private func setupObserving() {
    ...

    // Pan gesture state observing.
    observables.panGestureState.observer = { [weak self] state in
      self?.panGestureStateChanged(state: state)
    }
  }

  private func panGestureStateChanged(state: UIGestureRecognizerState) {
    switch state {
    case .began:
      panGestureBegan()
    case .ended:
      panGestureEnded()
    case .changed:
      panGestureChanged()
    default:
      break
    }
  }