import React from 'react';
import PropTypes from 'prop-types';

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>);
    }
}

Header.propTypes = {
    children: PropTypes.node
};