Reducer: 纯粹的函数，接收当前状态，以及最新的action，计算出最新的action.

selector: 纯函数，用于select，derive和compose pieces of state

Store和State的区别

State is accessed with the store, an observable of state and an observer of actions.

Store是state和action的Observable.

reducer第一次被系统调用：@ngrx/store/update-reducers:

type：@ngrx/store/update-reducers

store也是一个Observable：

store的source字段里，根据feature名称example找到state初始值0：