Middleware

redux中间件就是对dispatch进行处理，dispatch接收action creator后先进行处理后再讲action传给reducer

使用方法：

const createStoreWithMiddleware = applyMiddleware(
  thunkMiddleware,
  loggerMiddleware
)(createStore);

store = createStoreWithMiddleware(rootReducer, initialState);

执行过程：

编写middleware：

const customMiddleware = store => next => action => {
  if(action.type !== 'custom') return next(action)
  //do stuff!
}