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.