Bevy Platformer (thin router)

Genre router. Domain work lives in the Bevy skills below.

When to use

Load this to route a platformer to movement, camera, and tiles.

ECS shape

• Components hold the genre's per-entity flags as plain data. This file does not name new engine types.
• Resources hold run-wide counters (score, wave index, seed) via bevy-data-resources.
• Systems read those components on Update or the fixed timestep from bevy-master. Order them with .chain() when a later system must see the result.
• Schedule: gameplay mode changes go through bevy-state-machines. Do not invent a genre plugin.

Routing

• bevy-character-movement - Locomotion.
• bevy-camera-systems - Follow camera.
• bevy-tilemap - Ground tiles.
• bevy-2d-animation - Sprite clips.

Examples

[dependencies]
bevy = "0.20"

Coyote time and the jump buffer live on the mover. Gravity and the jump write velocity in one system so they cannot race.

#[derive(Component)]
struct Mover {
    velocity: Vec2,
    grounded: bool,
    coyote: f32,
    jump_buffer: f32,
}

fn arm_jump(keys: Res<ButtonInput<KeyCode>>, mut movers: Query<&mut Mover>) {
    if !keys.just_pressed(KeyCode::Space) {
        return;
    }
    for mut mover in &mut movers {
        mover.jump_buffer = 0.15;
    }
}

fn feel(mut movers: Query<&mut Mover>, time: Res<Time<Fixed>>) {
    let dt = time.delta_secs();
    for mut mover in &mut movers {
        if mover.grounded {
            mover.coyote = 0.12;
        } else {
            mover.coyote = (mover.coyote - dt).max(0.0);
        }
        mover.jump_buffer = (mover.jump_buffer - dt).max(0.0);
        if mover.jump_buffer > 0.0 && mover.coyote > 0.0 {
            mover.velocity.y = 420.0;
            mover.coyote = 0.0;
            mover.jump_buffer = 0.0;
            mover.grounded = false;
        }
        if !mover.grounded {
            let gravity = if mover.velocity.y < 0.0 { 2200.0 } else { 1400.0 };
            mover.velocity.y -= gravity * dt;
        }
    }
}

fn install_feel(app: &mut App) {
    app.add_systems(Update, arm_jump)
        .add_systems(FixedUpdate, feel);
}

arm_jump and feel both write Mover, and they sit on different schedules. If that borrow conflicts, move arm_jump onto FixedUpdate and .chain() it before feel.

Integration reads Mover and writes Transform. Chain it after feel.

#[derive(Component)]
struct Mover {
    velocity: Vec2,
}

fn integrate(mut bodies: Query<(&Mover, &mut Transform)>, time: Res<Time<Fixed>>) {
    let dt = time.delta_secs();
    for (mover, mut tf) in &mut bodies {
        tf.translation.x += mover.velocity.x * dt;
        tf.translation.y += mover.velocity.y * dt;
    }
}

A one-way ledge is a component with a top height. The mover only snaps when it is falling.

#[derive(Component)]
struct OneWay {
    half_width: f32,
    top: f32,
}

#[derive(Component)]
struct Mover {
    velocity: Vec2,
    grounded: bool,
}

fn land(
    mut bodies: Query<(&mut Mover, &Transform)>,
    platforms: Query<(&OneWay, &Transform)>,
) {
    for (mut mover, body_tf) in &mut bodies {
        if mover.velocity.y > 0.0 {
            continue;
        }
        let feet = body_tf.translation.truncate();
        for (plat, plat_tf) in &platforms {
            let top = plat_tf.translation.y + plat.top;
            let dx = (feet.x - plat_tf.translation.x).abs();
            if dx <= plat.half_width && (feet.y - top).abs() < 6.0 {
                mover.grounded = true;
                mover.velocity.y = 0.0;
            }
        }
    }
}

The camera copies the player translation and keeps its own z.

#[derive(Component)]
struct Player;

fn follow(
    player: Query<&Transform, With<Player>>,
    mut cameras: Query<&mut Transform, (With<Camera2d>, Without<Player>)>,
) {
    let Ok(player_tf) = player.single() else {
        return;
    };
    let Ok(mut cam) = cameras.single_mut() else {
        return;
    };
    cam.translation.x = player_tf.translation.x;
    cam.translation.y = player_tf.translation.y;
}

Scope

• Tile collision and sprite clips route to bevy-tilemap and bevy-2d-animation.
• land and feel both write Mover. Chain them on FixedUpdate when both are installed.

Expert Knowledge Delta

Target: Bevy 0.20+.

Core ECS types: Component, Resource, Query, Commands, Startup, Update, Fixed / FixedMain (bevy::time), .chain(), Plugin, DefaultPlugins, MinimalPlugins. Messages and observers are an ownership slot. See bevy-events-messaging for their APIs. Systems run in parallel unless .chain() orders the ones that conflict.

Who owns state

Piece	Owner	Who may mutate	Schedule
Data on one entity	Component	One system via Query	Update or FixedMain
One value for the whole app	Resource	One system	Startup to insert, then the schedule that reads it
Spawn	Commands	The spawning system	Startup for the initial world; later schedules for runtime spawn
Order that must not race	.chain()	The chained systems only	Same schedule
Plugin boundary	Plugin	build registers systems and resources	Before the app runs

Borrow rule: two systems that both mutably query the same component must be ordered (.chain()) or split so only one writes. Unrelated systems stay parallel. That is the ECS page rule, not a guess at a system-set API.

Decision

If you need	Do this	Do not
Per-entity data	Component + Query	A field stored off the entity
App-wide config or clock	Resource	Copy the value onto every entity
Stable simulation step	FixedMain and the Fixed clock	Tie simulation to raw frame Update
Startup spawn	Commands in Startup	Spawn from a render system
Cross-system notice that is not shared state	Message or observer slot in bevy-events-messaging	A new event type invented in this file
An API outside the documented set	Use the examples in this skill	A made-up crate or API

Load references/ecs-ownership.md for the ownership table.

NEVER

• Do not pin /0.20.0/ in docs URLs. WHY: links stay on latest; the 0.20 target is stated once in prose.

• Do not mutably query the same component from two unordered systems. WHY: the schedule runs systems in parallel unless .chain() orders them.

• Do not invent a physics, navigation, networking, or XR crate here. WHY: this skill only names documented Bevy crates and modules.

• Do not apply gravity on Update while coyote and the jump buffer tick on FixedUpdate. WHY: frame time then changes jump height.

• Do not write Mover from arm_jump and feel on two schedules. WHY: those systems alias the same component; put both on FixedUpdate and .chain() arm_jump before feel.

• No invented genre APIs. WHY: Bevy code stays idiomatic ECS and uses only documented Bevy APIs.

• Do not pick a physics or net crate here. WHY: Bevy code stays idiomatic ECS and uses only documented Bevy APIs.

Reference

The library pin on bevy-master stays 0.20.0. Official pages stay on the unversioned latest branch.

Official Documentation

• Assets - Official asset catalog. No third-party crate was selected.
• Examples - Example tree on the latest branch.
• ECS - Component, systems, Query, Startup, Update, and .chain().
• Plugins - Plugin, DefaultPlugins, MinimalPlugins, and add_plugins.

Related Skills

Prerequisites

• bevy-master - ECS ownership and schedules.
• bevy-ecs-composition - Spawn and query shape.

Complements

• godot-genre-platformer - Platformer on the Godot engine, same depth.
• bevy-character-movement - Locomotion.
• bevy-camera-systems - Follow camera.
• bevy-tilemap - Ground tiles.
• bevy-2d-animation - Sprite clips.

Downstream / consumers

• None. This router does not own a second implementation.

Master

• bevy-master - Bevy router.