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：
