Bevy Genre Platformer Agent Skill

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.