为什么在 React 中，当父组件重新渲染时子组件不会重新渲染（子组件没有被 React.memo 包裹）？

Question

在React Hooks - Understanding Component Re-renders这篇文章中，我了解到当我们在父组件中使用useContextHook 时，只有消耗上下文的子组件会重新渲染。

并且文章给出了上下文的两种消费方式。看看sn-p：

Efficient consumption of useContext\

import React from "react";
import ReactDOM from "react-dom";
import TickerComponent from "./tickerComponent";
import ThemedTickerComponent from "./themedTickerComponent";
import { ThemeContextProvider } from "./themeContextProvider";
import ThemeSelector from "./themeSelector";

import "./index.scss";
import logger from "./logger";

function App() {
  logger.info("App", `Rendered`);
  return (
    <ThemeContextProvider>
      <ThemeSelector />
      <ThemedTickerComponent id={1} />
      <TickerComponent id={2} />
    </ThemeContextProvider>
  );
}

import React, { useState } from "react";

const defaultContext = {
  theme: "dark",
  setTheme: () => {}
};

export const ThemeContext = React.createContext(defaultContext);

export const ThemeContextProvider = props => {
  const setTheme = theme => {
    setState({ ...state, theme: theme });
  };

  const initState = {
    ...defaultContext,
    setTheme: setTheme
  };

  const [state, setState] = useState(initState);

  return (
    <ThemeContext.Provider value={state}>
      {props.children}
    </ThemeContext.Provider>
  );
};

import React from "react";
import { useContext } from "react";
import { ThemeContext } from "./themeContextProvider";

function ThemeSelector() {
  const { theme, setTheme } = useContext(ThemeContext);
  const onThemeChanged = theme => {
    logger.info("ThemeSelector", `Theme selection changed (${theme})`);
    setTheme(theme);
  };
  return (
    <div style={{ padding: "10px 5px 5px 5px" }}>
      <label>
        <input
          type="radio"
          value="dark"
          checked={theme === "dark"}
          onChange={() => onThemeChanged("dark")}
        />
        Dark
      </label>
      &nbsp;&nbsp;
      <label>
        <input
          type="radio"
          value="light"
          checked={theme === "light"}
          onChange={() => onThemeChanged("light")}
        />
        Light
      </label>
    </div>
  );
}

module.exports = ThemeSelector;

import React from "react";
import { ThemeContext } from "./themeContextProvider";
import TickerComponent from "./tickerComponent";
import { useContext } from "react";

function ThemedTickerComponent(props) {
  const { theme } = useContext(ThemeContext);
  return <TickerComponent id={props.id} theme={theme} />;
}

module.exports = ThemedTickerComponent;

import React from "react";
import { useState } from "react";
import stockPriceService from "./stockPriceService";
import "./tickerComponent.scss";

function TickerComponent(props) {
  const [ticker, setTicker] = useState("AAPL");
  const currentPrice = stockPriceService.fetchPricesForTicker(ticker);
  const componentRef = React.createRef();

  setTimeout(() => {
    componentRef.current.classList.add("render");
    setTimeout(() => {
      componentRef.current.classList.remove("render");
    }, 1000);
  }, 50);

  const onChange = event => {
    setTicker(event.target.value);
  };

  return (
    <>
      <div className="theme-label">
        {props.theme ? "(supports theme)" : "(only dark mode)"}
      </div>
      <div className={`ticker ${props.theme || ""}`} ref={componentRef}>
        <select id="lang" onChange={onChange} value={ticker}>
          <option value="">Select</option>
          <option value="NFLX">NFLX</option>
          <option value="FB">FB</option>
          <option value="MSFT">MSFT</option>
          <option value="AAPL">AAPL</option>
        </select>
        <div>
          <div className="ticker-name">{ticker}</div>
          <div className="ticker-price">{currentPrice}</div>
        </div>
      </div>
    </>
  );
}

module.exports = TickerComponent;

Inefficient consumption of useContext

import React from "react";
import ReactDOM from "react-dom";
import { useContext } from "react";
import TickerComponent from "./tickerComponent";
import ThemedTickerComponent from "./themedTickerComponent";
import { ThemeContextProvider } from "./themeContextProvider";
import { ThemeContext } from "./themeContextProvider";

function App() {
  const { theme, setTheme } = useContext(ThemeContext);
  const onThemeChanged = theme => {
    setTheme(theme);
  };
  return (
    <>
      <div style={{ padding: "10px 5px 5px 5px" }}>
        <label>
          <input
            type="radio"
            value="dark"
            checked={theme === "dark"}
            onChange={() => onThemeChanged("dark")}
          />
          Dark
        </label>
        &nbsp;&nbsp;
        <label>
          <input
            type="radio"
            value="light"
            checked={theme === "light"}
            onChange={() => onThemeChanged("light")}
          />
          Light
        </label>
      </div>
      <ThemedTickerComponent id={1} />
      <TickerComponent id={2} theme="" />
    </>
  );
}

在低效使用useContext示例中，child组件TickerComponent (2)自parent后未使用上下文重新渲染<App /> 使用上下文并重新渲染。但是在 高效使用 useContext 示例中，child TickerComponent (2) 没有重新渲染，即使它是 parent <ThemeContxtProvider> 重新渲染因为上下文的消耗。

我了解到，没有 React.memo 的子级在父级重新渲染时会重新渲染，那么为什么在 useContext 的高效消费示例中不会发生这种情况？

Answer 1

你的问题是你正在考虑像这样的代码

function ComponentToRender() {
  const count = React.useRef(0)

  React.useEffect(() => {
    console.log('component rendered', count.current++)
  })

  return null
}

function App() {
  const [count, setCount] = useState(0);

  return (
    <div>
      <h2>You clicked {count} times!</h2>
      <button onClick={() => setCount(count + 1)}>Increment</button>
      <ComponentToRender />
    </div>
  );
}

和

function ComponentToRender() {
  const count = React.useRef(0)

  React.useEffect(() => {
    console.log('component rendered', count.current++)
  })

  return null
}

function Clicker({ children }) {
  const [count, setCount] = useState(0);

  return (
    <div>
      <h2>You clicked {count} times!</h2>
      <button onClick={() => setCount(count + 1)}>Increment</button>
      {children}
    </div>
  );
}

function App() {
  return (
    <Clicker>
      <ComponentToRender />
    </Clicker>
  );
}

等效。虽然它们做同样的事情，并且或多或少地以相同的方式表现，但第二个示例将只呈现一次ComponentToRender，即使在多次按下“增量”按钮之后也是如此。 （而第一个将在每次按下按钮时重新渲染。）

这个概念也适用于您的示例。您的“低效消耗”将触发 App 的重新渲染，并强制刷新该组件的每个直接子级。 “有效消费”没有，因为事实并非如此。在我的简化示例中，ComponentToRender 实际上是由App 渲染的，而不是Clicker。所以Clicker 的状态变化不会影响ComponentToRender（只是作为孩子传递）

在第二个示例中，App 的另一种写法是：

function App() {
  const componentToRenderWithinApp = <ComponentToRender />

  return (
    <Clicker>
      {componentToRenderWithinApp}
    </Clicker>
  );
}

这个相当于<Clicker><ComponentToRender /></Clicker>