reducer.js

import {
  createReactNavigationReduxMiddleware,
  createNavigationReducer,
} from 'react-navigation-redux-helpers';

export default (AppNavigator) => {
  const navReducer = createNavigationReducer(AppNavigator);
  const middleware = createReactNavigationReduxMiddleware(
    'root',
    state => state.nav,
  );

  return { navReducer, middleware };
};