11. 函数式组件 与 hook

src/components/SonComponent2.js

@@ -1,5 +1,6 @@
-import React, {useEffect, useState} from "react";
+import React, {useCallback, useContext, useEffect, useMemo, useRef, useState} from "react";
 
+export let ContextParent = React.createContext({})
 
 // 函数式组件
 // 1. 没有生命周期
@@ -28,7 +29,17 @@ export default function SonComponent2(props= {
         count: 0,
     })
 
+    // 类似 vue computed, 缓存运算结果
+    // 如果依赖项更新, 则重新计算
+    // 组件初始化时 不会 执行
+    let countMemo = useMemo(() => {
+        console.log("state.count", state.count)
+        return state.count
+    }, [state.count]);
+
     // 模拟 componentDidUpdate
+    // 类似 vue 的 watch
+    // 组件初始化时 就会 执行
     useEffect(() => {
         // 当count更新时执行的操作
         console.log('SonComponent2 组件后更新，执行的操作:', state);
@@ -42,16 +53,23 @@ export default function SonComponent2(props= {
         })
     }
 
-    function revokeChangeMsg(){
+    // 缓存函数
+    // 用于优化, 避免更新时重复创建
+    const revokeChangeMsg = useCallback(() => {
         let count = state.count
         setState({
             ...state,
             count: count > 0 ? --count : 0,
             tmp: props.changeMsg(state.tmp)
         })
-    }
+    }, [props, state]);
 
 
+    let ref = useRef()
+    useEffect(() => {
+        console.log("SonComponent2 ref", ref, ref.current)
+    }, []);
+
     return <div style={{border: '1px solid', width: '80%', overflow: 'auto'}}>
         子组件2
 
@@ -62,15 +80,25 @@ export default function SonComponent2(props= {
 
             <hr/>
 
+            <ContextParent.Provider value={{changeText: "修改msg", revokeText: "修改msg"}}>
                 <div>
                     {props.msg}
-
+                    <div>count: {countMemo}</div>
-                <div>
+                    <Consumer ref={ref} changeMsg={changeMsg} revokeChangeMsg={revokeChangeMsg}/>
-                    <button onClick={changeMsg}>修改msg</button>
-                    <button onClick={revokeChangeMsg}>撤销操作</button>
-                </div>
                 </div>
+            </ContextParent.Provider>
         </div>
     </div>
 
 }
+
+
+function Consumer(props){
+    let contextValue = useContext(ContextParent)
+    console.log("SonComponent2 Consumer contextValue", contextValue)
+
+    return <div>
+        <button ref={props.ref} onClick={props.changeMsg}>{contextValue.changeText}</button>
+        <button onClick={props.revokeChangeMsg}>{contextValue.revokeText}</button>
+    </div>
+}