关于react.js:useReduceruseContext替代redux方案

先简略温习一下redux工作流程

graph LR

A[action]-- dispatch.action-->B[store]
B--previousState,action -->C[reducers]

C-- newState -->B

B-- state -->D[组件]

react-hook替换redux计划

要求列表

• useReducer、useContext函数
• action：寄存批改state的action，此处与redux的思维统一
• reducer：用来解决不同action,此处咱们不提供初始状态的话，默认会去action找。
• rootReducer：当reducer过多的时候，咱们能够拆分reducer，拆分reducer后，应用combinReducers合并解决成一个大繁多的对象。

• 顶级组件：

  1. 组件利用provider提供context给子组件
     2.useReducer定义，引入reducer并提供初始化状态initialstate

• 子组件：

  1. useContext应用顶级组件提供的context
  2. 如果须要异步申请，应用useEffect

实现逻辑要害代码：

• cartReducer.js

  // 定义一个action type
  const CHANGE_CART_AMOUNT = "CHANGE_CART_AMOUNT";
  // 初始化状态数据
  const initialState ={
  cartList: []
  };
  // reducer解决action，返回newState
  export const cartReducer = (state, action) => {
  switch (action.type) {
    case CHANGE_CART_AMOUNT:
      let cartList = state?state.cart.cartList:[];
      let cartItem = action.payload;
      return {
        cartList: [...cartList, cartItem]
      };
  
    default:
      {
        return state;
      }
  }
  };

  当你除了一个cartReducer之外还有很多reducer，这个时候咱们须要拆分并对立治理。

• rootReducer.js

  import { cartReducer,initialStates } from './cartReducer';
  import combineReducers from './combineReducers';
  import { layoutInitialState, layoutReducer } from './layoutReducer';
  export const initialState = {
  layout: layoutInitialState,
  cart: initialStates()
  };
  //拆分reducer后，应用combinReducers合并解决成一个大繁多的对象， 
  export const rootReducer = combineReducers({
  layout: layoutReducer,
  cart: cartReducer 
  });
  

  这里的combineReducer依照redux逻辑，间接写，不是用redux的。
  redux的combineReducer工作原理参考:https://www.cnblogs.com/wy193…

• combineReducers.js

  //将多个reducer合并成一个reducer
  const combineReducers = reducers => {
  return (state = {}, action) => {
    const newState = {};
  
    for (let key in reducers) {
      newState[key] = reducers[key](state, action);
    }
  
    return newState;
  }; 
  };
  
  export default combineReducers;
  

• 顶级组件.jsx
  要害代码

  const AppContext = createContext({
  state: initialState,
  dispatch: () => {} 
  });
  //因为创立的contenxt AppContext须要在各个组件都用uesContext应用到，所以须要导出。
  export default AppContext;
  
   //Redux 是通过 createStore(reducer, initialState) 来创立一个 store 实例，
  // 实例封装了 state 的读写接口和监听接口：getState 、dispatch、subscribe
  
  // 在Redux中，store.dispatch 触发事件动作时，Redux 并不会为咱们被动从新渲染视图，而是须要咱们调用 store.subscribe 在监听函数中手动 render 视图
  
  //但 useReducer Hook 是没有应用 store 实例，
  //而是遵循 Hook 总是返回读写接口的规定，间接通过 [state, dispatch] = useReducer(reducer, initialState) 的形式返回状态的读写接口。
  
  const [state, dispatch] = useReducer(rootReducer,initialState);
  
  
  //因为布局数据简直不变动，咱们能够应用缓存，等变动再更新，利用dispatch去触发咱们下面定义的rootReducer
  const contextValue = useMemo(() => {
    return {
      state,
      dispatch
    };
  }, [state, dispatch]);
  
  //Provider传递参数
  return <AppContext.Provider value={contextValue}>
    </AppContext.Provider>;
    
  

• 子组件.jsx:在react-hook中，咱们只须要触发reducer就能实现像redux-store的dispatch成果一样。

  // 触发dispatch
  const {
    state,
    dispatch
  } = useAppContext();
  
  const handleCartAmountChange = useCallback((amount, product) => () => {
    dispatch({
      type: "CHANGE_CART_AMOUNT",
      payload: { ...product,
        qty: amount
      }
    });
  }, []);
  
  //获取state
  
  const {
    state
  } = useAppContext();
  
  useEffect(() => {
    setList(state?state.cart.cartList:[])
  },[state]);