A demo implementation of Reducer Injection is React

injectReducer.js

import React from 'react';
import PropTypes from 'prop-types';
import hoistNonReactStatics from 'hoist-non-react-statics';
import invariant from 'invariant';
import isEmpty from 'lodash/isEmpty';
import isFunction from 'lodash/isFunction';
import isString from 'lodash/isString';
import checkStore from './checkStore';
import createReducer from '../reducers';
import conformsTo from 'lodash/conformsTo';
import isFunction from 'lodash/isFunction';
import isObject from 'lodash/isObject';
import createReducer from './reducers';

function checkStore(store) {
  const shape = {
    dispatch: isFunction,
    subscribe: isFunction,
    getState: isFunction,
    replaceReducer: isFunction,
    runSaga: isFunction,
    injectedReducers: isObject,
    injectedSagas: isObject,
  };
  invariant(
    conformsTo(store, shape),
    '(app/utils...) injectors: Expected a valid redux store',
  );
}

function injectReducerFactory(store, isValid) {
  return function injectReducer(key, reducer) {
    if (!isValid) checkStore(store);

    invariant(
      isString(key) && !isEmpty(key) && isFunction(reducer),
      '(app/utils...) injectReducer: Expected `reducer` to be a reducer function',
    );

    // Check `store.injectedReducers[key] === reducer` for hot reloading when a key is the same but a reducer is different
    if (
      Reflect.has(store.injectedReducers, key) &&
      store.injectedReducers[key] === reducer
    )
      return;

    store.injectedReducers[key] = reducer; // eslint-disable-line no-param-reassign
    store.replaceReducer(createReducer(store.injectedReducers));
  };
}

function getInjectors(store) {
  checkStore(store);

  return {
    injectReducer: injectReducerFactory(store, true),
  };
}

export default ({ key, reducer }) => WrappedComponent => {
  class ReducerInjector extends React.Component {
    static WrappedComponent = WrappedComponent;
    static contextTypes = {
      store: PropTypes.object.isRequired,
    };
    static displayName = `withReducer(${WrappedComponent.displayName ||
      WrappedComponent.name ||
      'Component'})`;

    componentWillMount() {
      const { injectReducer } = this.injectors;

      injectReducer(key, reducer);
    }

    injectors = getInjectors(this.context.store);

    render() {
      return <WrappedComponent {...this.props} />;
    }
  }

  return hoistNonReactStatics(ReducerInjector, WrappedComponent);
};