转载

图片预加载

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。

两种思路

【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>

【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>

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>