以下内容为学习记录，可以参考 MDN 原文。
环境
- vscode 1.46
- Microsoft Edge 83
概述
本文通过编写一个弹球 demo 来展示 JavaScript 中对象的重要性。
我们的小球会在屏幕上弹跳，当它们碰到彼此时会变色。
定义 html
<!DOCTYPE html>
<html>
<head>
<meta charset="utf-8">
<title>弹跳彩球</title>
<link rel="stylesheet" href="style.css">
</head>
<body>
<h1>弹跳彩球</h1>
<canvas></canvas>
<script src="main.js"></script>
</body>
</html>
定义样式表
html, body {
margin: 0;
}
html {
font-family: sans-serif;
height: 100%;
}
body {
overflow: hidden;
height: inherit;
}
h1 {
font-size: 2rem;
letter-spacing: -1px;
position: absolute;
margin: 0;
top: -4px;
right: 5px;
color: transparent;
text-shadow: 0 0 4px white;
}
定义画布
// 设置画布
const canvas = document.querySelector("canvas");
const ctx = canvas.getContext("2d");
const width = canvas.width = window.innerWidth;
const height = canvas.height = window.innerHeight;
随机值函数
/**
* 生成随机数的函数
* @param {number} min 最小值
* @param {number} max 最大值
* @return {number} 随机值
*/
function random(min, max) {
const num = Math.floor(Math.random() * (max - min)) + min;
return num;
}
定义小球模型
function Ball(x, y, velX, velY, color, size) {
this.x = x;
this.y = y;
this.velX = velX;
this.velY = velY;
this.color = color;
this.size = size;
}
画小球
Ball.prototype.draw = function() {
ctx.beginPath();
ctx.fillStyle = this.color;
ctx.arc(this.x, this.y, this.size, 0, 2 * Math.PI);
ctx.fill();
}
更新小球位置
Ball.prototype.update = function() {
if ((this.x + this.size) >= width) {
this.velX = -(this.velX);
}
if ((this.x - this.size) <= 0) {
this.velX = -(this.velX);
}
if ((this.y + this.size) >= height) {
this.velY = -(this.velY);
}
if ((this.y - this.size) <= 0) {
this.velY = -(this.velY);
}
this.x += this.velX;
this.y += this.velY;
}
生成多个小球
let balls = [];
while (balls.length < 25) {
let size = random(10, 20);
let ball = new Ball(
// 为避免绘制错误，球至少离画布边缘球本身一倍宽度的距离
random(0 + size, width - size),
random(0 + size, height - size),
random(-7, 7),
random(-7, 7),
randomColor(),
size
);
balls.push(ball);
}
填充画布
function loop() {
ctx.fillStyle = 'rgba(0, 0, 0, 0.25)';
ctx.fillRect(0, 0, width, height);
for (let i = 0; i < balls.length; i++) {
balls[i].draw();
balls[i].update();
}
requestAnimationFrame(loop);
}
最后
loop();
Ball.prototype.collisionDetect = function() {
for (let j = 0; j < balls.length; j++) {
if (this !== balls[j]) {
const dx = this.x - balls[j].x;
const dy = this.y - balls[j].y;
const distance = Math.sqrt(dx * dx + dy * dy);
if (distance < this.size + balls[j].size) {
balls[j].color = this.color = randomColor();
}
}
}
}
balls[i].collisionDetect();