/*!
* 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);
