react-dnd/dnd-core

import {
  ADD_SOURCE,
  ADD_TARGET,
  REMOVE_SOURCE,
  REMOVE_TARGET,
} from '../actions/registry';

export default function refCount(state = 0, action) {
  switch (action.type) {
    case ADD_SOURCE:
    case ADD_TARGET:
      return state + 1;
    case REMOVE_SOURCE:
    case REMOVE_TARGET:
      return state - 1;
    default:
      return state;
  }
}