# 使用 css 实现 loading
使用 css 变量实现：
<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="UTF-8" />
<meta name="viewport" content="width=device-width, initial-scale=1.0" />
<meta http-equiv="X-UA-Compatible" content="ie=edge" />
<title>Document</title>
</head>
<style>
.box {
width: 200px;
height: 200px;
background: greenyellow;
padding: 20px;
position: relative;
--percent: 0;
}
.container {
width: calc(100% - 40px);
height: calc(100% - 40px);
background: white;
position: absolute;
}
.circle {
border-radius: 50%;
overflow: hidden;
}
.left,
.right {
position: absolute;
width: 50%;
height: 100%;
top: 0;
background: green;
}
.left-mask {
position: absolute;
width: 50%;
height: 100%;
top: 0;
left: 0;
background: greenyellow;
display: calc(50 - var(--percent));
}
.left {
left: 0;
transform-origin: right center;
transform: rotate(calc(var(--percent) / 50 * 180deg));
}
.right {
left: 50%;
transform-origin: left center;
transform: rotate(calc(var(--percent) / 50 * 180deg - 180deg));
}
</style>
<body>
<div class="box circle">
<div id="left" class="left"></div>
<div id="right" class="right"></div>
<div id="mask" class="left-mask"></div>
<div class="container circle"></div>
</div>
<div>
<button onclick="ro()">rotate</button>
<p id="txt"></p>
</div>
<script>
function ro() {
const random = Math.round(Math.random() * 100)
console.log(random)
document.getElementById('txt').innerText = random
if (random > 50) {
document.getElementById('mask').style.display = 'none'
} else {
document.getElementById('mask').style.display = 'block'
}
document
.getElementById('left')
.setAttribute('style', `--percent:${random <= 50 ? random : 50};`)
document
.getElementById('right')
.setAttribute('style', `--percent:${random - 100};`)
}
</script>
</body>
</html>
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