貌似之前本身也寫過圖片懶加載插件,可是新公司使用的是jQuery.lazyload插件,爲了更好的運用,本身仍是把源碼看了遍,分別記錄瞭如何使用,php
插件原理,各個配置屬性的完整解釋,demo實例,源碼分析(較簡短),源碼分析能夠配合使用,配置屬性,原理進行閱讀,如需轉載,請註明出處css
博客園 華子yjh瀏覽器
// 最簡單的使用,不帶參數 $('img').lazyload(); // 帶參數(配置對象),下面配置對象中的各個屬性值都是默認的 $('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" });
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),方便之後知足條件時觸發該事件;
在配置對象中有一個container屬性配置,默認爲window,若是img元素在該container容器視口中,則觸發appear事件;
爲了判斷img元素是否在container容器視口範圍中,造了以下四個輪子:app
$.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 $.leftofbegin = function(element, settings) {}; // 在視口左方
看看源碼中是如何利用這四個輪子:iphone
if ($.abovethetop(this, settings) || $.leftofbegin(this, settings)) { /* Nothing. */ } // 不知足在上方,左方;也不知足在下方,右方; 則觸發appear事件 else if (!$.belowthefold(this, settings) && !$.rightoffold(this, settings)) { $this.trigger("appear"); }
臨界值,這個值是針對container容器的,即距離container容器視口的臨界值ide
{ threshold: 0 }
事件,container容器默認綁定這個事件,在這個事件被觸發時,會不斷的判斷img元素是否知足觸發appear的條件,
所以當瀏覽器不停的滾動下來時,若是知足條件,則顯示圖片;oop
另外還有一點,若是這個事件不是scroll事件,則選中的img元素都會綁定這個事件,綁定的這個事件中一樣會觸發內部appear事件;源碼分析
{ event: 'scroll' }
顯示方法,默認爲show,也能夠設置爲fadeIn,API中隱藏了一個配置屬性effectspeed,動畫運行的時間動畫
{ effect: "show" }
img元素的一個data屬性,用於存放圖片的真實地址this
{ data_attribute: "original", }
忽略隱藏的img元素
{ skip_invisible: true }
圖片佔位符,img元素默認src屬性爲1*1像素的透明圖片
{ placeholder: "data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAAAEAAAABCAYAAAAfFcSJAAAAAXNSR0IArs4c6QAAAARnQU1BAACxjwv8YQUAAAAJcEhZcwAADsQAAA7EAZUrDhsAAAANSURBVBhXYzh8+PB/AAffA0nNPuCLAAAAAElFTkSuQmCC" }
在img觸發appear事件時執行的回調
{ appear: null }
在img觸發load事件時執行的回調
{ load: null }
最後一個配置屬性failure_limit
{ failure_limit: 0 }
爲了便於理解,咱們先來看一段與其有關的源碼:
var counter = 0; elements.each(function() { if ($.abovethetop(this, settings) || $.leftofbegin(this, settings)) { // ... } else if (!$.belowthefold(this, settings) && !$.rightoffold(this, settings)) { // ... } else { if (++counter > settings.failure_limit) { return false; } } });
什麼意思呢,若是找到的是第 failure_limit 個img元素,且不在container視口上方,左方及視口內(能夠容許在視口下方,右方),則中斷循環
看完原理和配置屬性,是否以爲很簡單呢,來看看幾個demo吧
demo1 下拉滾動: http://jsfiddle.net/ddEPL/
demo2 Tab切換: http://jsfiddle.net/ddEPL/1/ http://jsfiddle.net/ddEPL/8/
/* * Lazy Load - jQuery plugin for lazy loading images * * Copyright (c) 2007-2013 Mika Tuupola * * Licensed under the MIT license: * http://www.opensource.org/licenses/mit-license.php * * Project home: * http://www.appelsiini.net/projects/lazyload * * Version: 1.9.3 * */ (function($, 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 = $(this); // 若是圖片隱藏,且忽略隱藏,則中斷循環 if (settings.skip_invisible && !$this.is(":visible")) { return; } if ($.abovethetop(this, settings) || $.leftofbegin(this, settings)) { /* Nothing. */ } // img知足在container視口中,則顯示 else if (!$.belowthefold(this, settings) && !$.rightoffold(this, settings)) { $this.trigger("appear"); /* if we found an image we'll load, reset the counter */ counter = 0; } // 若是找到的是第(failure_limit + 1)個img元素,且不在container視口上方,左方及視口內(能夠容許在視口下方,右方), // 則中斷循環 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. */ // 爲container綁定scroll事件 if (0 === settings.event.indexOf("scroll")) { $container.bind(settings.event, function() { return update(); }); } this.each(function() { var self = this; var $self = $(self); self.loaded = false; /* If no src attribute given use data:uri. */ // 設置佔位符 if ($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綁定appear,觸發後則移除該事件 $self.one("appear", function() { if (!this.loaded) { // 存在回調則觸發 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,過濾掉已經加載的img元素,避免下次在update中輪循 var temp = $.grep(elements, function(element) { return !element.loaded; }); elements = $(temp); // 存在回調則觸發 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. */ // 綁定不是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);