JavaScript image preloading example analysis_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:

Lightbox effects use preloading to center-display images. They need to wait until they are fully loaded before they can be displayed, resulting in a poor experience (such as the full-screen effect of filick albums). JavaScript cannot obtain img file header data. Is this really the case? This article uses a clever method to let javascript obtain it.

This is an example of how most people use preloading to get the image size:

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;
    };
  };
};

JavaScript code:

// 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
 * @version 2011.05.27
 * <a href="http://my.oschina.net/arthor" class="referer" target="_blank">@author</a> TangBin
 * <a href="http://my.oschina.net/see" class="referer" target="_blank">@see</a>   http://www.planeart.cn/&#63;p=1121
 * @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);
    };
  };
})();

Call example:

imgReady('http://www.google.com.hk/intl/zh-CN/images/logo_cn.png', function () {
  alert('size ready: width=' + this.width + '; height=' + this.height);
});

I hope this article will be helpful to everyone’s JavaScript programming design.