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.