Lazy Load 源码学习注释笔记

/*!
 * Lazy Load - jQuery plugin for lazy loading images
 *
 * Copyright (c) 2007-2015 Mika Tuupola
 *
 * Licensed under the MIT license:
 *   http://www.opensource.org/licenses/mit-license.php
 *
 * Project home:
 *   http://www.appelsiini.net/projects/lazyload
 *
 * Version:  1.9.7
 *
 */

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

    $.fn.lazyload = function(options) {
        var elements = this;
        //this指向谁?指向函数的所有者。例如 $("img.lazy").lazyload();,this代表的是$("img.lazy")选择器中的元素数组
        var $container;
        var settings = {
            threshold       : 0,
            failure_limit   : 0,
            event           : "scroll",//初始触发事件时scroll
            effect          : "show",
            container       : window,
            data_attribute  : "original",
            skip_invisible  : false,
            appear          : null,
            load            : null,
            placeholder     : "data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAAAEAAAABCAYAAAAfFcSJAAAAAXNSR0IArs4c6QAAAARnQU1BAACxjwv8YQUAAAAJcEhZcwAADsQAAA7EAZUrDhsAAAANSURBVBhXYzh8+PB/AAffA0nNPuCLAAAAAElFTkSuQmCC"
        };

        function update() {
            var counter = 0;

            elements.each(function() {
                var $this = $(this);//this指的是每个img元素
                if (settings.skip_invisible && !$this.is(":visible")) {
                    return;
                }
                if ($.abovethetop(this, settings) ||
                    $.leftofbegin(this, settings)) {//如果指定的容器有一部分超出了屏幕最上端或最左端
                        /* Nothing. */
                } else if (!$.belowthefold(this, settings) &&
                    !$.rightoffold(this, settings)) {


                        $this.trigger("appear");
                        /*以上是加载条件,即:图片已经暴露,或者到了配置的暴露极限值*/
                        /* if we found an image we'll load, reset the counter */
                        counter = 0;
                } 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. */
        if (0 === settings.event.indexOf("scroll")) {

            $container.on(settings.event, function() {//1.只要有对应事件触发,就会调用update

                return update();
            });
        }

        this.each(function() {//对选择器中的元素进行遍历处理

            var self = this;//到了这里,*this*就代表每一个元素了,为了避免混淆,把this赋值给self
            var $self = $(self);

            self.loaded = false;

            /* If no src attribute given use data:uri. */
            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. */
            $self.one("appear", function() {//一旦出现在浏览器的view port中(注意这里用了one,意思是,只在第一次触发此事件时执行以下的动作)
               
                if (!this.loaded) {//这个图片尚未定义为:已加载
                    if (settings.appear) {//跳过,初始为null
                        var elements_left = elements.length;
                        settings.appear.call(self, elements_left, settings);
                    }
                    //动态创建DOM元素
                   /* $("<img />")
                        .one("load", function() {*///动态创建的img元素加载完成时的动作
                            var original = $self.attr("data-" + settings.data_attribute);
                            //alert(111)
                            $self.hide();//设置为display:none,意为阻止加载
                            if ($self.is("img")) {
                                $self.attr("src", original);
                            } else {
                                $self.css("background-image", "url('" + original + "')");
                            }

                            $self[settings.effect](settings.effect_speed);
                            // $self['show'](argument),等同于 $self.show(argument),此时图片开始加载

                            self.loaded = true;//标记为已经加载

                            /* Remove image from array so it is not looped next time. */
                            //为了优化,下次循环时不要带上
                            var temp = $.grep(elements, function(element) {
                                return !element.loaded;
                            });
                            elements = $(temp);//轻装上阵?

                            if (settings.load) {//初始为null,跳过

                                var elements_left = elements.length;
                                settings.load.call(self, elements_left, settings);
                            }
                        /*})
                        .attr("src", $self.attr("data-" + settings.data_attribute));*///赋给src属性,去加载吧!我的图片!
                }
            });
            /* When wanted event is triggered load original image */
            /* by triggering appear.                              */
            if (0 !== settings.event.indexOf("scroll")) {//如果settings.event中,scroll并不是在首部出现

                $self.on(settings.event, function() {
                    if (!self.loaded) {

                        $self.trigger("appear");
                    }
                });
            }
        });

        /* Check if something appears when window is resized. */
        $window.on("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.on("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}) */
    //以下会判断,是否满足了加载条件,满足会返回false
    $.belowthefold = function(element, settings) {
        var fold;

        if (settings.container === undefined || settings.container === window) {

            fold = (window.innerHeight ? window.innerHeight : $window.height()) + $window.scrollTop();//文档超出屏幕的高度+window本身的高度
            //console.log(fold)
        } else {
            fold = $(settings.container).offset().top + $(settings.container).height();
        }
        /*
        offset() 方法返回或设置匹配元素相对于文档的偏移（位置）。
        如果文档在屏幕上方的高度+屏幕本身的高度,大于元素在文档中的纵向偏移量(从文档顶端到达元素的纵向距离),那就说明元素已经暴露出来了,则返回true
        如果设定了threshold,则表示:元素还没出现时,差多少px就开始加载
        */
        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);