这篇文章主要介绍了JavaScript如何实现简单的倒计时效果的相关知识，内容详细易懂，操作简单快捷，具有一定借鉴价值，相信大家阅读完这篇JavaScript如何实现简单的倒计时效果文章都会有所收获，下面我们一起来看看吧。
具体代码如下
<!DOCTYPE html>
<html>
<head lang="en">
<meta charset="utf8">
<meta http-equiv="X-UA-Compatible" cotent="IE=edge,chrome=1">
<meta name="viewport" content="width=device-width, minimum-scale=1.0, maximum-scale=2.0 initial-scale=1.0, user-scalable=yes">
<meta name="keywords" content="count down">
<meta name="description" content="count down">
<title>倒计时(Count Down)</title>
</head>
<body>
<script>
var halfPI = Math.PI / 2;
var doublePI = Math.PI * 2;
var ctx;
//画布大小
var canvasSize = 200;
var halfCanvasSize = canvasSize / 2;
//画布的外切圆半径
var excircleRadius = Math.SQRT2 * halfCanvasSize;
//线条宽度
var lineWidth = 2;
//蒙版颜色
var maskColor = "rgba(90,90,90,0.4)";
//单个数字的字体
var singleNumFont = halfCanvasSize * 1.6 + "px Calibri";
//两个数字的字体
var doubleNumFont = halfCanvasSize * 1.2 + "px Calibri";
//每秒刷新的帧数
var stepCount = 30;
//定时器执行间隔时间
var interval = 1000 / stepCount;
//每帧前进的弧度
var radianStep = doublePI / stepCount;
//定时器id
var intervalId = 0;
//倒计时
var countdownTime = 0;
//当前角度
var radian = 0;
function init() {
var canvas = document.createElement("canvas");
canvas.innerHTML = "Your browser can not support canvas";
canvas.width = canvasSize;
canvas.height = canvasSize;
canvas.style.backgroundColor = "darkkhaki";
canvas.style.display = "block";
var restartBtn = document.createElement("button");
restartBtn.innerHTML = "restart";
//注意:canvas的width不能写单位, 而按钮的width必须写单位, 否则无效
restartBtn.style.width = "200px";
restartBtn.addEventListener("click", function() {
restart();
});
document.body.appendChild(canvas);
document.body.appendChild(restartBtn);
ctx = canvas.getContext("2d");
ctx.lineWidth = lineWidth;
ctx.textAlign = "center";
ctx.textBaseline = "middle";
restart();
}
function strokeLine(x1, y1, x2, y2) {
ctx.beginPath();
ctx.moveTo(x1, y1);
ctx.lineTo(x2, y2);
ctx.stroke();
}
function strokeCircle(cx, cy, r) {
ctx.beginPath();
ctx.arc(cx, cy, r, 0, doublePI);
ctx.stroke();
}
//渲染背景
function renderBg() {
ctx.clearRect(0, 0, canvasSize, canvasSize);
//绘制横竖线
ctx.strokeStyle = "black";
strokeLine(halfCanvasSize, 0, halfCanvasSize, canvasSize);
strokeLine(0, halfCanvasSize, canvasSize, halfCanvasSize);
//绘制两个内圈
ctx.strokeStyle = "Silver";
strokeCircle(halfCanvasSize, halfCanvasSize, halfCanvasSize * 0.9);
strokeCircle(halfCanvasSize, halfCanvasSize, halfCanvasSize * 0.7);
}
//渲染蒙版
function renderMask(radian) {
//绘制扇形的边界线
var x = halfCanvasSize + excircleRadius * Math.sin(radian);
var y = halfCanvasSize - excircleRadius * Math.cos(radian);
ctx.strokeStyle = "black";
strokeLine(halfCanvasSize, halfCanvasSize, x, y);
//绘制扇形
ctx.fillStyle = maskColor;
ctx.beginPath();
ctx.moveTo(halfCanvasSize, halfCanvasSize);
ctx.arc(halfCanvasSize, halfCanvasSize, excircleRadius, -halfPI, -halfPI + radian);
ctx.closePath();
ctx.fill();
}
//渲染数字
function renderNum(num) {
ctx.fillStyle = "black";
ctx.font = num < 10 ? singleNumFont : doubleNumFont;
ctx.fillText(num, halfCanvasSize, halfCanvasSize);
}
//渲染
function render() {
renderBg();
renderMask(radian);
renderNum(countdownTime);
}
//更新
function update() {
radian += radianStep;
if(radian >= (doublePI + radianStep)) {
radian = 0;
countdownTime--;
}
if(countdownTime < 0) {
stop();
}
}
//循环
function loop() {
render();
update();
}
//停止
function stop() {
if(intervalId === 0) {
return;
}
clearInterval(intervalId);
intervalId = 0;
}
//开始倒计时
function restart() {
stop();
countdownTime = 10;
radian = 0;
intervalId = setInterval(loop, interval);
}
init();
</script>
</body>
</html>
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