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[Java教程]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>

 【效果演示】