示例：
<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="UTF-8" />
<style>
.observedDiv {
width: 40vw;
height: 50vh;
background-color: lightblue;
}
</style>
</head>
<body>
<div id="observedDiv" class="observedDiv">这个 div 的宽高会被监听。</div>
<script>
const observedDiv = document.getElementById('observedDiv')
const resizeObserver = new ResizeObserver(entries => {
for (let entry of entries) {
const { width, height } = entry.contentRect
console.log(`新的宽度: ${width}px，新的高度: ${height}px`)
}
})
// 开启监听
resizeObserver.observe(observedDiv)
setTimeout(() => {
console.log('停止监听了')
// 卸载监听-指定dom
resizeObserver.unobserve(observedDiv)
// 卸载监听-所有
// resizeObserver.disconnect();
}, 5000)
</script>
</body>
</html>
