function preload(imgs, callback, end) {
        var len = imgs.length;
        var loaded = 0;
        for (var i = 0; i < len; i++) {
            (function(i) {
                var img = new Image();
                img.onerror = img.onload = function() {
                    loaded++;
                    end ? loaded === len && callback(loaded, i) : callback(loaded, i)
                };
                img.src = imgs[i];
            })(i);
        }
    }