With our only unique skill of note being moving around, it's hard to feel emotionally invested in these poor critters running in circles forever in an unbreakable cage. An inevitable component of fantasy gaming is required: magic.

Now, with the way the system is currently set up, "pressing this button to dash forwards 4 spaces" would be extremely easy. We can do better - a system which would normally be painful to implement, but which takes advantage of Rust's pattern matching and enums, as well as Bevy's system ordering... Enter - Spell Crafting.

Design Capsule
Spells will be composed of a series of Forms and Functions. Forms choose tiles on the screen, and Functions execute an effect on those tiles. In the case of a lasso, for example, the Form is a projectile and the Function is getting constricted.

Create a new file, spells.rs.

// spells.rs
use bevy::prelude::*;

pub struct SpellPlugin;

impl Plugin for SpellPlugin {
    fn build(&self, app: &mut App) {}
}

Don't forget to link it into main.rs.

// main.rs

// SNIP
mod spells;

// SNIP
use spells::SpellPlugin;

fn main() {
    App::new()
        .add_plugins(DefaultPlugins.set(ImagePlugin::default_nearest()))
        .add_plugins((
            SpellPlugin, // NEW!
            EventPlugin,
            GraphicsPlugin,
            MapPlugin,
            InputPlugin,
        ))
        .run();
}

It is important to add SpellPlugin as the first plugin for the sake of this tutorial. This will cause a minor bug later on, which will be explained and fixed afterwards.

D.I.Y. Wizard

Now, we may start to populate this new plugin with some starting structs and enums. I named the individual components that form a Spell "Axiom" because:

1. Calling them "Components" would get confusing fast with Bevy components.
2. They are things that happen, an enforceable truth.
3. The word "Axiom" is just dripping with flair and style.

// spells.rs
#[derive(Event)]
/// Triggered when a creature (the `caster`) casts a `spell`.
pub struct CastSpell {
    pub caster: Entity,
    pub spell: Spell,
}

#[derive(Component, Clone)]
/// A spell is composed of a list of "Axioms", which will select tiles or execute an effect onto
/// those tiles, in the order they are listed.
pub struct Spell {
    pub axioms: Vec<Axiom>,
}

#[derive(Debug, Clone)]
/// There are Form axioms, which target certain tiles, and Function axioms, which execute an effect
/// onto those tiles.
pub enum Axiom {
    // FORMS

    // Target the caster's tile.
    Ego,

    // FUNCTIONS

    // The targeted creatures dash in the direction of the caster's last move.
    Dash,
}

We will begin with the very simple spell "Ego, Dash". When cast, the caster dashes in the direction of their last move. Note that I didn't use "Self" for the self-target, because it's already taken by Rust as a keyword, and "Ego" sounds very cool.

The implementation will rely on a struct with yet another cute name: Synapses. Named after the transmission of signals between neurons, they are like a snowball rolling down a hill and accumulating debris.

When a new SynapseData is created, it is blank except for the fact that it knows its caster, how the caster moved last turn (caster_momentum) and where that caster is (caster_position). It still has no tiles to target, and no effects to execute onto the game. As it "rolls" down the list of Axioms, it will accumulate targets and effects to execute on those targets. The effects are simply replicas of Events, like "teleport this entity here" or "summon this entity here" - named EventDispatch.

// spells.rs
/// The tracker of everything which determines how a certain spell will act.
struct SynapseData {
    /// Where a spell will act.
    targets: Vec<Position>,
    /// How a spell will act.
    effects: Vec<EventDispatch>,
    /// Who cast the spell.
    caster: Entity,
    /// In which direction did the caster move the last time they did so?
    caster_momentum: OrdDir,
    /// Where is the caster on the map?
    caster_position: Position,
}

impl SynapseData {
    /// Create a blank SynapseData.
    fn new(caster: Entity, caster_momentum: OrdDir, caster_position: Position) -> Self {
        SynapseData {
            targets: Vec::new(),
            effects: Vec::new(),
            caster,
            caster_momentum,
            caster_position,
        }
    }
}

/// An enum with replicas of common game Events, to be translated into the real Events
/// and dispatched to the main game loop.
pub enum EventDispatch {
    TeleportEntity {
        destination: Position,
        entity: Entity,
    },
}

Now, a prototype Bevy system can be written to handle all of this.

// spells.rs
/// Work through the list of Axioms of a spell, translating it into Events to launch onto the game.
fn gather_effects(
    mut cast_spells: EventReader<CastSpell>,
    mut sender: EventWriter<SpellEffect>,
    caster: Query<(&Position, &OrdDir)>,
    map: Res<Map>,
) {
    for cast_spell in cast_spells.read() {
        // First, get the list of Axioms.
        let axioms = &cast_spell.spell.axioms;
        // And the caster's position and last move direction.
        let (caster_position, caster_momentum) = caster.get(cast_spell.caster).unwrap();

        // Create a new synapse to start "rolling down the hill" accumulating targets and effects.
        let mut synapse_data =
            SynapseData::new(cast_spell.caster, *caster_momentum, *caster_position);

        // Loop through each axiom.
        for axiom in axioms.iter() {
            // For Forms, add targets.
            axiom.target(&mut synapse_data, &map);
            // For Functions, add effects that operate on those targets.
            axiom.execute(&mut synapse_data, &map);
        }

        // Once all Axioms are processed, dispatch everything to the system that will translate
        // all effects into proper events.
        sender.send(SpellEffect {
            events: synapse_data.effects,
        });
    }
}

You may have noticed the following:

1. The yet uninitialized SpellEffect event.
2. The OrdDir struct is not a component and does not appear on any entities, meaning Query<(&Position, &OrdDir)> will return nothing.
3. axiom.target and axiom.execute functions not yet existing.

Let us work through these problems one by one.

1. Transforming Spell Effects into Events

Our spell effects currently do nothing - they are only replicas of real Events, such as TeleportEntity. They need to be translated into the real deal.

// spells.rs

#[derive(Event)]
/// An event dictating that a list of Events must be sent to the game loop
/// after the completion of a spell.
pub struct SpellEffect {
    events: Vec<EventDispatch>,
}

/// Translate a list of EventDispatch into their "real" Event counterparts and send them off
/// into the main game loop to modify the game's creatures.
pub fn dispatch_events(
    mut receiver: EventReader<SpellEffect>,
    mut teleport: EventWriter<TeleportEntity>,
) {
    for effect_list in receiver.read() {
        for effect in &effect_list.events {
            // Each EventDispatch enum is translated into its Event counterpart.
            match effect {
                EventDispatch::TeleportEntity {
                    destination,
                    entity,
                } => {
                    teleport.send(TeleportEntity::new(*entity, destination.x, destination.y));
                }
            };
        }
    }
}

With this new system, every completed spell with dispatch all corresponding Events once it is concluded!

2. Tracking Creatures' Last Move (Momentum)

OrdDir already exists, but it is currently nothing but a simple enum. It could be elevated into a much greater Component...

// main.rs
#[derive(Component, PartialEq, Eq, Copy, Clone, Debug)] // CHANGED: Added Component.
pub enum OrdDir {
    Up,
    Right,
    Down,
    Left,
}

It will also need to be a crucial part of each Creature.

// creature.rs
#[derive(Bundle)]
pub struct Creature {
    pub position: Position,
    pub momentum: OrdDir, // NEW!
    pub sprite: SpriteBundle,
    pub atlas: TextureAtlas,
}

This will instantly rain down errors into the crate - all Creatures must now receive this new Component.

// map.rs
fn spawn_player(
    mut commands: Commands,
    asset_server: Res<AssetServer>,
    atlas_layout: Res<SpriteSheetAtlas>,
) {
            // SNIP
            atlas: TextureAtlas {
                layout: atlas_layout.handle.clone(),
                index: 0,
            },
            momentum: OrdDir::Up, // NEW!
        },
        Player,
    ));
}

fn spawn_cage(
    mut commands: Commands,
    asset_server: Res<AssetServer>,
    atlas_layout: Res<SpriteSheetAtlas>,
) {
            // SNIP
            atlas: TextureAtlas {
                layout: atlas_layout.handle.clone(),
                index,
            },
            momentum: OrdDir::Up, // NEW!
        });
        if tile_char == 'H' {
            creature.insert(Hunt);
        }
    }
}

All good, but all Creatures are now eternally "facing" upwards regardless of their actions. Let us track this with a new Event.

// events.rs
#[derive(Event)]
pub struct AlterMomentum {
    pub entity: Entity,
    pub direction: OrdDir,
}

fn alter_momentum(mut events: EventReader<AlterMomentum>, mut creature: Query<&mut OrdDir>) {
    for momentum_alteration in events.read() {
        *creature.get_mut(momentum_alteration.entity).unwrap() = momentum_alteration.direction;
    }
}

This event receives its trigger, right now, from when the player steps. It won't track any other creatures... for now.

// events.rs
fn player_step(
    mut events: EventReader<PlayerStep>,
    mut teleporter: EventWriter<TeleportEntity>,
    mut momentum: EventWriter<AlterMomentum>, // NEW!
    player: Query<(Entity, &Position), With<Player>>,
    hunters: Query<(Entity, &Position), With<Hunt>>,
    map: Res<Map>,
) {
        // SNIP
        teleporter.send(TeleportEntity::new(
            player_entity,
            player_pos.x + off_x,
            player_pos.y + off_y,
        ));
        // NEW!
        momentum.send(AlterMomentum {
            entity: player_entity,
            direction: event.direction,
        });
        // End NEW.

        for (hunter_entity, hunter_pos) in hunters.iter() {
        // SNIP
        }
    }
}

3. Actually Making The Spell Do Something

Now, for the true main course...

// spells.rs
impl Axiom {
    fn target(&self, synapse_data: &mut SynapseData, map: &Map) {
        match self {
            // Target the caster's tile.
            Self::Ego => {
                synapse_data.targets.push(synapse_data.caster_position);
            }
            _ => (),
        }
    }
}

The ̀target function should handle all potential "Forms" in a spell targeting certain tiles. Anything that isn't a "Form" gets flushed down the final _ => ().

Ego is quite simple. Push the caster_position into the targets, done.

As for Dash... it is a little more involved. Its implementation will reside inside another impl Axiom function, execute.

// events.rs
impl Axiom {
    // SNIP
    /// Execute Function-type Axioms. Returns true if this produced an actual effect.
    fn execute(&self, synapse_data: &mut SynapseData, map: &Map) -> bool {
        match self {
            Self::Dash => {
                // For each (Entity, Position) on a targeted tile...
                for (dasher, dasher_pos) in synapse_data.get_all_targeted_entity_pos_pairs(map) {
                    // The dashing creature starts where it currently is standing.
                    let mut final_dash_destination = dasher_pos;
                    // It will travel in the direction of the caster's last move.
                    let (off_x, off_y) = synapse_data.caster_momentum.as_offset();
                    // The dash has a maximum travel distance of 10.
                    let mut distance_travelled = 0;
                    while distance_travelled < 10 {
                        distance_travelled += 1;
                        // Stop dashing if a solid Creature is hit.
                        if !map.is_passable(
                            final_dash_destination.x + off_x,
                            final_dash_destination.y + off_y,
                        ) {
                            break;
                        }
                        // Otherwise, keep offsetting the dashing creature's position.
                        final_dash_destination.shift(off_x, off_y);
                    }

                    // Once finished, release the Teleport event.
                    synapse_data.effects.push(EventDispatch::TeleportEntity {
                        destination: final_dash_destination,
                        entity: dasher,
                    });
                }
                true
            }
            // Forms (which do not have an in-game effect) return false.
            _ => false,
        }
    }
 }

There is only one unimplemented function in this block, get_all_targeted_entity_pos_pairs, which inspects the Map to pull out all the corresponding key-value pairs.

// spells.rs
impl SynapseData {

    // SNIP

    fn get_all_targeted_entity_pos_pairs(&self, map: &Map) -> Vec<(Entity, Position)> {
        let mut targeted_pairs = Vec::new();
        for target in &self.targets {
            if let Some(entity) = map.get_entity_at(target.x, target.y) {
                targeted_pairs.push((*entity, *target));
            }
        }
        targeted_pairs
    }
}

The Test Run

After all this, the first spell Ego, Dash is ready to enter our grimoire - and while that was a lot, future spell effects will be a lot easier to implement from now on. Simply add more entries in the match statements of target and execute!

One last thing: actually casting it.

// input.rs
/// Each frame, if a button is pressed, move the player 1 tile.
fn keyboard_input(
    player: Query<Entity, With<Player>>, // NEW!
    mut spell: EventWriter<CastSpell>, // NEW!
    mut events: EventWriter<PlayerStep>,
    input: Res<ButtonInput<KeyCode>>,
) {
    // NEW!
    if input.just_pressed(KeyCode::Space) {
        spell.send(CastSpell {
            caster: player.get_single().unwrap(),
            spell: Spell {
                axioms: vec![Axiom::Ego, Axiom::Dash],
            },
        });
    }
    // End NEW.

    // SNIP
}

Finally, cargo ruǹ will allow us to escape the sticky grasp of the Hunter by pressing the spacebar! What's that? It crashed? Of course, we now need to register absolutely everything that was just added into Bevy's ECS.

This is extremely easy to forget and is mostly indicated by "struct is never constructed"-type warnings. If you are ever testing your changes and things seem to be going wrong, check first that you registered your systems, events and resources!

With that said:

// events.rs
impl Plugin for EventPlugin {
    fn build(&self, app: &mut App) {
        app.add_event::<PlayerStep>();
        app.add_event::<TeleportEntity>();
        app.add_event::<AlterMomentum>(); // NEW!
        app.add_systems(Update, player_step);
        app.add_systems(Update, teleport_entity);
        app.add_systems(Update, alter_momentum); // NEW!
    }
}

// spells.rs
impl Plugin for SpellPlugin {
    fn build(&self, app: &mut App) {
        app.add_event::<CastSpell>();
        app.add_event::<SpellEffect>();
        app.add_systems(Update, gather_effects);
        app.add_systems(Update, dispatch_events);
    }
}

And there's just one last thing I'd like to change for now: knocking down the light-speed movement down a notch.

// input.rs
    if input.just_pressed(KeyCode::KeyW) { // CHANGED to just_pressed
        events.send(PlayerStep {
            direction: OrdDir::Up,
        });
    }
    if input.just_pressed(KeyCode::KeyD) { // CHANGED to just_pressed
        events.send(PlayerStep {
            direction: OrdDir::Right,
        });
    }
    if input.just_pressed(KeyCode::KeyA) { // CHANGED to just_pressed
        events.send(PlayerStep {
            direction: OrdDir::Left,
        });
    }
    if input.just_pressed(KeyCode::KeyS) { // CHANGED to just_pressed
        events.send(PlayerStep {
            direction: OrdDir::Down,
        });
    }

Try again. cargo run. Pressing the space bar will now allow you to escape your sticky little friend!

Intermediate Wizardry 201

The player dashing around is fun and good... but what about a projectile that knocks back whatever critter it hits? This sounds slightly far-fetched, but it actually takes almost no code that we have not already seen. Enter... MomentumBeam, Dash.

// spells.rs
pub enum Axiom {
    // FORMS

    // Target the caster's tile.
    Ego,

    // NEW!
    // Fire a beam from the caster, towards the caster's last move. Target all travelled tiles,
    // including the first solid tile encountered, which stops the beam.
    MomentumBeam,
    // End NEW.

    // SNIP
}

It, of course, receives its own implementation.

// spells.rs
    fn target(&self, synapse_data: &mut SynapseData, map: &Map) {
        match self {
        // SNIP
        // NEW!
            // Shoot a beam from the caster towards its last move, all tiles passed through
            // become targets, including the impact point.
            Self::MomentumBeam => {
                // Start the beam where the caster is standing.
                let mut start = synapse_data.caster_position;
                // The beam travels in the direction of the caster's last move.
                let (off_x, off_y) = synapse_data.caster_momentum.as_offset();
                let mut distance_travelled = 0;
                let mut output = Vec::new();
                // The beam has a maximum distance of 10.
                while distance_travelled < 10 {
                    distance_travelled += 1;
                    start.shift(off_x, off_y);
                    // The new tile is always added, even if it is impassable...
                    output.push(start);
                    // But if it is impassable, it is the last added tile.
                    if !map.is_passable(start.x, start.y) {
                        break;
                    }
                }
                // Add these tiles to `targets`.
                synapse_data.targets.append(&mut output);
            }
        // End NEW.
        }
    }

You may notice that this is extremely similar to the Dash logic... Its differences are the inclusion of the final impact tile (which is solid), and how it collects all travelled tiles in an output vector, added to targets.

// Do not add this block, it is merely a demonstration.
// The dashing creature starts where it currently is standing.
let mut final_dash_destination = dasher_pos;
// It will travel in the direction of the caster's last move.
let (off_x, off_y) = synapse_data.caster_momentum.as_offset();
// The dash has a maximum travel distance of 10.
let mut distance_travelled = 0;
while distance_travelled < 10 {
    distance_travelled += 1;
    // Stop dashing if a solid Creature is hit.
    if !map.is_passable(
        final_dash_destination.x + off_x,
        final_dash_destination.y + off_y,
    ) {
        break;
    }
    // Otherwise, keep offsetting the dashing creature's position.
    final_dash_destination.shift(off_x, off_y);
}

In software development, "don't repeat yourself" is a common wisdom, but in games development, sometimes, it must be done within reason. Think, what if we add later a magic forcefield that blocks beams but not movement? In that case, if we had done something like this to adhere to "don't repeat yourself":

// Do not add this block, it is merely a demonstration.
Self::Dash => {
    for (dasher, dasher_pos) in synapse_data.get_all_targeted_entity_pos_pairs(map) {
        // Create a fake synapse just to use a beam.
        let mut artificial_synapse = SynapseData::new_from_synapse(synapse_data);
        // Set the fake synapse's caster and caster position to be the targeted creatures.
        (
            artificial_synapse.caster,
            artificial_synapse.caster_position,
        ) = (dasher, dasher_pos);
        // Fire the beam with the caster's momentum.
        Self::MomentumBeam.target(&mut artificial_synapse, map);
        // Get the penultimate tile, aka the last passable tile in the beam's path.
        let destination_tile = artificial_synapse
            .targets
            .get(artificial_synapse.targets.len().wrapping_sub(2));
        // If that penultimate tile existed, teleport to it.
        if let Some(destination_tile) = destination_tile {
            synapse_data.effects.push(EventDispatch::TeleportEntity {
                destination: *destination_tile,
                entity: dasher,
            });
        }
    }
    true
}

Here, a fake beam is invented, which needs a fake "synapse" to go alongside it, and it specifically extracts the penultimate tile (as the last one is the solid impact point) to dash to. Should the "anti beam forcefield" be invented later, this would need an added exception, potentially implemented as an extra parameter passed to the target function... lots of complexity for not much reward.

And just like that, with only 11 added lines of code (which were very similar to our Dash implementation), the projectile is ready:

// input.rs
    if input.just_pressed(KeyCode::Space) {
        spell.send(CastSpell {
            caster: player.get_single().unwrap(),
            spell: Spell {
                axioms: vec![Axiom::MomentumBeam, Axiom::Dash],
            },
        });
    }

cargo run. Not only can you teach your sticky companion some manners, you can even break the walls of the cage, and escape into the abyss beyond.

The next chapter will allow for even greater modularity over this system.

Cleaning Our Room

Before continuing, it must be noted that the main.rs file is slowly reaching critical mass with its 161 lines of code. Before it swallows the Sun, it would be wise to divide it into multiple files, using Plugins.

As an example, let's bundle up everything that has something to do with displaying things on screen into a single GraphicsPlugin.

Create a new file in src/graphics.rs. Write within:

// graphics.rs

use bevy::prelude::*;
// Note the imports from main.rs
use crate::{Player, OrdDir, Position};

pub struct GraphicsPlugin;

impl Plugin for GraphicsPlugin {
    fn build(&self, app: &mut App) {
        app.init_resource::<SpriteSheetAtlas>();
        app.add_systems(Startup, setup_camera);
        app.add_systems(Update, adjust_transforms);
    }
}

Then, add the resource and the two systems, as they appeared in Part 2 of the tutorial:

// graphics.rs
#[derive(Resource)]
pub struct SpriteSheetAtlas { // Note the pub!
    handle: Handle<TextureAtlasLayout>,
}

impl FromWorld for SpriteSheetAtlas {
    fn from_world(world: &mut World) -> Self {
        let layout = TextureAtlasLayout::from_grid(UVec2::splat(16), 8, 1, None, None);
        let mut texture_atlases = world
            .get_resource_mut::<Assets<TextureAtlasLayout>>()
            .unwrap();
        Self {
            handle: texture_atlases.add(layout),
        }
    }
}

fn setup_camera(mut commands: Commands) {
    commands.spawn(Camera2dBundle {
        transform: Transform::from_xyz(0., 0., 0.),
        ..default()
    });
}

/// Each frame, adjust every entity's display location to be offset
/// according to the player's location.
fn adjust_transforms(
    player: Query<&Position, With<Player>>,
    mut npcs: Query<(&Position, &mut Transform), Without<Player>>,
) {
    // There should only be one player on any given frame.
    let player_pos = player.get_single().expect("0 or 2+ players");
    // Get the player's position.
    let (px, py) = (player_pos.x, player_pos.y);
    // For each Position and Transform of each non-player creature...
    for (npc_pos, mut npc_tran) in npcs.iter_mut() {
        // Measure their offset distance from the player's location.
        let (off_x, off_y) = (npc_pos.x - px, npc_pos.y - py);
        // Adjust their visual position to match this offset.
        (npc_tran.translation.x, npc_tran.translation.y) = (
            // Multiplied by the graphical size of a tile, which is 64x64.
            off_x as f32 * 4. * 16.,
            off_y as f32 * 4. * 16.,
        );
    }
}

This can finally be connected to main.rs:

// main.rs
mod graphics; // NEW!

use graphics::GraphicsPlugin; // NEW!

fn main() {
    App::new()
        .add_plugins(DefaultPlugins.set(ImagePlugin::default_nearest()))
        .add_plugins(GraphicsPlugin) // NEW!
        // Note that the following have been removed:
        // - SpriteSheetAtlas
        // - setup_camera
        // - adjust_transforms
        .add_systems(Startup, spawn_player)
        .add_systems(Startup, spawn_cage)
        .add_systems(Update, keyboard_input)
        .run();
}

Note that this reorganization comes with the necessity of many import (use) statements. In the future of this tutorial, inter-file imports will no longer be represented in the code snippets. rust-analyzer offers auto-importing of unimported items as a code action, and compiler errors for this particular issue are clear and offer precise suggestions. Also remember to clean as you go, and remove unused imports marked by warnings.

I have organized the rest of the Part 2 components, bundles, systems and resources in the following way:

creature.rs (No plugin! Only struct definitions.)

• Player
• Creature

input.rs

• keyboard_input

map.rs

• Position
• spawn_player
• spawn_cage

And, as it was only just done:

graphics.rs

• SpriteSheetAtlas
• setup_camera
• adjust_transforms

We will also add pub markers to the structs and enums moved over (but not the systems). As Components and Resourcès tend to travel around quite a bit, they will often need to be imported across other Plugins. Not to worry, missing a pub will simply have the compiler complain a bit and provide a helpful error message to correct the issue, mentioning that "this struct is inaccessible".

This leads to this main() function:

// main.rs
fn main() {
    App::new()
        .add_plugins(DefaultPlugins.set(ImagePlugin::default_nearest()))
        .add_plugins((GraphicsPlugin, MapPlugin, InputPlugin))
        .run();
}

Note the tuple in the second add_plugins̀. Just as it was shown in Part 2 for commands.spawn(), many Bevy functions can take either a single item or a tuple of items as an argument!

Compile everything with cargo run to make sure all is neat and proper, and to fix potential still-private or unimported structs/struct fields.

If it works, you may notice strange black lines on the periphery of the walls:

This can happen when working with a 2D spritesheet in Bevy. To fix it, disable Multi Sample Anti-aliasing:

impl Plugin for GraphicsPlugin {
    fn build(&self, app: &mut App) {
        app.init_resource::<SpriteSheetAtlas>();
        app.insert_resource(Msaa::Off); // NEW!
        app.add_systems(Startup, setup_camera);
        app.add_systems(Update, adjust_transforms);
    }
}

Much better. If you'd like to see how the fully reorganized code looks like, check in tutorial/source_code/3-getting-chased-around/3.1-reorganized.

Detecting the Happening of Things - Events

You may remember keyboard_input and how it adjusts the Player's position:

// input.rs
// SNIP
if input.pressed(KeyCode::KeyW) {
    player.y += 1;
}
// SNIP

This is very weak programming! As the game expands, we might need to detect when the player steps on slippery goo or when it collides with another entity. We'll need to implement these checks on each possible direction to step in, have error-prone repeated code blocks, and end up with a towering heap of function arguments that looks like this:

fn dispense_functions(
    mut creatures: ParamSet<(
        Query<(&Transform, &mut Species, &mut SoulBreath, &mut AxiomEffects, 
        	&mut Animator<Transform>, &mut Position, Has<RealityAnchor>)>,
        Query<&Position>,
        Query<&Species>,
        Query<&SoulBreath>,
        Query<(&Position, &Transform), With<RealityAnchor>>,
    )>,
    mut plant: Query<&mut Plant>,
    faction: Query<&Faction>,
    check_wound: Query<Entity, With<Wounded>>,
    mut next_state: ResMut<NextState<TurnState>>,
    mut world_map: ResMut<WorldMap>,
    mut souls: Query<(&mut Animator<Transform>, &Transform, 
    	&mut TextureAtlasSprite, &mut Soul), Without<Position>>,
    ui_center: Res<CenterOfWheel>,
    time: Res<SoulRotationTimer>,
    mut events: EventWriter<LogMessage>,
    mut zoom: ResMut<ZoomInEffect>,
    mut commands: Commands,
    mut current_crea_display: ResMut<CurrentEntityInUI>,
    texture_atlas_handle: Res<SpriteSheetHandle>,
){ /* endless misery */ }

Yes, this is a real function, from one of my old (and bad) Bevy projects. We wish to avoid this. Enter: Events!

This revolution will be neatly contained in a new plugin, EventPlugin, inside a new file, events.rs. It will serve as a repository of the "actions" being taken within our game. The player taking a step is one such action of interest.

// events.rs
pub struct EventPlugin;

impl Plugin for EventPlugin {
    fn build(&self, app: &mut App) {
        app.add_event::<PlayerStep>();
    }
}

#[derive(Event)]
pub struct PlayerStep {
    pub direction: OrdDir,
}

Don't forget to link all this to main.rs.

mod creature;
mod events; // NEW!
mod graphics;
mod input;
mod map;

use bevy::prelude::*;
use events::EventPlugin; // NEW!
use graphics::GraphicsPlugin;
use input::InputPlugin;
use map::MapPlugin;

fn main() {
    App::new()
        .add_plugins(DefaultPlugins.set(ImagePlugin::default_nearest()))
        .add_plugins((EventPlugin, GraphicsPlugin, MapPlugin, InputPlugin)) // CHANGED
        .run();
}

Note the new struct: OrdDir, short for "Ordinal Direction". This will be a very common enum throughout the game's code - so common, in fact, that I have opted to place it within ̀main.rs̀̀̀. This is personal preference and it could have very well been integrated into one of the plugins.

// main.rs
#[derive(PartialEq, Eq, Copy, Clone, Debug)]
pub enum OrdDir {
    Up,
    Right,
    Down,
    Left,
}

impl OrdDir {
    pub fn as_offset(self) -> (i32, i32) {
        let (x, y) = match self {
            OrdDir::Up => (0, 1),
            OrdDir::Right => (1, 0),
            OrdDir::Down => (0, -1),
            OrdDir::Left => (-1, 0),
        };
        (x, y)
    }
}

And, at last, the very first ̀Event-based system can be implemented:

// events.rs
fn player_step(
    // Incoming events must be read with an EventReader.
    mut events: EventReader<PlayerStep>,
    // Fetch the Position of the Player.
    mut player: Query<&mut Position, With<Player>>,
) {
    // There should only be one player.
    let mut player_pos = player.get_single_mut().expect("0 or 2+ players");
    // Unpack the event queue - not that it will be very long in this case!
    for event in events.read() {
        // Calculate how to modify the player's Position from the OrdDir.
        let (off_x, off_y) = event.direction.as_offset();
        // Change the player's position.
        player_pos.shift(off_x, off_y);
    }
}

Register it.

// events.rs
impl Plugin for EventPlugin {
    fn build(&self, app: &mut App) {
        app.add_event::<PlayerStep>();
        app.add_systems(Update, player_step); // NEW!
    }
}

First, note the EventReader argument, which is a requirement to unpack the contents of received Event̀s, which are getting produced by... nothing at the moment. An EventReader, of course, needs a companion EventWriter. This is how the previously unwieldy keyboard_input system can be reworked!

// input.rs
fn keyboard_input(
    mut events: EventWriter<PlayerStep>,
    input: Res<ButtonInput<KeyCode>>,
) {
    if input.pressed(KeyCode::KeyW) {
        events.send(PlayerStep {
            direction: OrdDir::Up,
        });
    }
    if input.pressed(KeyCode::KeyD) {
        events.send(PlayerStep {
            direction: OrdDir::Right,
        });
    }
    if input.pressed(KeyCode::KeyA) {
        events.send(PlayerStep {
            direction: OrdDir::Left,
        });
    }
    if input.pressed(KeyCode::KeyS) {
        events.send(PlayerStep {
            direction: OrdDir::Down,
        });
    }
}

Instead of this system handling the player's motion - and being responsible for the implementation of all the subtleties that may imply, the heavy work is now all offshored to an Event specialized in handling this task!

cargo ruǹ's results should be fairly disappointing - as, from a non-developer perspective, nothing about the game has fundamentally changed - at least not our ability to phase through walls at lightspeed. However, our codebase will be much more extensible for the near future - not to mention that this Event is only the first of many.

Enforcing Basic Physics - Collisions & The Map

A wall should wall things. It's in the name.

There are multiple ways to implement this - the simplest would be to query every single creature with a Position on the player's move, check if any of them occupies the destination tile, and abort the move if that's the case. Computers today are decently fast, but that is still a very naive implementation.

The alternative is to keep a tidy phone book of everyone's location! Enter - the Map Resource.

// map.rs
/// The position of every creature, updated automatically.
#[derive(Resource)]
pub struct Map {
    pub creatures: HashMap<Position, Entity>,
}

impl Map {
    /// Which creature stands on a certain tile?
    pub fn get_entity_at(&self, x: i32, y: i32) -> Option<&Entity> {
        self.creatures.get(&Position::new(x, y))
    }

    /// Is this tile passable?
    pub fn is_passable(&self, x: i32, y: i32) -> bool {
        self.get_entity_at(x, y).is_none()
    }
}

It's a HashMap which contains only entries where a creature exists, which gives it the ability to fetch whoever is standing on, say, (27, 4) in record time with no ̀Query or iterating over entities required!

When importing the HashMap, I suggest using the use bevy::utils::HashMap instead of Rust's std implementation. The Bevy version bases itself off of hashbrown, which is weaker to flooding hacks but more performant - an interesting characteristic for game development, unless one is making the next CIA agent training simulator.

Don't forget to register this new Resourcè.

// map.rs
pub struct MapPlugin;

impl Plugin for MapPlugin {
    fn build(&self, app: &mut App) {
        // NEW!
        app.insert_resource(Map {
            creatures: HashMap::new(),
        });
        // End NEW.
        app.add_systems(Startup, spawn_player);
        app.add_systems(Startup, spawn_cage);
    }
}

It's now possible to test the waters before venturing into a new tile, thus avoiding any further phasing incidents.

// events.rs
fn player_step(
    mut events: EventReader<PlayerStep>,
    mut player: Query<&mut Position, With<Player>>,
    map: Res<Map>,
) {
    let mut player_pos = player.get_single_mut().expect("0 or 2+ players");
    for event in events.read() {
        let (off_x, off_y) = event.direction.as_offset();
        // REPLACES player_pos.shift(off_x, off_y)
        // Get the destination tile.
        let destination = Position::new(player_pos.x + off_x, player_pos.y + off_y);
        // Check if the destination tile is empty.
        if map.is_passable(destination.x, destination.y) {
            // If yes, authorize the move.
            player_pos.shift(off_x, off_y);
        }
        // End REPLACES.
    }
}

Don't cargo run just yet! Our ̀Map is completely empty and unaware of the existence of walls. This can be fixed with a single new system.

// map.rs
/// Newly spawned creatures earn their place in the HashMap.
fn register_creatures(
    mut map: ResMut<Map>,
    // Any entity that has a Position that just got added to it -
    // currently only possible as a result of having just been spawned in.
    displaced_creatures: Query<(&Position, Entity), Added<Position>>,
) {
    for (position, entity) in displaced_creatures.iter() {
        // Insert the new creature in the Map. Position implements Copy,
        // so it can be dereferenced (*), but `.clone()` would have been
        // fine too.
        map.creatures.insert(*position, entity);
    }
}

The most unique part about this new system is the ̀Added filter, which fetches only entities who have newly received the Position component and not been handled by this system yet. Right now, it means all newly created creatures will be processed by this system once, and then ignored afterwards.

Register it.

// map.rs
pub struct MapPlugin;

impl Plugin for MapPlugin {
    fn build(&self, app: &mut App) {
        app.insert_resource(Map {
            creatures: HashMap::new(),
        });
        app.add_systems(Startup, spawn_player);
        app.add_systems(Startup, spawn_cage);
        app.add_systems(Update, register_creatures); // NEW!
    }
}

Activate cargo run... and the walls finally have tangibility!

A Very Sticky Critter - The First NPC

It's about time the Player got some company. Not a particularly affable one, I must admit, but we all start from somewhere.

// map.rs, spawn_cage
let cage = "##########H......##.......##.......##.......##.......##.......##.......##########";

Edit the wall placement string to include a (H)unter. Yes, this is messy - a proper map generator will be the topic of a future chapter.

This Hunter also earns itself a separate sprite:

// map.rs, spawn_cage
let position = Position::new(idx as i32 % 9, idx as i32 / 9);
let index = match tile_char {
    '#' => 3,
    'H' => 4, // NEW!
    _ => continue,
};

And the ability to be differentiated from walls, with a new Hunt component...

// creature.rs
#[derive(Component)]
pub struct Hunt;

...added to any 'H' character in the initial spawn function.

// map.rs, spawn_cage
let mut creature = commands.spawn(Creature { // CHANGED - note the variable assignment
    position,
    sprite: SpriteBundle {
        texture: asset_server.load("spritesheet.png"),
        transform: Transform::from_scale(Vec3::new(4., 4., 0.)),
        ..default()
    },
    atlas: TextureAtlas {
        layout: atlas_layout.handle.clone(),
        index,
    },
});
if tile_char == 'H' {
    creature.insert(Hunt);
}

cargo run, and our new companion is here. Excellent. Now, to give it motion of its own...

The first problem is that motion, in our game, is currently only supported by player_step, which solely refers to the player character and nothing else. There should be a more generic Event, capable of controlling absolutely any creature to move around...

// events.rs
#[derive(Event)]
struct TeleportEntity {
    destination: Position,
    entity: Entity,
}

impl TeleportEntity {
    fn new(entity: Entity, x: i32, y: i32) -> Self {
        Self {
            destination: Position::new(x, y),
            entity,
        }
    }
}

Its matching system has a lot of similarity to player_step.

// events.rs
fn teleport_entity(
    mut events: EventReader<TeleportEntity>,
    mut creature: Query<&mut Position>,
    map: Res<Map>,
) {
    for event in events.read() {
        let mut creature_position = creature
            // Get the Position of the Entity targeted by TeleportEntity.
            .get_mut(event.entity)
            .expect("A TeleportEntity was given an invalid entity");
        // If motion is possible...
        if map.is_passable(event.destination.x, event.destination.y) {
            // ...move that Entity to TeleportEntity's destination tile.
            creature_position.update(event.destination.x, event.destination.y);
        } else {
            // Nothing here just yet, but this is where collisions between creatures
            // will be handled.
            continue;
        }
    }
}

Don't forget to register all of this...

// events.rs
impl Plugin for EventPlugin {
    fn build(&self, app: &mut App) {
        app.add_event::<PlayerStep>();
        app.add_event::<TeleportEntity>(); // NEW!
        app.add_systems(Update, player_step);
        app.add_systems(Update, teleport_entity); // NEW!
    }
}

...and, of course, to actually use it in player_step so all entity motion of any kind is handled by this specialized system.

// events.rs
fn player_step(
    mut events: EventReader<PlayerStep>,
    mut teleporter: EventWriter<TeleportEntity>, // NEW!
    // CHANGED, no longer needs mutable access, and also fetches the Entity component.
    player: Query<(Entity, &Position), With<Player>>,
) {
    // CHANGED, no longer needs mutable access, and also fetches the Entity component.
    let (player_entity, player_pos) = player.get_single().expect("0 or 2+ players");
    for event in events.read() {
        let (off_x, off_y) = event.direction.as_offset();
        // CHANGED, Send the event to TeleportEntity instead of handling the motion directly.
        teleporter.send(TeleportEntity::new(
            player_entity,
            player_pos.x + off_x,
            player_pos.y + off_y,
        ));
    }
}

And there we go! player_step is now only an intermediate point leading to a central teleport_entity system, which can handle any and all creature motion. This means every creature will be on the same footing, with no repeated code!

Just like when player_step was first added, cargo run on this will not change gameplay whatsoever. However, all this has finally allowed us to gift motion to our new Hunter.

First, define a very naive "algorithm" to move towards a point on the map. Start with this helper function to calculate a distance between two points:

// map.rs
fn manhattan_distance(a: Position, b: Position) -> i32 {
    (a.x - b.x).abs() + (a.y - b.y).abs()
}

And then, a way to find the best move among all four orthogonal options:

// map.rs
impl Map {

    // SNIP - all other impl Map functions
    
    /// Find all adjacent accessible tiles to start, and pick the one closest to end.
    pub fn best_manhattan_move(&self, start: Position, end: Position) -> Option<Position> {
        let mut options = [
            Position::new(start.x, start.y + 1),
            Position::new(start.x, start.y - 1),
            Position::new(start.x + 1, start.y),
            Position::new(start.x - 1, start.y),
        ];

        // Sort all candidate tiles by their distance to the `end` destination.
        options.sort_by(|&a, &b| manhattan_distance(a, end).cmp(&manhattan_distance(b, end)));

        options
            .iter()
            // Only keep either the destination or unblocked tiles.
            .filter(|&p| *p == end || self.is_passable(p.x, p.y))
            // Remove the borrow.
            .copied()
            // Get the tile that manages to close the most distance to the destination.
            // If it exists, that is. Otherwise, this is just a None.
            .next()
    }
}

Finally, implement that Hunt implies chasing the player around.

// events.rs
fn player_step(
    mut events: EventReader<PlayerStep>,
    mut teleporter: EventWriter<TeleportEntity>,
    player: Query<(Entity, &Position), With<Player>>,
    hunters: Query<(Entity, &Position), With<Hunt>>, // NEW!
    map: Res<Map>, // NEW! Bringing back the map, so "pathfinding" can be done.
) {
    let (player_entity, player_pos) = player.get_single().expect("0 or 2+ players");
    for event in events.read() {
        let (off_x, off_y) = event.direction.as_offset();
        teleporter.send(TeleportEntity::new(
            player_entity,
            player_pos.x + off_x,
            player_pos.y + off_y,
        ));

        // NEW!
        for (hunter_entity, hunter_pos) in hunters.iter() {
            // Try to find a tile that gets the hunter closer to the player.
            if let Some(move_target) = map.best_manhattan_move(*hunter_pos, *player_pos) {
                // If it is found, cause another TeleportEntity event.
                teleporter.send(TeleportEntity {
                    destination: move_target,
                    entity: hunter_entity,
                });
            }
        }
        // End NEW.
    }
}

cargo run, and let the hunt begin!

There is only the slight issue that our Hunter is rather on the incorporeal side of things. Indeed, as it moves, the Map fails to update and the Hunter is still considered to have phantasmatically remained in its spawn location. Not to mention that the centre of the cage, where we spawned, is also mysteriously blocked by an invisible wall.

There exists another filter like Added, named Changed, which triggers whenever a specified component is not only added for the first time, but also when an already existing instance is modified - such as in the case of moving a creature around. However, using it would be unwise. Here is why - the following happen in order:

• The user presses a button on their keyboard to move.
• PlayerStep is triggered. Two TeleportEntity are sent out.
• The Player's TeleportEntity happens first, moving the Player onto coordinates (2, 3). The Map is NOT updated yet, because it is located in a different system (register_creatures), and ̀teleport_entity isn't done yet, as it has another event to get through.
• The Hunter's TeleportEntity happens, moving the Hunter onto coordinates (2, 3) too! This appears to be a legal move to the game, because the Map̀ hadn't been updated yet.
• teleport_entity is done, and register_creatures triggers, editing Map to "knock out" the Player and leave only the Hunter, while the Player is now off the Map and completely untargetable.

To fix this, we need to modify the Map immediately after a creature moves. Leave register_creatures set to Added, and instead, modify teleport_entity:

// events.rs
fn teleport_entity(
    mut events: EventReader<TeleportEntity>,
    mut creature: Query<&mut Position>,
    mut map: ResMut<Map>, // CHANGED, this needs mutability now.
) {
    for event in events.read() {
        let mut creature_position = creature
            .get_mut(event.entity)
            .expect("A TeleportEntity was given an invalid entity");
        if map.is_passable(event.destination.x, event.destination.y) {
            map.move_creature(*creature_position, event.destination); // NEW!
            creature_position.update(event.destination.x, event.destination.y);
        } else {
            continue;
        }
    }
}

map.move_creature is a new impl Map function.

// map.rs
impl Map {
    /// Move a pre-existing entity around the Map.
    pub fn move_creature(&mut self, old_pos: Position, new_pos: Position) {
        // As the entity already existed in the Map's records, remove it.
        let entity = self.creatures.remove(&old_pos).expect(&format!(
            "The map cannot move a nonexistent Entity from {:?} to {:?}.",
            old_pos, new_pos
        ));
        self.creatures.insert(new_pos, entity);
    }
}

And with that, everything is going according to plan.

The next chapter of this tutorial will introduce... something to do that isn't moving around in aimless circles.