[英]In React, when a parent component re-renders, isn't it true that the children with unchanged props should not need to re-render?
[英]Why, in React, do children not re-render when parent component re-renders(children are not wrapped by React.memo)?
在這篇文章React Hooks - 了解組件重新渲染中,我了解到當我們在父組件中使用useContext
Hook 時,只有使用上下文的子組件會重新渲染。
並且文章給出了context的兩種消費方式。 看一下片段:
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>
<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;
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>
<label>
<input
type="radio"
value="light"
checked={theme === "light"}
onChange={() => onThemeChanged("light")}
/>
Light
</label>
</div>
<ThemedTickerComponent id={1} />
<TickerComponent id={2} theme="" />
</>
);
}
在useContext 的低效消費示例中,子組件TickerComponent (2)
沒有使用上下文重新渲染,因為父<App />
使用上下文並重新渲染。 但是在Efficient consumption of useContext示例中,子TickerComponent (2) 沒有重新渲染,即使它的父<ThemeContxtProvider>
因為上下文的消耗而重新渲染。
我了解到沒有 React.memo 的孩子會在父母重新渲染時重新渲染,那么為什么在高效使用 useContext示例中沒有發生這種情況?
你的問題是你正在考慮像這樣的代碼
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>
);
}
相等的。 雖然它們做同樣的事情,並且或多或少地以相同的方式表現,但第二個示例將僅渲染ComponentToRender
一次,即使在多次按下“增量”按鈕之后也是如此。 (而第一個將在每次按下按鈕時重新呈現。)
該概念也適用於您的示例。 您的“低效消費”將觸發App
的重新渲染,並強制刷新該組件的每個直接子組件。 “有效消費”沒有,因為事實並非如此。 在我的簡化示例中, ComponentToRender
實際上是由App
呈現的,而不是Clicker
。 因此, Clicker
的 state 中的更改不會影響ComponentToRender
(剛剛作為子項傳遞)
App
的另一種寫法,在第二個例子中,是:
function App() {
const componentToRenderWithinApp = <ComponentToRender />
return (
<Clicker>
{componentToRenderWithinApp}
</Clicker>
);
}
這個相當於<Clicker><ComponentToRender /></Clicker>
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