image的加载事件(onload)和加载状态(complete)

http://blog.sina.com.cn/s/blog_73c8ed1901013k70.html

 

if (img.complete) {
  callback(img.width, img.height);
} else {
  img.onload = function () {
    callback(img.width, img.height);
    img.onload = null;
  };
};