基于javascript实现图片预加载

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。