JS实现移动端图片延迟加载

/**
 * lazy.js
 * 图片延迟加载
 * Created by 赵小磊 on 2016/1/15.
 */

(function (window, document) {
    var lazy = function (elem, options) {
        var photos = typeof elem == "string" ? document.querySelectorAll(elem) : elem,
            len = photos.length,
            options = options || {},
            size = options.size || 5,
            attr = options.attr || "data-original",
            callback = options.callback || "",
            page = 0;
        function loadPhoto() {
            var scrollTop = document.body.scrollTop || document.documentElement.scrollTop,
                docHeight = document.body.scrollHeight,
                winHeight = document.documentElement.clientHeight,
                i = 0,
                sum = 0;
            if (scrollTop + winHeight >= docHeight - 50) {
                page++;
                sum = size * page;
                for (i; i < sum; i++) {
                    if (i < len) {
                        var photo = photos[i],
                            photoSrc = photo.getAttribute(attr);
                        if (photoSrc) {
                            photo.src = photoSrc;
                            photo.removeAttribute(attr);
                            if (i == sum - 1) {
                                photo.onload = function () {
                                    if (docHeight <= winHeight) {
                                        loadPhoto();
                                    }
                                }
                            }
                            if (callback && i == len - 1) {
                                callback();
                            }
                        }
                    }
                }
            }
        }
        window.addEventListener("load", loadPhoto, false);
        window.addEventListener("scroll", loadPhoto, false);
        window.addEventListener("touchmove", loadPhoto, false);
    };
    window.lazy = lazy;
})(window, document);

var lazyPhoto = new lazy(".topic img", {
            size: 3,
            callback: function () {
                //console.log("加载完了");
            }
        });