图片下载器, 支持 callback

imageLoader.js

;(function(root, factory) {
  if(typeof define === 'function' && define.amd) {
    define([], factory);
  } else if(typeof exports === 'object') {
    module.exports = factory();
  } else {
    root.imageLoader = factory();
  }
})(this, function() {
  'use strict';
  
  var list = [];
  var loading = {};
  var cache = false;
  
  function callback(ret, src) {
    var cbs = loading[src];
    if (cbs) {
      for (var i = 0; i < cbs.length; ++i) {
        var cb = cbs[i];
        if (typeof cb === 'function') {
          cb(ret);
        }
      }
      delete loading[src];
      if (cache && !ret) {
        list.push([src, cbs]);
      }
    }
  }

  // 使用回调函数
  // 以便支持 image 标签的 src 属性以及 background-image 样式
  function load(src, cb, force) {
    if (loading[src]) {
      if (force) Array.prototype.push.apply(loading[src], cb);
      return;
    }
    loading[src] = [cb];
    var img = document.createElement('img');
    img.onload = function() {
      img.onerror = img.onload = null;
      callback(true, src);
    };
    img.onerror = function() {
      img.onerror = img.onload = null;
      callback(false, src);
    };
    img.src = src;
  }

  // 手动调用. ex: offline -> online 时调用.
  function reload() {
    var item, stack = list.slice(0);
    list.length = 0;
    while(item = stack.pop()) {
      load(item[0], item[1]);
    }
  }

  return {
    load: load, 
    reload: reload,
    // Global setting
    cache: function(use) {
      cache = !!use;
    }
  };
  
});