前言：

貌似以前自己也寫過圖片懶載入外掛，但是新公司使用的是jQuery.lazyload外掛，為了更好的運用，自己還是把原始碼看了遍，分別記錄瞭如何使用，

外掛原理，各個配置屬性的完整解釋，demo例項，原始碼分析(較簡短)，原始碼分析可以配合使用，配置屬性，原理進行閱讀，如需轉載，請註明出處

部落格園 華子yjh

一、如何使用

// 最簡單的使用，不帶引數
$('img').lazyload();


// 帶引數(配置物件)，下面配置物件中的各個屬性值都是預設的
$('img').lazyload({
    threshold       : 0,
    failure_limit   : 
 
0,
    event           : "scroll",
    effect          : "show",
    container       : window,
    data_attribute  : "original",
    skip_invisible  : true,
    appear          : null,
    load            : null,
    placeholder     : "data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAAAEAAAABCAYAAAAfFcSJAAAAAXNSR0IArs4c6QAAAARnQU1BAACxjwv8YQUAAAAJcEhZcwAADsQAAA7EAZUrDhsAAAANSURBVBhXYzh8+PB/AAffA0nNPuCLAAAAAElFTkSuQmCC"
});
 

二、內部原理

首先選中的img元素都綁定了一個appear事件(處理img顯示真實的圖片地址)，方便以後滿足條件時觸發該事件；
在配置物件中有一個container屬性配置，預設為window，如果img元素在該container容器視口中，則觸發appear事件；
為了判斷img元素是否在container容器視口範圍中，造瞭如下四個輪子：

$.belowthefold = function(element, settings) {};    // 在視口下方
$.rightoffold = function(element, settings) {};        // 在視口右方
 

$.abovethetop = function(element, settings) {};        // 在視口上方
$.leftofbegin = function(element, settings) {};        // 在視口左方

看看原始碼中是如何利用這四個輪子：

if ($.abovethetop(this, settings) || $.leftofbegin(this, settings)) {
    /* Nothing. */
}
// 不滿足在上方，左方；也不滿足在下方，右方； 則觸發appear事件
else if (!$.belowthefold(this, settings) && !$.rightoffold(this, settings)) {
    $this.trigger("appear");
}

三、配置物件中的其他屬性

臨界值，這個值是針對container容器的，即距離container容器視口的臨界值

{
    threshold: 0
}

事件，container容器預設繫結這個事件，在這個事件被觸發時，會不斷的判斷img元素是否滿足觸發appear的條件，
因此當瀏覽器不停的滾動下來時，如果滿足條件，則顯示圖片；

另外還有一點，如果這個事件不是scroll事件，則選中的img元素都會繫結這個事件，繫結的這個事件中同樣會觸發內部appear事件；

{
    event: 'scroll'
}

顯示方法，預設為show，也可以設定為fadeIn，API中隱藏了一個配置屬性effectspeed，動畫執行的時間

{
    effect: "show"
}

img元素的一個data屬性，用於存放圖片的真實地址

{
    data_attribute: "original",
}

忽略隱藏的img元素

{
    skip_invisible: true
}

圖片佔位符，img元素預設src屬性為1*1畫素的透明圖片

{
    placeholder: "data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAAAEAAAABCAYAAAAfFcSJAAAAAXNSR0IArs4c6QAAAARnQU1BAACxjwv8YQUAAAAJcEhZcwAADsQAAA7EAZUrDhsAAAANSURBVBhXYzh8+PB/AAffA0nNPuCLAAAAAElFTkSuQmCC"
}

在img觸發appear事件時執行的回撥

{
    appear: null
}

在img觸發load事件時執行的回撥

{
    load: null
}

最後一個配置屬性failure_limit

{
    failure_limit: 0
}

為了便於理解，我們先來看一段與其有關的原始碼：

var counter = 0;

elements.each(function() {
    if ($.abovethetop(this, settings) || $.leftofbegin(this, settings)) {
        // ...
    } else if (!$.belowthefold(this, settings) && !$.rightoffold(this, settings)) {
        // ...
    } else {
        if (++counter > settings.failure_limit) {
            return false;
        }
    }
});

什麼意思呢，如果找到的是第 failure_limit 個img元素，且不在container視口上方，左方及視口內（可以允許在視口下方，右方），則中斷迴圈

三、demo

看完原理和配置屬性，是否覺得很簡單呢，來看看幾個demo吧

四、原始碼分析

/*
 * Lazy Load - jQuery plugin for lazy loading images
 *
 * Copyright (c) 2007-2013 Mika Tuupola
 *
 * Licensed under the MIT license:
 *   http://www.opensource.org/licenses/mit-license.php
 *
 * Project home:
 *   http://www.appelsiini.net/projects/lazyload
 *
 * Version:  1.9.3
 *
 */

(function($, window, document, undefined) {
    var $window = $(window);

    $.fn.lazyload = function(options) {
        var elements = this;
        var $container;
        var settings = {
            threshold       : 0,
            failure_limit   : 0,
            event           : "scroll",
            effect          : "show",
            container       : window,
            data_attribute  : "original",
            skip_invisible  : true,
            appear          : null,
            load            : null,
            placeholder     : "data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAAAEAAAABCAYAAAAfFcSJAAAAAXNSR0IArs4c6QAAAARnQU1BAACxjwv8YQUAAAAJcEhZcwAADsQAAA7EAZUrDhsAAAANSURBVBhXYzh8+PB/AAffA0nNPuCLAAAAAElFTkSuQmCC"
        };

        function update() {
            var counter = 0;

            elements.each(function() {
                var $this = $(this);
                // 如果圖片隱藏，且忽略隱藏，則中斷迴圈
                if (settings.skip_invisible && !$this.is(":visible")) {
                    return;
                }
                if ($.abovethetop(this, settings) || $.leftofbegin(this, settings)) {
                        /* Nothing. */
                } 
                // img滿足在container視口中，則顯示
                else if (!$.belowthefold(this, settings) && !$.rightoffold(this, settings)) {
                        $this.trigger("appear");
                        /* if we found an image we'll load, reset the counter */
                        counter = 0;
                }
                // 如果找到的是第（failure_limit + 1）個img元素，且不在container視口上方，左方及視口內（可以允許在視口下方，右方），
                // 則中斷迴圈
                else {
                    if (++counter > settings.failure_limit) {
                        return false;
                    }
                }
            });

        }

        if(options) {
            /* Maintain BC for a couple of versions. */
            if (undefined !== options.failurelimit) {
                options.failure_limit = options.failurelimit;
                delete options.failurelimit;
            }
            if (undefined !== options.effectspeed) {
                options.effect_speed = options.effectspeed;
                delete options.effectspeed;
            }

            $.extend(settings, options);
        }

        /* Cache container as jQuery as object. */
        $container = (settings.container === undefined ||
                      settings.container === window) ? $window : $(settings.container);

        /* Fire one scroll event per scroll. Not one scroll event per image. */
        // 為container繫結scroll事件
        if (0 === settings.event.indexOf("scroll")) {
            $container.bind(settings.event, function() {
                return update();
            });
        }

        this.each(function() {
            var self = this;
            var $self = $(self);

            self.loaded = false;

            /* If no src attribute given use data:uri. */
            // 設定佔位符
            if ($self.attr("src") === undefined || $self.attr("src") === false) {
                if ($self.is("img")) {
                    $self.attr("src", settings.placeholder);
                }
            }

            /* When appear is triggered load original image. */
            // one繫結appear，觸發後則移除該事件
            $self.one("appear", function() {
                if (!this.loaded) {
                    // 存在回撥則觸發
                    if (settings.appear) {
                        var elements_left = elements.length;
                        settings.appear.call(self, elements_left, settings);
                    }
                    $("<img />")
                        .bind("load", function() {

                            var original = $self.attr("data-" + settings.data_attribute);
                            $self.hide();
                            if ($self.is("img")) {
                                $self.attr("src", original);
                            } else {
                                $self.css("background-image", "url('" + original + "')");
                            }
                            $self[settings.effect](settings.effect_speed);

                            self.loaded = true;

                            /* Remove image from array so it is not looped next time. */
                            // 更新elements，過濾掉已經載入的img元素，避免下次在update中輪循
                            var temp = $.grep(elements, function(element) {
                                return !element.loaded;
                            });
                            elements = $(temp);
                            
                            // 存在回撥則觸發
                            if (settings.load) {
                                var elements_left = elements.length;
                                settings.load.call(self, elements_left, settings);
                            }
                        })
                        .attr("src", $self.attr("data-" + settings.data_attribute));
                }
            });

            /* When wanted event is triggered load original image */
            /* by triggering appear.                              */
            // 繫結不是scroll的事件，用於觸發appear事件
            if (0 !== settings.event.indexOf("scroll")) {
                $self.bind(settings.event, function() {
                    if (!self.loaded) {
                        $self.trigger("appear");
                    }
                });
            }
        });

        /* Check if something appears when window is resized. */
        $window.bind("resize", function() {
            update();
        });

        /* With IOS5 force loading images when navigating with back button. */
        /* Non optimal workaround. */
        if ((/(?:iphone|ipod|ipad).*os 5/gi).test(navigator.appVersion)) {
            $window.bind("pageshow", function(event) {
                if (event.originalEvent && event.originalEvent.persisted) {
                    elements.each(function() {
                        $(this).trigger("appear");
                    });
                }
            });
        }

        /* Force initial check if images should appear. */
        $(document).ready(function() {
            update();
        });

        return this;
    };

    /* Convenience methods in jQuery namespace.           */
    /* Use as  $.belowthefold(element, {threshold : 100, container : window}) */

    $.belowthefold = function(element, settings) {
        var fold;

        if (settings.container === undefined || settings.container === window) {
            fold = (window.innerHeight ? window.innerHeight : $window.height()) + $window.scrollTop();
        } else {
            fold = $(settings.container).offset().top + $(settings.container).height();
        }

        return fold <= $(element).offset().top - settings.threshold;
    };

    $.rightoffold = function(element, settings) {
        var fold;

        if (settings.container === undefined || settings.container === window) {
            fold = $window.width() + $window.scrollLeft();
        } else {
            fold = $(settings.container).offset().left + $(settings.container).width();
        }

        return fold <= $(element).offset().left - settings.threshold;
    };

    $.abovethetop = function(element, settings) {
        var fold;

        if (settings.container === undefined || settings.container === window) {
            fold = $window.scrollTop();
        } else {
            fold = $(settings.container).offset().top;
        }

        return fold >= $(element).offset().top + settings.threshold  + $(element).height();
    };

    $.leftofbegin = function(element, settings) {
        var fold;

        if (settings.container === undefined || settings.container === window) {
            fold = $window.scrollLeft();
        } else {
            fold = $(settings.container).offset().left;
        }

        return fold >= $(element).offset().left + settings.threshold + $(element).width();
    };

    $.inviewport = function(element, settings) {
         return !$.rightoffold(element, settings) && !$.leftofbegin(element, settings) &&
                !$.belowthefold(element, settings) && !$.abovethetop(element, settings);
     };

    /* Custom selectors for your convenience.   */
    /* Use as $("img:below-the-fold").something() or */
    /* $("img").filter(":below-the-fold").something() which is faster */

    $.extend($.expr[":"], {
        "below-the-fold" : function(a) { return $.belowthefold(a, {threshold : 0}); },
        "above-the-top"  : function(a) { return !$.belowthefold(a, {threshold : 0}); },
        "right-of-screen": function(a) { return $.rightoffold(a, {threshold : 0}); },
        "left-of-screen" : function(a) { return !$.rightoffold(a, {threshold : 0}); },
        "in-viewport"    : function(a) { return $.inviewport(a, {threshold : 0}); },
        /* Maintain BC for couple of versions. */
        "above-the-fold" : function(a) { return !$.belowthefold(a, {threshold : 0}); },
        "right-of-fold"  : function(a) { return $.rightoffold(a, {threshold : 0}); },
        "left-of-fold"   : function(a) { return !$.rightoffold(a, {threshold : 0}); }
    });

})(jQuery, window, document);