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import React, { useEffect, useCallback, useRef } from 'react';
import ReactDOM from 'react-dom';
import PropTypes from 'prop-types';
const elClassHidden = 'header--hidden';
const header = document.getElementById('header');
export default function Header({ children }) {
let dHeight = useRef(0);
let wHeight = useRef(0);
let wScrollCurrent = useRef(0);
let wScrollBefore = useRef(0);
let wScrollDiff = useRef(0);
useEffect(() => {
window.addEventListener('scroll', () => (!window.requestAnimationFrame)
? throttle(250, updateHeader)
: window.requestAnimationFrame(updateHeader), false);
}, [updateHeader]);
let throttle = (delay, fn) => {
var last, deferTimer;
return function () {
var context = this, args = arguments, now = +new Date;
if (last && now < last + delay) {
clearTimeout(deferTimer);
deferTimer = setTimeout(
function () {
last = now;
fn.apply(context, args);
},
delay);
} else {
last = now;
fn.apply(context, args);
}
};
};
let updateHeader = useCallback(() => {
dHeight.current = document.body.offsetHeight;
wHeight.current = window.innerHeight;
wScrollCurrent.current = window.pageYOffset;
wScrollDiff.current = wScrollBefore.current - wScrollCurrent.current;
if (wScrollCurrent.current <= 0) {
// scrolled to the very top; element sticks to the top
header.classList.remove(elClassHidden);
} else if (wScrollDiff > 0 && header.classList.contains(elClassHidden)) {
// scrolled up; element slides in
header.classList.remove(elClassHidden);
} else if (wScrollDiff.current < 0) {
// scrolled down
if (wScrollCurrent.current + wHeight.current >= dHeight.current && header.classList.contains(elClassHidden)) {
// scrolled to the very bottom; element slides in
header.classList.remove(elClassHidden);
} else {
// scrolled down; element slides out
header.classList.add(elClassHidden);
}
}
wScrollBefore.current = wScrollCurrent.current;
}, []);
return ReactDOM.createPortal(children, header);
}
Header.propTypes = {
children: PropTypes.node
};
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