>();
    }
}

What about all the tiles, the texts, the sprites, the covers, etc ?

Since we spawned every board entity as children to our board_entity, using despawn_recursive will also despawn its children:

the background
the tiles
the tile texts
the tile sprites
the tile covers
etc.

App

Let's define some basic states:

// main.rs
#[derive(Debug, Clone, Eq, PartialEq, Hash)]
pub enum AppState {
    InGame,
    Out,
}

fn main() {
    // ..
    .add_state(AppState::InGame)
    .add_plugin(BoardPlugin {
        running_state: AppState::InGame,
    })
}

If we run the app now, nothing has changed, but if we edit the states we can completely control our board systems:

// main.rs
use bevy::log;

fn main() {
    // ..
    // State handling
    .add_system(state_handler);
    // ..
}

fn state_handler(mut state: ResMut<State<AppState>>, keys: Res<Input<KeyCode>>) {
    if keys.just_pressed(KeyCode::C) {
        log::debug!("clearing detected");
        if state.current() == &AppState::InGame {
            log::info!("clearing game");
            state.set(AppState::Out).unwrap();
        }
    }
    if keys.just_pressed(KeyCode::G) {
        log::debug!("loading detected");
        if state.current() == &AppState::Out {
            log::info!("loading game");
            state.set(AppState::InGame).unwrap();
        }
    }
}

Everything should be familiar here,

state is wrapped in ResMut<>, because states are handled like any resource but with an additional wrapper: State<>
keys is an Input<> argument, allowing to check for keyboard interaction using KeyCode (it can be used with MouseButton for mouse interaction)

Now pressing C should cleanup the board entirely, and pressing G should generate a new board.

Exercise

States can be tricky, so it's good to practice using it.

Implement the following features:

When I press Escape the game pauses and I can't interact with the board, if I press Escape again the game resumes.
When I press G a new board generates, without having to press C first.

Give me your answers on Twitter at @ManevilleF

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Author: Félix de Maneville
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