javascript 为什么渲染每两次按键才触发一次?

vmdwslir  于 2023-03-21  发布在  Java
关注(0)|答案(1)|浏览(153)

问题:

目标:每次按下“W+C+N”键,屏幕上都会随机出现一个图像。
实际结果:图像只会每两个按键出现一次。例如,如果我在“W+C+N”上按4次,只会出现2个新图像。

我试过/猜测问题出在哪里:

我的useEffect的误用,它要么来自:

  • 对removeEventListener方法的误解,它是否“禁用”了handleKey函数中的setcoordinates?
  • 未指定正确的依赖项[handleKey]:handleKey函数是通过按键激活的,并且在新的坐标上改变。所以每次传递新的坐标时重新渲染似乎是正确的
  • VS代码警告:“”handleKey”函数使useState Hook的依赖项在每次呈现时都发生变化。请将其移到useEffect回调中。”我确实尝试过,但它不再工作了。

我的真实代码

import egg from "./logo.png";
import { useState, useEffect } from "react";

function App() {
  let map = []; //array of keys to store
  const [coordinates, setcoordinates] = useState([]); //array of image coordinates

  const handleKey = (event) => {
    onkeydown = onkeyup = function (event) {
      map[event.keyCode] = event.type === "keydown";
    };
    //random image coordinates
    if (map[87] && map[67] && map[78]) {
      const newCoordinates = [...coordinates];
      let random_left = Math.floor(Math.random() * window.innerWidth);
      let random_top = Math.floor(Math.random() * window.innerHeight);
      newCoordinates.push({ left: random_left, top: random_top });
      setcoordinates(newCoordinates);
    }
  };
  useEffect(() => {
    window.addEventListener("keypress", handleKey);
    return () => {
      window.removeEventListener("keypress", handleKey);
    };
  }, [handleKey]);
  return (
    <div className="App">
      <img src={egg} alt="o easter egg" />
      <h1>Rocambole</h1>
      <h2>Good luck!</h2>
      {console.log(coordinates)}
      {coordinates.map((item, index) => {
        return (
          <img
            key={index}
            src={egg}
            alt="egg"
            className="newImg"
            style={{ top: `${item.top}px`, left: `${item.left}px` }}
          />
        );
      })}
    </div>
  );
}

export default App;
9avjhtql

9avjhtql1#

尝试使用函数更新状态,这样可以确保内存中注册的事件侦听器每次都获得最新状态,并且还可以将函数handleKey移到useEffect中:

import egg from "./logo.png";
import { useState, useEffect } from "react";

function App() {
  let map = []; //array of keys to store
  const [coordinates, setcoordinates] = useState([]); //array of image coordinates
  useEffect(() => {
    const handleKey = (event) => {
      onkeydown = onkeyup = function (event) {
        map[event.keyCode] = event.type === "keydown";
      };
      //random image coordinates
      if (map[87] && map[67] && map[78]) {
        let random_left = Math.floor(Math.random() * window.innerWidth);
        let random_top = Math.floor(Math.random() * window.innerHeight);
   
        setcoordinates(coordinates =>[...coordinates, { left: random_left, top: random_top }]);
      }
    };
    window.addEventListener("keypress", handleKey);
    return () => {
      window.removeEventListener("keypress", handleKey);
    };
  }, []);
  return (
    <div className="App">
      <img src={egg} alt="o easter egg" />
      <h1>Rocambole</h1>
      <h2>Good luck!</h2>
      {console.log(coordinates)}
      {coordinates.map((item, index) => {
        return (
          <img
            key={index}
            src={egg}
            alt="egg"
            className="newImg"
            style={{ top: `${item.top}px`, left: `${item.left}px` }}
          />
        );
      })}
    </div>
  );
}

export default App;

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