在什么情况下它会在使用 react 自定义钩子时重新渲染

Question

I tried a sample in using react hook, make it a custom hook. The problem is the simple hook useCount() goes fine, but the hook useCarHighlight() intending to switch highlight line would not cause re-render. I see it is the same of the two, is anything wrong I should attention for about this?

I made a sandbox here: https://codesandbox.io/s/typescript-j2xtf

Some code below:

// index.tsx

import * as React from "react";
import * as ReactDOM from "react-dom";
import useCarHighlight, { Car } from "./useCarHighlight";
import useCount from "./useCount";

const myCars: Car[] = [
  { model: "C300", brand: "benz", price: 29000, ac: "auto ac" },
  { model: "Qin", brand: "byd", price: 9000 }
];

const App = () => {
  const { cars, setHighlight } = useCarHighlight(myCars, "Qin");
  const { count, increase, decrease } = useCount(10);
  console.log(
    `re-render at ${new Date().toLocaleTimeString()}, 
    Current highlight: ${
      cars.find(c => c.highlight)?.model
    }`
  );
  return (
    <div>
      <ul>
        {cars.map(car => {
          const { model, highlight, brand, price, ac = "no ac" } = car;
          return (
            <li
              key={model}
              style={{ color: highlight ? "red" : "grey" }}
            >{`[${brand}] ${model}: $ ${price}, ${ac}`}</li>
          );
        })}
      </ul>
      <button onClick={() => setHighlight("C300")}>highlight C300</button>
      <button onClick={() => setHighlight("Qin")}>highlight Qin</button>
      <hr />

      <h1>{`Count: ${count}`}</h1>
      <button onClick={() => increase()}>+</button>
      <button onClick={() => decrease()}>-</button>
    </div>
  );
};

ReactDOM.render(<App />, document.getElementById("root"));



// useCarHighlight.ts

import { useState } from "react";

export type Car = {
  model: string;
  brand: string;
  price: number;
  ac?: "auto ac" | "manual ac";
};

export default function(
  initialCars: Car[],
  initialSelectedModel: string
): {
  cars: Array<Car & { highlight: boolean }>;
  setHighlight: (selMod: string) => void;
} {
  const carsHighlight = initialCars.map(car => ({
    ...car,
    highlight: initialSelectedModel === car.model
  }));

  const [cars, setCars] = useState(carsHighlight);
  const setHighlight = (selMod: string) => {
    cars.forEach(car => {
      car.highlight = car.model === selMod;
    });
    setCars(cars);
  };

  return {
    cars,
    setHighlight
  };
}




// useCount.ts
import { useState } from "react";

export default function useCount(initialCount: number) {
  const [state, setState] = useState(initialCount);
  const increase = () => setState(state + 1);
  const decrease = () => setState(state - 1);
  return {
    count: state,
    increase,
    decrease
  };
}

Answer 1

Unlike class components, mutating state of hooks does not queue a re-render, when using hooks you have to update your state in an immutable way.

Also, when calculating the next state based on the previous state it is recommended to use a functional update and read the previous state from the first argument of the function.

const setHighlight = (selMod: string) => {
  setCars(prevState =>
    prevState.map(car => ({
      ...car,
      highlight: car.model === selMod
    }))
  );
};

Here is a good resource about immutable update patterns

Answer 2

Dont use forEach in setHighlight , use map instead

  const setHighlight = (selMod: string) => {
    const newCars = cars.map(car => ({
      ...car,
      highlight: car.model === selMod
    }));
    setCars(newCars);
  };

Answer 3

Use map instead of forEach as the address of car object isn't getting changed when you update highlight property in car.

const setHighlight = (selMod: string) => {
let carsTemp = cars.map(car => ({
  ...car,
  highlight : car.model === selMod
}));
setCars(carsTemp);};