React 基礎~渲染效能/ useCallback

• Even if the memo is set to a child component, the child component can still be re-rendered.

• It is a case that we pass a function as a props to the child component.

・src/Example.js

import React, { useCallback, useState } from "react";
import Child from "./Child";

const Example = () => {
  console.log("Parent render");

  const [countA, setCountA] = useState(0);
  const [countB, setCountB] = useState(0);

  const clickHandler = () => {
    setCountB((pre) => pre + 1);
  };

  return (
    <div className="parent">
      <div>
        <h3>Parent component</h3>
        <div>
          <button
            onClick={() => {
              setCountA((pre) => pre + 1);
            }}
          >
            button A
          </button>
          <span>Update parent component</span>
        </div>
      </div>
      <div>
        <p>The count of click button A：{countA}</p>
      </div>
      <Child countB={countB} onClick={clickHandler} />
    </div>
  );
};

export default Example;

・src/Child.js

import { memo } from "react";

const ChildMemo = memo(({ countB, onClick }) => {
  console.log("%cChild render", "color: red;");

  return (
    <div className="child">
      <h2>Child component</h2>
      <div>
        <button onClick={onClick}>button B</button>
        <span>Uodate child component</span>
      </div>
      <span>The count of click button B ：{countB}</span>
    </div>
  );
});

export default ChildMemo;

• The reason the child component is re-rendered is because of the clickHandler function in src/Example.js

  const clickHandler = () => {
    setCountB((pre) => pre + 1);
  };

• This function is passed to the child component as an onClick props.

 <Child countB={countB} onClick={clickHandler} />

• If we wrap the child component with a useCallback, we can avoid this case.

  const clickHandler = useCallback(() => {
    setCountB((pre) => pre + 1);
  }, []);

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