LazyLoad.js及scrollLoading.js

目前图片延迟加载主要分两大块，一是触发加载（根据滚动条位置加载图片）；二是自动预加载（加载完首屏后n秒后自动加载其他位置的图片）。现在最流行使用的就是LazyLoad.js及scrollLoading.js这两个用Jquery写的小插件。

一、LazyLoad实现：

网上找到一个更好点的处理方式，以下方式转自http://www.poluoluo.com/jzxy/201110/145261.html：

//首先用JQ判断浏览器类型及版本，如果是IE8以下的浏览器就用effect=show，否则就用effect=fadeIn，判断浏览器版本函数如下
function checkbrowse() {
var ua = navigator.userAgent.toLowerCase();
var is = (ua.match(/\b(chrome|opera|safari|msie|firefox)\b/) || ['', 'mozilla'])[1];
var r = '(?:' + is + '|version)[\\/: ]([\\d.]+)';
var v = (ua.match(new RegExp(r)) || [])[1];
jQuery.browser.is = is;
jQuery.browser.ver = v;
return {
'is': jQuery.browser.is,
'ver': jQuery.browser.ver
}
}

//改造Jquery.LazyLoad.js函数，根据浏览器版本来显示不同的滤镜效果
var public = checkbrowse();
var showeffect = "";
if ((public['is'] == 'msie' && public['ver'] < 8.0)) {
showeffect = "show"
} else {
showeffect = "fadeIn"
}
jQuery(document).ready(function($) {
$("img").lazyload({
placeholder: "http://demo.jb51.net/js/2011/lazyload/Js/lazyload/grey.gif",
effect: showeffect,
failurelimit: 10
})
});
</script>

html:

而LazyLoad只是给你一个延迟加载的假象，每次图片都会加载两次，完全就是一个假象的延迟加载。可以修改LazyLoad文件实现真实延迟加载，本人比较懒，就直接在网上找了一个修改好的，嘿嘿

/*
* Lazy Load - jQuery plugin for lazy loading images
*
* Copyright (c) 2007-2009 Mika Tuupola
*
* Licensed under the MIT license:
*   http://www.opensource.org/licenses/mit-license.php
*
* Project home:
*   http://www.appelsiini.net/projects/lazyload
*
* Version:  1.5.0
*
*/
(function ($) {
    $.fn.lazyload = function (options) {
        var settings = {
            threshold: 0,
            failurelimit: 0,
            event: "scroll",
            effect: "show",
            container: window
        };
        if (options) {
            $.extend(settings, options);
        }
        /* Fire one scroll event per scroll. Not one scroll event per image. */
        var elements = this;
        if ("scroll" == settings.event) {
            $(settings.container).bind("scroll", function (event) {
                var counter = 0;
                elements.each(function () {
                    if ($.abovethetop(this, settings) ||
$.leftofbegin(this, settings)) {
                        /* Nothing. */
                    } else if (!$.belowthefold(this, settings) &&
!$.rightoffold(this, settings)) {
                        $(this).trigger("appear");
                    } else {
                        if (counter++ > settings.failurelimit) {
                            return false;
                        }
                    }
                });
                /* Remove image from array so it is not looped next time. */
                var temp = $.grep(elements, function (element) {
                    return !element.loaded;
                });
                elements = $(temp);
            });
        }
        this.each(function () {
            var self = this;
            //删除的代码

            /* When appear is triggered load original image. */
            $(self).one("appear", function () {
                if (!this.loaded) {
                    $("<img />")
.bind("load", function () {
    $(self)
.hide()
.attr("src", $(self).attr("original"))
[settings.effect](settings.effectspeed);
    self.loaded = true;
})
.attr("src", $(self).attr("original"));
                };
            });
            /* When wanted event is triggered load original image */
            /* by triggering appear.                              */
            if ("scroll" != settings.event) {
                $(self).bind(settings.event, function (event) {
                    if (!self.loaded) {
                        $(self).trigger("appear");
                    }
                });
            }
        });
        /* Force initial check if images should appear. */
        $(settings.container).trigger(settings.event);
        return this;
    };
    /* Convenience methods in jQuery namespace.           */
    /* Use as  $.belowthefold(element, {threshold : 100, container : window}) */
    $.belowthefold = function (element, settings) {
        if (settings.container === undefined || settings.container === window) {
            var fold = $(window).height() + $(window).scrollTop();
        } else {
            var fold = $(settings.container).offset().top + $(settings.container).height();
        }
        return fold <= $(element).offset().top - settings.threshold;
    };
    $.rightoffold = function (element, settings) {
        if (settings.container === undefined || settings.container === window) {
            var fold = $(window).width() + $(window).scrollLeft();
        } else {
            var fold = $(settings.container).offset().left + $(settings.container).width();
        }
        return fold <= $(element).offset().left - settings.threshold;
    };
    $.abovethetop = function (element, settings) {
        if (settings.container === undefined || settings.container === window) {
            var fold = $(window).scrollTop();
        } else {
            var fold = $(settings.container).offset().top;
        }
        return fold >= $(element).offset().top + settings.threshold + $(element).height();
    };
    $.leftofbegin = function (element, settings) {
        if (settings.container === undefined || settings.container === window) {
            var fold = $(window).scrollLeft();
        } else {
            var fold = $(settings.container).offset().left;
        }
        return fold >= $(element).offset().left + settings.threshold + $(element).width();
    };
    /* Custom selectors for your convenience.   */
    /* Use as $("img:below-the-fold").something() */
    $.extend($.expr[':'], {
        "below-the-fold": "$.belowthefold(a, {threshold : 0, container: window})",
        "above-the-fold": "!$.belowthefold(a, {threshold : 0, container: window})",
        "right-of-fold": "$.rightoffold(a, {threshold : 0, container: window})",
        "left-of-fold": "!$.rightoffold(a, {threshold : 0, container: window})"
    });
})(jQuery);