一、定义
预加载图片是提升用户体验的一个好办法，提前加载用户所需的图片，保证图片快速、无缝发布，使用户在浏览器网站时获得更好用户体验。常用于图片画廊等应用中。
[注意]若使用即时加载，加载的图片与页面的其他内容一起加载会增加页面的整体加载时间，所以使用window.onload比较合适。
二、两种思路
1、使用背景图像
使用页面无用元素的背景图片预加载
<style>body{ margin: 0;}img{ width: 100px; height: 100px;}ul{ margin: 0; padding: 0; list-style: none;}.list li{ height: 0; width: 0;}</style></head><body><button>载入图片</button><img src="img/test.png" alt="测试"><ul class="list"> <li id="preload1"></li> <li id="preload2"></li> <li id="preload3"></li> <li id="preload4"></li></ul><script>var oBtn = document.getElementsByTagName('button')[0];var oImg0 = document.images[0];var array = ["img/img1.gif","img/img2.gif","img/img3.gif","img/img4.gif"]var iNow = -1;oBtn.onclick = function(){ iNow++; iNow = iNow%4; oImg0.src = array[iNow];}function preLoadImg(){ preload1.style.background = "url('img/img1.gif')"; preload2.style.background = "url('img/img2.gif')"; preload3.style.background = "url('img/img3.gif')"; preload4.style.background = "url('img/img4.gif')";}window.onload = function(){ preLoadImg(); }</script></body>
2、使用Image()
通过new Image()或document.createElement('img')创建<img>标签，然后通过<img>src赋值语句来加载图片
<style>body{ margin: 0;}img{ width: 100px; height: 100px;}</style></head><body><button>载入图片</button><img src="img/test.png" alt="测试"><script>var oBtn = document.getElementsByTagName('button')[0];var oImg0 = document.images[0];var array = ["img/img1.gif","img/img2.gif","img/img3.gif","img/img4.gif"]var iNow = -1;oBtn.onclick = function(){ iNow++; iNow = iNow%4; oImg0.src = array[iNow];}var aImages = [];function preLoadImg(array){ for(var i = 0, len = preLoadImg.arguments[0].length; i < len; i++){ aImages[i] = new Image(); aImages[i].src = preLoadImg.arguments[0][i]; }}window.onload = function(){ preLoadImg(array); }</script></body>
三、onload事件
利用图像的onload事件可以确切地知道图片是否被真正加载，并可能在后续执行一系列对图片的操作功能，如获取当前图片的实际宽高及索引等
[注意1]图片的src赋值语句必须放在图片的onload事件后面。否则可能出现图片已经加载完毕、但事件绑定尚未完成的情况
<button>载入图片</button><script>var oBtn = document.getElementsByTagName('button')[0];oBtn.onclick = function(){ preLoadImg('img/test.png');}function preLoadImg(url){ var oImg = document.createElement('img'); //在本机环境下，IE8-浏览器下oImg的onload事件放在src后面将无法载入图片 oImg.src = url; oImg.onload = function(){ document.body.appendChild(oImg); oImg.onload = null; oImg = null; } }</script>
[注意2]Image对象的onload属性引用了一个匿名函数对象，而匿名函数通过其作用域引用Image对象，这种循环引用会有IE6中导致内存泄漏，因此，应该解除循环引用。
【递归写法】
<!DOCTYPE html><html lang="en"><head><meta charset="UTF-8"><title>Document</title><style>body{ margin: 0;}img{ width: 100px; height: 100px;}</style></head><body><button>载入图片</button><img src="img/test.png" alt="测试"><script>var oBtn = document.getElementsByTagName('button')[0];var oImg0 = document.images[0];var array = ["img/img1.gif","img/img2.gif","img/img3.gif","img/img4.gif"]var iNow = -1;oBtn.onclick = function(){ iNow++; iNow = iNow%4; oImg0.src = array[iNow];}var oImg = document.createElement('img');var iDown = 0; preLoadImg();function preLoadImg(){ oImg.onload = function(){ iDown++; alert('第' + iDown + '张图片的宽:' + this.width + ' 高:' + this.height); if(iDown < array.length){ preLoadImg(); }else{ oImg.onload = null; oImg = null; } } oImg.src = array[iDown]; }</script></body></html>
【考虑onerror的更完善写法】
<!DOCTYPE html><html lang="en"><head><meta charset="UTF-8"><title>Document</title><style>body{ margin: 0;}img{ width: 100px; height: 100px;}</style></head><body><button>载入图片</button><img src="img/test.png" alt="测试"><script>var oBtn = document.getElementsByTagName('button')[0];var oImg0 = document.images[0];var array = ["img/img1.gif","img/img2.gif","img/img3.gif","img/img4.gif"]var iNow = -1;oBtn.onclick = function(){ iNow++; iNow = iNow%4; oImg0.src = array[iNow];}var iDown = 0;var oImage = new Image();function preLoadImg(arr){ function loadImgTest(arr){ iDown++; if(iDown < arr.length){ preLoadImg(arr); }else{ alert('ok'); oImg.onload = null; oImg = null; } } oImage.onload = function(){ loadImgTest(arr); }; oImage.onerror = function(){ loadImgTest(arr); }; oImage.src = arr[iDown];}preLoadImg(array);</script></body></html>
【循环写法】
<!DOCTYPE html><html lang="en"><head><meta charset="UTF-8"><title>Document</title><style>body{ margin: 0;}img{ width: 100px; height: 100px;}</style></head><body><button>载入图片</button><img src="img/test.png" alt="测试"><script>var oBtn = document.getElementsByTagName('button')[0];var oImg0 = document.images[0];var array = ["img/img1.gif","img/img2.gif","img/img3.gif","img/img4.gif"]var iNow = -1;oBtn.onclick = function(){ iNow++; iNow = iNow%4; oImg0.src = array[iNow];}function preLoadImg(arr,callback){ var aImages = []; var iDown = 0; for(var i = 0; i < arr.length; i++){ aImages[i] = new Image(); aImages[i].onload = function(){ loadImgTest(arr,callback); }; aImages[i].onerror = function(){ loadImgTest(arr,callback); }; aImages[i].src = arr[iDown]; } function loadImgTest(arr,callback){ iDown++; if(iDown == arr.length){ alert('ok'); callback && callback.call(aImages); } } }preLoadImg(array,function(){ console.log(this[0].width);});</script></body></html>应用：预加载模糊变清晰<!DOCTYPE html><html lang="en"><head><meta charset="UTF-8"><title>Document</title><style>body{ margin: 0;}img{ width: 500px; height: 500px;}</style></head><body><button>载入图片</button><img src="#" alt="测试"><script>var oBtn = document.getElementsByTagName('button')[0];var oImg0 = document.images[0];var arrayB = ["img/img1.gif","img/img2.gif","img/img3.gif","img/img4.gif"];var arrayL = ["img/img1.jpg","img/img2.jpg","img/img3.jpg","img/img4.jpg"];var iNow = -1;oBtn.onclick = function(){ iNow++; iNow = iNow%4; oImg0.src = arrayL[iNow]; aftLoadImg(arrayB,oImg0);}var aImages = [];window.onload = function(){ preLoadImg(arrayL); }function preLoadImg(arr){ for(var i = 0, len = arr.length; i < len; i++){ aImages[i] = new Image(); aImages[i].src = arr[i]; }}function aftLoadImg(arr,obj){ var oImg = new Image(); oImg.onload = function(){ obj.src = arr[iNow]; } oImg.src = arr[iNow];}</script></body></html>
希望本文所述对大家的javascript程序设计有所帮助。
