onStopDrag.js

onStopDrag = (id, { dx, dy }) => {
    // dx and dy are captured in the closure!
    this.client.propose(draft => {
      const box = draft.boxes.find(b => b.id === id);
      box.x += dx;
      box.y += dy;
      if (box.x < 0 || box.x > 400 || box.y < 0 || box.y > 400)
        // throwing here is safe; as original state will be untouched.
        // This can happen when actions are being replayed
        throw "Invalid drop coordinates: " + JSON.stringify(box);
    });
  };