• Provider

     //最後匯出的是createProvider()。所以一開始storeKey應該是以預設值‘store’傳進去的
     function createProvider(storeKey = 'store', subKey) {
     const subscriptionKey = subKey || `${storeKey}Subscription`
  
     class Provider extends Component {
         //設定context，能讓子元件拿到store
         //相當於返回 {store: this.store}
         getChildContext() {
           return { [storeKey]: this[storeKey], [subscriptionKey]: null }
         }
  
         constructor(props, context) {
           super(props, context)
           //this.store = props.store
           this[storeKey] = props.store;
         }
  
         render() {
           //只能有一個子元件
           return Children.only(this.props.children)
         }
     }
     
     //props和context型別驗證
     Provider.propTypes = {
         store: storeShape.isRequired,
         children: PropTypes.element.isRequired,
     }
     Provider.childContextTypes = {
   
  

             [storeKey]: storeShape.isRequired,
            [subscriptionKey]: subscriptionShape,
        }
    
            return Provider
    }

通常的做法是我們先通過redux建立好store，然後賦給Provider元件的store屬性。在Provider元件內部，拿到這個store，設定為context屬性，這樣就能讓它的所有元件都能通過context拿到store。

   <Provider store={store}>
       <App />
   </Provider>