- FMP 即 First Meaningful Paint，有意义的首屏时间
- 通过 MutationObserver 观察 DOM 结构变化以及视口范围计算实现
实现方案
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const details = [];
const ignoreEleList = ['script', 'style', 'link', 'br'];
let observeDom;
let firstScreenTiming;
// 查看当前元素的祖先元素是否在数组中
function isEleInArray(target, arr) {
if (!target || target === document.documentElement) {
return false;
} else if (arr.indexOf(target) !== -1) {
return true;
} else {
return isEleInArray(target.parentElement, arr);
}
}
function isInFirstScreen(target) {
if (!target || !target.getBoundingClientRect) return false;
const rect = target.getBoundingClientRect();
const screenHeight = window.innerHeight;
const screenWidth = window.innerWidth;
return rect.left >= 0
&& rect.left < screenWidth
&& rect.top >= 0
&& rect.top < screenHeight;
}
function updateTiming() {
if (observeDom) {
observeDom.disconnect();
}
for (let i = 0; i < details.length; i++) {
const detail = details[i]
for (let j = 0; j < detail.roots.length; j++) {
if (isInFirstScreen(detail.roots[j])) {
firstScreenTiming = detail.time;
break;
}
}
if (typeof firstScreenTiming === 'number') {
break;
}
}
console.log('ccc firstScreenTiming', firstScreenTiming);
}
if (window.MutationObserver) {
observeDom = new MutationObserver((mutations => {
if (!mutations || !mutations.forEach) return;
const detail = {
time: performance.now(),
roots: [],
};
mutations.forEach(mutation => {
if (!mutation || !mutation.addedNodes || !mutation.addedNodes.forEach) return;
mutation.addedNodes.forEach(ele => {
if (ele.nodeType === 1 && ignoreEleList.indexOf(ele.nodeName.toLocaleLowerCase()) === -1) {
if (!isEleInArray(ele, detail.roots)) {
detail.roots.push(ele);
}
}
});
});
if (detail.roots.length) {
details.push(detail);
}
}));
observeDom.observe(document, {
childList: true,
subtree: true,
});
}
window.addEventListener('load', function () {
updateTiming()
});
效果
由图可见，页面是 Node 直出，在页面 DOM 元素开始渲染的时候会触发 MutationObserver 事件，即可获取到渲染事件。