5.匀速运动的小球.html

<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>匀速运动的小球</title>
    <style>
        * {
            padding: 0;
            margin: 0;
            box-sizing: border-box;
        }
        #canvas {
            background-color: beige;
        }
    </style>
</head>
<body>
<canvas id="canvas"></canvas>
<script>
    const canvas = document.getElementById('canvas')
    canvas.width = window.innerWidth
    canvas.height = window.innerHeight

    const ctx = canvas.getContext('2d') // 在2d上下文中绘制

    class Ball {
        constructor(r = 30, color = 'red', g = 0.1) {
            this.r = r
            this.color = color
            this.timer = null
            this.x = 0
            this.y = 0
        }
        draw(ctx) {
            const { x, y, color, r } = this
            ctx.save()
            ctx.beginPath()
            ctx.fillStyle = color
            ctx.arc(x, y, r, 0, Math.PI * 2)
            ctx.fill()
            ctx.restore()
        }
    }

    const ball = new Ball()
    ball.x = 500
    ball.y = 150
    ball.draw(ctx)

    const v = 0.2
    let time = Date.now()

    let y = ball.y

    !(function render() {
        const now = Date.now()
        const diff = now - time
        time = now

        /**
         * @设置一个初始时间，在渲染时，每次获取当前时间，计算时间差 => 表明渲染函数每次执行的间隔时间
         * @同时更新旧的时间
         * @对于计算的时间差 => 让初始速度 * 这个时间差 即为匀速运动
        * */

        ctx.clearRect(0, 0, canvas.width, canvas.height)
        ball.y += v * diff
        console.log('diff: ', ball.y - y)
        ball.draw(ctx)
        y = ball.y
        requestAnimationFrame(render)
    })()
</script>
</body>
</html>