关于移动端 Rem 布局的一些总结

!function(win, lib) {
    var timer,
        doc     = win.document,
        docElem = doc.documentElement,

        vpMeta   = doc.querySelector('meta[name="viewport"]'),
        flexMeta = doc.querySelector('meta[name="flexible"]'),
 
        dpr   = 0,
        scale = 0,
 
        flexible = lib.flexible || (lib.flexible = {});
 
    // 设置了 viewport meta
    if (vpMeta) {
 
        console.warn("将根据已有的meta标签来设置缩放比例");
        var initial = vpMeta.getAttribute("content").match(/initial\-scale=([\d\.]+)/);
 
        if (initial) {
            scale = parseFloat(initial[1]); // 已设置的 initialScale
            dpr = parseInt(1 / scale);      // 设备像素比 devicePixelRatio
        }
 
    }
    // 设置了 flexible Meta
    else if (flexMeta) {
        var flexMetaContent = flexMeta.getAttribute("content");
        if (flexMetaContent) {
 
            var initial = flexMetaContent.match(/initial\-dpr=([\d\.]+)/),
                maximum = flexMetaContent.match(/maximum\-dpr=([\d\.]+)/);
 
            if (initial) {
                dpr = parseFloat(initial[1]);
                scale = parseFloat((1 / dpr).toFixed(2));
            }
 
            if (maximum) {
                dpr = parseFloat(maximum[1]);
                scale = parseFloat((1 / dpr).toFixed(2));
            }
        }
    }
 
    // viewport 或 flexible
    // meta 均未设置
    if (!dpr && !scale) {
        // QST
        // 这里的 第一句有什么用 ?
        // 和 Android 有毛关系 ?
        var u = (win.navigator.appVersion.match(/android/gi), win.navigator.appVersion.match(/iphone/gi)),
            _dpr = win.devicePixelRatio;
 
        // 所以这里似乎是将所有 Android 设备都设置为 1 了
        dpr = u ? ( (_dpr >= 3 && (!dpr || dpr >= 3))
                        ? 3
                        : (_dpr >= 2 && (!dpr || dpr >= 2))
                            ? 2
                            : 1
                  )
                : 1;
 
        scale = 1 / dpr;
    }
 
    docElem.setAttribute("data-dpr", dpr);
 
    // 插入 viewport meta
    if (!vpMeta) {
        vpMeta = doc.createElement("meta");
         
        vpMeta.setAttribute("name", "viewport");
        vpMeta.setAttribute("content",
            "initial-scale=" + scale + ", maximum-scale=" + scale + ", minimum-scale=" + scale + ", user-scalable=no");
 
        if (docElem.firstElementChild) {
            docElem.firstElementChild.appendChild(vpMeta)
        } else {
            var div = doc.createElement("div");
            div.appendChild(vpMeta);
            doc.write(div.innerHTML);
        }
    }
 
    function setFontSize() {
        var winWidth = docElem.getBoundingClientRect().width;
 
        if (winWidth / dpr > 540) {
            (winWidth = 540 * dpr);
        }
 
        // 根节点 fontSize 根据宽度决定
        var baseSize = winWidth / 10;
 
        docElem.style.fontSize = baseSize + "px";
        flexible.rem = win.rem = baseSize;
    }
 
    // 调整窗口时重置
    win.addEventListener("resize", function() {
        clearTimeout(timer);
        timer = setTimeout(setFontSize, 300);
    }, false);
 
     
    // 这一段是我自己加的
    // orientationchange 时也需要重算下吧
    win.addEventListener("orientationchange", function() {
        clearTimeout(timer);
        timer = setTimeout(setFontSize, 300);
    }, false);
 
 
    // pageshow
    // keyword: 倒退 缓存相关
    win.addEventListener("pageshow", function(e) {
        if (e.persisted) {
            clearTimeout(timer);
            timer = setTimeout(setFontSize, 300);
        }
    }, false);
 
    // 设置基准字体
    if ("complete" === doc.readyState) {
        doc.body.style.fontSize = 12 * dpr + "px";
    } else {
        doc.addEventListener("DOMContentLoaded", function() {
            doc.body.style.fontSize = 12 * dpr + "px";
        }, false);
    }
  
    setFontSize();
 
    flexible.dpr = win.dpr = dpr;
 
    flexible.refreshRem = setFontSize;
 
    flexible.rem2px = function(d) {
        var c = parseFloat(d) * this.rem;
        if ("string" == typeof d && d.match(/rem$/)) {
            c += "px";
        }
        return c;
    };
 
    flexible.px2rem = function(d) {
        var c = parseFloat(d) / this.rem;
         
        if ("string" == typeof d && d.match(/px$/)) {
            c += "rem";
        }
        return c;
    }
}(window, window.lib || (window.lib = {}));

!(function(doc, win) {
    var docEle = doc.documentElement,
        evt = "onorientationchange" in window ? "orientationchange" : "resize",
        fn = function() {
            var width = docEle.clientWidth;
            width && (docEle.style.fontSize = 20 * (width / 320) + "px");
        };
     
    win.addEventListener(evt, fn, false);
    doc.addEventListener("DOMContentLoaded", fn, false);
 
}(document, window));