Bevy Game Loop Harvest Agent Skill

Bevy harvest loop

When to use

Load this when crops advance on a timer and a harvest message records the yield.

ECS shape

• Components: growth elapsed and a stage index you define.
• Resources: the last visit and the season scale live in resources (bevy-data-resources).
• Systems: step growth on FixedUpdate. Use .chain() so harvest reads the stage growth just wrote.
• Messages: Harvested is the one-shot yield fact. Inventory applies it in bevy-inventory-system.

Idioms

| Need | Bevy shape |

| --- | --- |

| Growth | Elapsed time on a component, stepped on the fixed clock. |

| Extract | A Harvested message. Inventory consumes it. |

| Season | A resource scale. No calendar type was opened. |

Scope

• A crop component stores stage and elapsed time. Growth steps on Time<Fixed>.
• Offline catch-up reads Time::elapsed_secs against a resource that remembers the last visit.

Examples

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

[dependencies]
bevy = "0.20"

Growth advances one stage per interval. Harvest only fires when the last stage is ready.

#[derive(Component)]
struct Crop {
    stage: u8,
    elapsed: f32,
    stage_seconds: f32,
    stages: u8,
}

#[derive(Message)]
struct Harvested {
    plot: Entity,
    yield_item: u32,
}

fn grow(time: Res<Time<Fixed>>, mut crops: Query<&mut Crop>) {
    let dt = time.delta_secs();
    for mut crop in &mut crops {
        if crop.stage + 1 >= crop.stages {
            continue;
        }
        crop.elapsed += dt;
        if crop.elapsed >= crop.stage_seconds {
            crop.elapsed = 0.0;
            crop.stage += 1;
        }
    }
}

fn harvest(
    mut commands: Commands,
    crops: Query<(Entity, &Crop)>,
    mut harvested: MessageWriter<Harvested>,
) {
    for (entity, crop) in &crops {
        if crop.stage + 1 < crop.stages {
            continue;
        }
        harvested.write(Harvested {
            plot: entity,
            yield_item: 7,
        });
        commands.entity(entity).despawn();
    }
}

pub struct HarvestPlugin;

impl Plugin for HarvestPlugin {
    fn build(&self, app: &mut App) {
        app.add_message::<Harvested>()
            .add_systems(FixedUpdate, (grow, harvest).chain());
    }
}

Catch-up applies the seconds since the last recorded visit, then stores the new mark.

#[derive(Component)]
struct Crop {
    stage: u8,
    elapsed: f32,
    stage_seconds: f32,
    stages: u8,
}

#[derive(Resource)]
struct LastVisit {
    elapsed_secs: f32,
}

fn catch_up(time: Res<Time>, mut last: ResMut<LastVisit>, mut crops: Query<&mut Crop>) {
    let now = time.elapsed_secs();
    let gap = (now - last.elapsed_secs).max(0.0);
    last.elapsed_secs = now;
    if gap <= 0.0 {
        return;
    }
    for mut crop in &mut crops {
        let mut left = gap;
        while left > 0.0 && crop.stage + 1 < crop.stages {
            let need = (crop.stage_seconds - crop.elapsed).max(0.0);
            if left < need {
                crop.elapsed += left;
                left = 0.0;
            } else {
                left -= need;
                crop.elapsed = 0.0;
                crop.stage += 1;
            }
        }
    }
}

A season resource scales how fast every crop in the run advances.

#[derive(Resource)]
struct Season {
    growth_scale: f32,
}

#[derive(Component)]
struct Crop {
    stage: u8,
    elapsed: f32,
    stage_seconds: f32,
    stages: u8,
}

fn season_grow(time: Res<Time<Fixed>>, season: Res<Season>, mut crops: Query<&mut Crop>) {
    let dt = time.delta_secs() * season.growth_scale.max(0.0);
    for mut crop in &mut crops {
        if crop.stage + 1 >= crop.stages {
            continue;
        }
        crop.elapsed += dt;
        if crop.elapsed >= crop.stage_seconds {
            crop.elapsed -= crop.stage_seconds;
            crop.stage += 1;
        }
    }
}

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 scene-tree script on the actor |

| 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 | Read the official manual linked below | 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 scene-tree node or signal bus for this mechanic. 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.

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-inventory-system - Where the yield goes.
• bevy-data-resources - Tables and definitions.
• bevy-events-messaging - One-shot facts.

Complements

• bevy-state-machines - Mode flags.

Downstream / consumers

• None yet.

Master

• bevy-master - Bevy router.