Complete example of javascript image preloading_javascript skills
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 ? 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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