// vX - sprite's velocity in x direction
const getVx = value<Velocity>('character.vX');
const setVx = action<Velocity>('character.vX');
const onVxChange = react<Velocity>('character.vX');

// or simpler
const [ getVx, setVx, onVxChange ] = state<Velocity>('character.vX');

// read current value, e.g. inside the ticker to update sprite's x position:
const ticker = () => {
  sprite.x += getVx();
}

// set new value
const BOOST = 2;
setVx(prevVx => prevVx * BOOST);

// react to state change
onVxChange((newValue, prevValue) => {
  // ...
});