Bevy Mechanic Revival Agent Skill

Bevy revival mechanic

When to use

Load this when an actor is downed, can be revived, or respawns.

ECS shape

• Components: a downed flag and a revive elapsed you define.
• Resources: run-wide tables live in a resource (bevy-data-resources), not an autoload.
• Systems: mutate those components on Update or the fixed timestep. Use .chain() when damage must land before death.
• Messages: one-shot facts (hit, pickup, quest advance) go through bevy-events-messaging. This file does not add a new event type name.

Scope

Downed time, revive progress, and the last checkpoint are components. Overlap is a distance test here; a physics solver can replace that test later.

Examples

[package]
name = "revival"
version = "0.1.0"
edition = "2021"

[dependencies]
bevy = "0.20"

Health hits zero, the actor is downed, and an ally within range fills the revive bar. A full bar clears the downed flag and restores health.

#[derive(Component)]
struct Health {
    current: f32,
    max: f32,
}

#[derive(Component)]
struct Downed {
    elapsed: f32,
}

#[derive(Component)]
struct Reviving {
    progress: f32,
    needed: f32,
}

#[derive(Component)]
struct Ally;

fn apply_down(
    mut commands: Commands,
    mut actors: Query<(Entity, &Health), Without<Downed>>,
) {
    for (entity, health) in &mut actors {
        if health.current <= 0.0 {
            commands.entity(entity).insert(Downed { elapsed: 0.0 });
        }
    }
}

fn tick_revive(
    time: Res<Time<Fixed>>,
    allies: Query<&Transform, With<Ally>>,
    mut downed: Query<(Entity, &Transform, &mut Health, &mut Downed, Option<&mut Reviving>)>,
    mut commands: Commands,
) {
    let dt = time.delta_secs();
    for (entity, transform, mut health, mut downed, reviving) in &mut downed {
        downed.elapsed += dt;
        let near = allies.iter().any(|ally| {
            ally.translation.distance(transform.translation) < 2.0
        });
        if !near {
            commands.entity(entity).remove::<Reviving>();
            continue;
        }
        let progress = reviving.map(|mut rev| {
            rev.progress += dt;
            rev.progress
        });
        if let Some(progress) = progress {
            if progress >= 2.0 {
                health.current = health.max;
                commands.entity(entity).remove::<Downed>().remove::<Reviving>();
            }
        } else {
            commands.entity(entity).insert(Reviving {
                progress: 0.0,
                needed: 2.0,
            });
        }
    }
}

The checkpoint resource stores the respawn pose. Death after the bleed-out moves the actor there and refills health.

#[derive(Resource)]
struct Checkpoint {
    translation: Vec3,
}

#[derive(Component)]
struct Health {
    current: f32,
    max: f32,
}

#[derive(Component)]
struct Downed {
    elapsed: f32,
}

fn move_to_checkpoint(
    mut query: Query<(Entity, &mut Transform, &mut Health, &Downed)>,
    checkpoint: Res<Checkpoint>,
    mut commands: Commands,
) {
    for (entity, mut transform, mut health, downed) in &mut query {
        if downed.elapsed < 8.0 {
            continue;
        }
        transform.translation = checkpoint.translation;
        health.current = health.max;
        commands.entity(entity).remove::<Downed>();
    }
}

pub struct RevivalPlugin;

impl Plugin for RevivalPlugin {
    fn build(&self, app: &mut App) {
        app.insert_resource(Checkpoint {
            translation: Vec3::ZERO,
        })
        .add_systems(FixedUpdate, move_to_checkpoint);
    }
}

Touching a marker overwrites the checkpoint.

#[derive(Component)]
struct CheckpointMarker;

#[derive(Component)]
struct Player;

#[derive(Resource)]
struct Checkpoint {
    translation: Vec3,
}

fn capture_checkpoint(
    players: Query<&Transform, With<Player>>,
    markers: Query<&Transform, With<CheckpointMarker>>,
    mut checkpoint: ResMut<Checkpoint>,
) {
    let Ok(player) = players.single() else {
        return;
    };
    for marker in &markers {
        if player.translation.distance(marker.translation) < 1.2 {
            checkpoint.translation = marker.translation;
        }
    }
}

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 Godot node script |

| 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 | Check the official docs first | 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.

• No Godot node, resource, or signal types for this mechanic. WHY: Bevy code stays idiomatic ECS and uses only documented Bevy APIs.

• No Godot types in Bevy code. WHY: Bevy code stays idiomatic ECS and uses only documented Bevy APIs.
• No invented APIs or unnamed third-party crates as the chosen solver. 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 - Hub and schedules.
• bevy-ecs-composition - Components and systems.
• bevy-combat-system - What drops health to zero.
• bevy-data-resources - Tables and definitions.
• bevy-events-messaging - One-shot facts.

Complements

• bevy-state-machines - Mode flags.
• godot-mechanic-revival - Revival on the Godot engine, same depth.

Downstream / consumers

• None yet.

Master

• bevy-master - Bevy router.