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    function log(url, callback) {
        // 防止请求被垃圾回收
        var image = new Image(),
            id = "_tbtx_" + Math.random();

        window[id] = image;
        image.onload = image.onerror = function() {
            window[id] = null;
            if (callback) {
                callback();
            }
        };
        image.src = url;
    }