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import React from 'react';
const elClassHidden = 'header--hidden'
const elClassBackground = 'header--background';
export default class Header extends React.Component {
constructor(props) {
super(props);
this.dHeight = 0;
this.wHeight = 0;
this.wScrollCurrent = 0;
this.wScrollBefore = 0;
this.wScrollDiff = 0;
this.header = React.createRef()
}
componentDidMount() {
const header = this.header.current;
window.addEventListener('scroll', this.throttle(250, () => {
this.dHeight = document.body.offsetHeight;
this.wHeight = window.innerHeight;
this.wScrollCurrent = window.pageYOffset;
this.wScrollDiff = this.wScrollBefore - this.wScrollCurrent;
if (this.wScrollCurrent <= 0) {
// scrolled to the very top; element sticks to the top
header.classList.remove(elClassHidden)
header.classList.remove(elClassBackground)
} else if (this.wScrollDiff > 0 && header.classList.contains(elClassHidden)) {
// scrolled up; element slides in
header.classList.remove(elClassHidden)
header.classList.add(elClassBackground)
} else if (this.wScrollDiff < 0) {
// scrolled down
if (this.wScrollCurrent + this.wHeight >= this.dHeight && header.classList.contains(elClassHidden)) {
// scrolled to the very bottom; element slides in
header.classList.remove(elClassHidden)
header.classList.add(elClassBackground)
} else {
// scrolled down; element slides out
header.classList.add(elClassHidden)
}
}
this.wScrollBefore = this.wScrollCurrent;
}), { passive: true });
}
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);
}
};
};
render() {
return (<header ref={this.header}>{this.props.children}</header>);
}
}
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