rem helper

rem.js

(function (doc, win) {
    var docEl = doc.documentElement,
        isIPhone = window.navigator.appVersion.match(/iphone/gi),
        platform = navigator.platform,
        justMobile = !/win32/i.test(platform),
        fontSize, scale;

    recalc();
    function recalc() {
        var clientWidth = docEl.clientWidth;
        var dpr = window.devicePixelRatio;

        // iOS下，对于2和3的屏，用2倍的方案，其余的用1倍方案。其他设备下，仍旧使用1倍的方案
        if (!isIPhone || !justMobile)) {
            dpr = 1;
            clientWidth = clientWidth ? clientWidth : (window.screen.width || 360);
        }
        scale = 1 / (dpr > 1 ? 2 : dpr);

        fontSize = 20 * (clientWidth / 320) / scale;
        fontSize = (fontSize > 54) ? 54: fontSize;
        // 如果fontSize是小数，取小于它的偶数
        if (~~fontSize !== fontSize) {
            fontSize = ~~(fontSize / 2) * 2
        }

        docEl.style.fontSize = fontSize + 'px';
        docEl.setAttribute('data-dpr', dpr);

        //设置viewport
        var viewport = document.querySelector('meta[name="viewport"]');
        var viewport_content = 'initial-scale=' + scale + ', maximum-scale=' + scale + ', minimum-scale=' + scale + ', user-scalable=no';

        viewport && viewport.setAttribute('content', viewport_content);
    };
})(document, window);