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
我们的弹球 demo 很有趣, 但是现在我们想让它更具有互动性，我们为它添加一个由玩家控制的“恶魔圈”，
如果恶魔圈抓到弹球会把它会吃掉。我们还想测验你面向对象的水平，首先创建一个通用 Shape() 对象，
然后由它派生出弹球和恶魔圈。最后，我们为 demo 添加一个计分器来记录剩下的球数。
定义 html
<!DOCTYPE html>
<html>
<head>
<meta charset="utf-8">
<title>Bouncing balls</title>
<link rel="stylesheet" href="style.css">
</head>
<body>
<h1>bouncing balls</h1>
<p>Ball count: </p>
<canvas></canvas>
<script src="main.js"></script>
</body>
</html>
定义样式表
html, body {
margin: 0;
}
html {
font-family: 'Helvetica Neue', Helvetica, Arial, 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;
}
p {
position: absolute;
margin: 0;
top: 35px;
right: 5px;
color: #aaa;
}
定义画布
const para = document.querySelector("p");
let count = 0;
// setup canvas
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;
}
定义形状模型
// define shape constructor
function Shape(x, y, velX, velY, exists) {
this.x = x;
this.y = y;
this.velX = velX;
this.velY = velY;
this.exists = exists;
}
定义小球模型
// define Ball constructor, inheriting from Shape
function Ball(x, y, velX, velY, exists, color, size) {
Shape.call(this, x, y, velX, velY, exists);
this.color = color;
this.size = size;
}
Ball.prototype = Object.create(Shape.prototype);
Ball.prototype.constructor = Ball;
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;
}
小球碰撞检测
// define ball collision detection
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].exists) {
balls[j].color = this.color = "rgb(" + random(0, 255) + "," +
random(0, 255) + "," + random(0, 255) + ")";
}
}
}
};
定义恶魔圈
// define EvilCircle constructor, inheriting from Shape
function EvilCircle(x, y, exists) {
Shape.call(this, x, y, 20, 20, exists);
this.color = "white";
this.size = 10;
}
EvilCircle.prototype = Object.create(Shape.prototype);
EvilCircle.prototype.constructor = EvilCircle;
画恶魔圈
// define EvilCircle draw method
EvilCircle.prototype.draw = function() {
ctx.beginPath();
ctx.strokeStyle = this.color;
ctx.lineWidth = 3;
ctx.arc(this.x, this.y, this.size, 0, 2 * Math.PI);
ctx.stroke();
};
恶魔圈边界
// define EvilCircle checkBounds method
EvilCircle.prototype.checkBounds = function() {
if ((this.x + this.size) >= width) {
this.x -= this.size;
}
if ((this.x - this.size) <= 0) {
this.x += this.size;
}
if ((this.y + this.size) >= height) {
this.y -= this.size;
}
if ((this.y - this.size) <= 0) {
this.y += this.size;
}
};
控制恶魔圈
EvilCircle.prototype.setControls = function() {
const _this = this;
window.onkeydown = function(e) {
if (e.key === "a") {
_this.x -= _this.velX;
} else if (e.key === "d") {
_this.x += _this.velX;
} else if (e.key === "w") {
_this.y -= _this.velY;
} else if (e.key === "s") {
_this.y += _this.velY;
}
};
};
恶魔圈碰撞检测
// define EvilCircle collision detection
EvilCircle.prototype.collisionDetect = function() {
for (let j = 0; j < balls.length; j++) {
if (balls[j].exists) {
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].exists = false;
count--;
para.textContent = "Ball count: " + count;
}
}
}
};
生成多个小球
// define array to store balls and populate it
const balls = [];
while (balls.length < 25) {
const size = random(10, 20);
const ball = new Ball(
// ball position always drawn at least one ball width
// away from the adge of the canvas, to avoid drawing errors
random(0 + size, width - size),
random(0 + size, height - size),
random(-7, 7),
random(-7, 7),
true,
"rgb(" + random(0, 255) + "," + random(0, 255) + "," + random(0, 255) + ")",
size
);
balls.push(ball);
count++;
para.textContent = "Ball count: " + count;
}
最后
// define loop that keeps drawing the scene constantly
const evil = new EvilCircle(random(0, width), random(0, height), true);
evil.setControls();
function loop() {
ctx.fillStyle = "rgba(0,0,0,0.25)";
ctx.fillRect(0, 0, width, height);
for (let i = 0; i < balls.length; i++) {
if (balls[i].exists) {
balls[i].draw();
balls[i].update();
balls[i].collisionDetect();
}
}
evil.draw();
evil.checkBounds();
evil.collisionDetect();
requestAnimationFrame(loop);
}
loop();