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Complete example of javascript image preloading_javascript skills

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WBOYOriginal
2016-05-16 15:26:311595browse

The example in this article describes the method of javascript image preloading. Share it with everyone for your reference, the details are as follows: 

<!DOCTYPE >
<html xmlns="http://www.w3.org/1999/xhtml">
<head>
 <meta http-equiv="Content-Type" content="text/html; charset=utf-8"/>
 <title>图片预加载</title>
</head>
<body>
<img id="song" width="300" src="http://www.baidu.com/img/bdlogo.gif" _src="http://p2009c.zbjimg.com/task/2009-12/03/188273/middlexznxwkia.gif" />
</body>
</html>
<script type="text/javascript">
 //默认图片
 var defurl = 'http://www.baidu.com/img/bdlogo.gif';
 //一张大尺寸图片
 var imgurl = 'http://www.planeart.cn/demo/imgReady/vistas24.jpg';
 //一张gif图片
 var gifurl = 'http://p2009c.zbjimg.com/task/2009-12/03/188273/middlexznxwkia.gif';
 //图片预加载
 function loadImage(url, callback) {
  var img = new Image(); //创建一个Image对象，实现图片的预下载
  img.onload = function(){
   img.onload = null;//gif图片在ie下会循环请求
   callback(img);
  }
  img.src = url;
 }
 var img = document.getElementById('song');
 var url = img.getAttribute('_src')
 loadImage(gifurl,function(){
  //alert('ok');
  img.src = url;
 })
 /**
  * 网络上通用的图片预加载函数
  * @param url
  * @param callback
  */
 var imgLoad = function (url, callback) {
  var img = new Image();
  img.src = url;
  if (img.complete) {
   callback(img.width, img.height);
  } else {
   img.onload = function () {
    callback(img.width, img.height);
    img.onload = null;
   };
  };
 };
 // 更新：
 // 05.27: 1、保证回调执行顺序：error > ready > load；2、回调函数this指向img本身
 // 04-02: 1、增加图片完全加载后的回调 2、提高性能
 /**
  * 图片头数据加载就绪事件 - 更快获取图片尺寸
  *
  * 原理：没有缓存的情况下，用计时器不断验证图片的大小是否发生变化，如果不在变化则为ready
  *  如果有缓存则w3c浏览器会调用 complete，ie则会走 onload，都不在走 计时器那部分
  *
  * @version 2011.05.27
  * @author TangBin
  * @param {String} 图片路径
  * @param {Function} 尺寸就绪
  * @param {Function} 加载完毕 (可选)
  * @param {Function} 加载错误 (可选)
  * @example imgReady('http://www.google.com.hk/intl/zh-CN/images/logo_cn.png', function () {
alert('size ready: width=' + this.width + '; height=' + this.height);
});
  */
 var imgReady = (function () {
  var list = [], intervalId = null,
   // 用来执行队列
   tick = function () {
    var i = 0;
    for (; i < list.length; i++) {
     list[i].end &#63; list.splice(i--, 1) : list[i]();
    }
    ;
    !list.length && stop();
   },
   // 停止所有定时器队列
   stop = function () {
    clearInterval(intervalId);
    intervalId = null;
   };
  return function (url, ready, load, error) {
   var onready, width, height, newWidth, newHeight,
    img = new Image();
   img.src = url;
   // 如果图片被缓存，则直接返回缓存数据
   if (img.complete) {
    ready.call(img);
    load && load.call(img);
    return;
   }
   ;
   width = img.width;
   height = img.height;
   // 加载错误后的事件
   img.onerror = function () {
    error && error.call(img);
    onready.end = true;
    img = img.onload = img.onerror = null;
   };
   // 图片尺寸就绪
   onready = function () {
    newWidth = img.width;
    newHeight = img.height;
    if (newWidth !== width || newHeight !== height ||
     // 如果图片已经在其他地方加载可使用面积检测
     newWidth * newHeight > 1024
     ) {
     ready.call(img);
     onready.end = true;
    }
    ;
   };
   onready();
   // 完全加载完毕的事件
   img.onload = function () {
     // onload在定时器时间差范围内可能比onready快
    // 这里进行检查并保证onready优先执行
    !onready.end && onready();
    load && load.call(img);
    // IE gif动画会循环执行onload，置空onload即可
    img = img.onload = img.onerror = null;
   };
    // 加入队列中定期执行
   if (!onready.end) {
    list.push(onready);
    // 无论何时只允许出现一个定时器，减少浏览器性能损耗
    if (intervalId === null) intervalId = setInterval(tick, 40);
   }
   ;
  };
 })();
 var img_url = 'http://www.planeart.cn/demo/imgReady/vistas24.jpg';
 imgReady(img_url, function () {
  //console.info(this.width);
  //console.info(this.height);
  alert(this.width + '\n' + this.height);
  document.getElementById('song').src = img_url;
 })
</script>

I hope this article will be helpful to everyone in JavaScript programming.

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