Bevy Tweening Agent Skill

Bevy tweening

When to use

Load this when a value should ease over time. A tween player is yours to build in ECS from elapsed time.

ECS shape

• Feature: EasingCurve samples an ease between two values. It is not a player that stores entity state.
• Components: a tween you own: start, end, elapsed, and duration.
• Systems: step elapsed time on Update with Time::delta_secs, then write Transform.
• Animation: property animation over a clip is AnimationClip on bevy-animation. That is a clip, not a one-shot UI tween.

[dependencies]
bevy = "0.20"

One system advances elapsed time. The eased t is EasingCurve::sample_clamped. Finished tweens are despawned by the marker, not by a callback type.

use bevy::curve::Curve;
use bevy::curve::easing::{EaseFunction, EasingCurve};

#[derive(Component)]
struct Slide {
    start: Vec3,
    end: Vec3,
    elapsed: f32,
    duration: f32,
}

fn spawn_slide(mut commands: Commands) {
    commands.spawn((
        Transform::from_translation(Vec3::ZERO),
        Slide {
            start: Vec3::ZERO,
            end: Vec3::new(2.0, 1.0, 0.0),
            elapsed: 0.0,
            duration: 0.6,
        },
    ));
}

fn step_slides(
    time: Res<Time>,
    mut slides: Query<(Entity, &mut Slide, &mut Transform)>,
    mut commands: Commands,
) {
    let curve = EasingCurve::new(0.0, 1.0, EaseFunction::QuadraticOut);
    for (entity, mut slide, mut transform) in &mut slides {
        slide.elapsed = (slide.elapsed + time.delta_secs()).min(slide.duration);
        let t = if slide.duration <= 0.0 {
            1.0
        } else {
            slide.elapsed / slide.duration
        };
        let eased = curve.sample_clamped(t);
        transform.translation = slide.start.lerp(slide.end, eased);
        if slide.elapsed >= slide.duration {
            commands.entity(entity).remove::<Slide>();
        }
    }
}

pub struct JuicePlugin;

impl Plugin for JuicePlugin {
    fn build(&self, app: &mut App) {
        app.add_systems(Startup, spawn_slide)
            .add_systems(Update, step_slides);
    }
}

A punch scale uses the same clock and a different ease. When elapsed reaches duration, the marker comes off so later systems stop writing that transform.

use bevy::curve::Curve;
use bevy::curve::easing::{EaseFunction, EasingCurve};

#[derive(Component)]
struct Punch {
    elapsed: f32,
    duration: f32,
    amount: f32,
}

fn step_punch(time: Res<Time>, mut punches: Query<(Entity, &mut Punch, &mut Transform)>, mut commands: Commands) {
    let curve = EasingCurve::new(0.0, 1.0, EaseFunction::CubicOut);
    for (entity, mut punch, mut transform) in &mut punches {
        punch.elapsed = (punch.elapsed + time.delta_secs()).min(punch.duration);
        let t = punch.elapsed / punch.duration.max(0.0001);
        let eased = curve.sample_clamped(t);
        let scale = 1.0 + punch.amount * (1.0 - eased);
        transform.scale = Vec3::splat(scale);
        if punch.elapsed >= punch.duration {
            commands.entity(entity).remove::<Punch>();
        }
    }
}

Delay then move is two components. The delay system inserts Slide only after its timer ends, so the order is data, not a sequence asset.

#[derive(Component)]
struct Slide {
    start: Vec3,
    end: Vec3,
    elapsed: f32,
    duration: f32,
}

#[derive(Component)]
struct Wait {
    remaining: f32,
    end: Vec3,
}

fn release_waits(time: Res<Time>, mut waits: Query<(Entity, &mut Wait)>, mut commands: Commands) {
    for (entity, mut wait) in &mut waits {
        wait.remaining -= time.delta_secs();
        if wait.remaining <= 0.0 {
            commands.entity(entity).insert(Slide {
                start: Vec3::ZERO,
                end: wait.end,
                elapsed: 0.0,
                duration: 0.4,
            });
            commands.entity(entity).remove::<Wait>();
        }
    }
}

Choose the owner

| Need | Owner |

| --- | --- |

| Property tween | Slide plus EasingCurve writing Transform. |

| Sequence | A Wait component that inserts Slide when it elapses. |

| Finish | Remove the marker when elapsed reaches duration. Clip events stay on bevy-animation. |

Scope

• EasingCurve::sample_clamped is the ease used above. EaseFunction::QuadraticOut and EaseFunction::CubicOut are two of the built-in curves.
• The player is the Slide / Punch / Wait components. A third-party tween crate is optional and is not required for this pattern.

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 value that is not a component |

| 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 Tween node. WHY: Bevy code stays idiomatic ECS and uses only documented Bevy APIs.
• No named tween crate as the chosen solution. 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 Documentation

• Assets - Official asset catalog.
• 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-animation - Clip-based property animation.
• bevy-master - Fixed clock name.

Complements

• godot-tweening - Equivalent Godot skill.

Downstream / consumers

• None yet.

Master

• bevy-master - Bevy router.