interaction-pt1.js

function interaction(gestureTranslation, gestureState) {
  const start = new Value(0);
  const dragging = new Value(0);
  const position = new Value(0);

  return cond(
    eq(gestureState, State.ACTIVE),
    [
      cond(eq(dragging, 0), [set(dragging, 1), set(start, position)]),
      set(position, add(start, gestureTranslation)),
    ],
    [set(dragging, 0), position]
  );
}