Updaters

Updaters are callbacks that run every frame. They let you drive tattva properties with arbitrary logic — useful for reactive layouts, physics-like motion, and procedural animation that doesn't fit the timeline model.

Adding an updater

scene.add_updater(tattva_id, |scene, id, dt| {
    // dt = delta time in seconds since last frame
    let t = scene.scene_time;
    scene.set_position_2d(id, glam::Vec2::new(t.sin() * 3.0, t.cos() * 2.0));
});

The closure receives:

• scene — mutable reference to the full scene
• id — the TattvaId this updater is attached to
• dt — frame delta time in seconds

Circular motion example

let radius = 3.0;
let speed = 1.0; // radians per second

scene.add_updater(particle_id, move |scene, id, _dt| {
    let t = scene.scene_time * speed;
    scene.set_position_2d(id, glam::Vec2::new(t.cos() * radius, t.sin() * radius));
});

Reactive tattvas

One tattva can read another's state and respond to it. This is how force field visualizations work — each vector arrow reads the particle position and updates its own rotation:

scene.add_updater(vector_id, move |scene, vid, _dt| {
    if let Some(source) = scene.get_tattva_any(particle_id) {
        let props = murali::frontend::props::DrawableProps::read(source.props());
        let particle_pos = props.position;
        drop(props);

        let delta = vector_pos - particle_pos;
        let angle = delta.y.atan2(delta.x);

        scene.set_rotation(vid, glam::Quat::from_rotation_z(angle));
    }
});

Updaters vs timeline animations

	Timeline	Updater
Scheduling	Time-based, declarative	Runs every frame
Use case	Defined start/end animations	Reactive, physics, procedural
Easing	Built-in	Manual
Seeking	Supported or explicitly rejected	Returns SeekError

Use timeline animations for anything with a clear start and end. Use updaters for continuous reactive behavior.

Updaters receive frame dt, so Murali cannot reconstruct them at an arbitrary time without replaying every frame with the original step sequence. A scene containing updaters therefore rejects seeks with SeekError::FrameDependentUpdaters. Move seekable behavior into a core animation or a reversible timeline callback.