function first_screen () {
    var imgs = document.getElementsByTagName("img"), fs = +new Date;
    var fsItems = [], that = this;
    function getOffsetTop(elem) {
        var top = 0;
        top = window.pageYOffset ? window.pageYOffset : document.documentElement.scrollTop;
        try{
            top += elem.getBoundingClientRect().top;
        }catch(e){

        }finally{
            return top;
        }

    }
    var loadEvent = function() {
        //gif避免
        if (this.removeEventListener) {
            this.removeEventListener("load", loadEvent, false);
        }
        fsItems.push({
            img : this,
            time : +new Date
        });
    }
    for (var i = 0; i < imgs.length; i++) {
        (function() {
            var img = imgs[i];

            if (img.addEventListener) {

                !img.complete && img.addEventListener("load", loadEvent, false);
            } else if (img.attachEvent) {

                img.attachEvent("onreadystatechange", function() {
                    if (img.readyState == "complete") {
                        loadEvent.call(img, loadEvent);
                    }

                });
            }

        })();
    }
    function firstscreen_time() {
        var sh = document.documentElement.clientHeight;
        for (var i = 0; i < fsItems.length; i++) {
            var item = fsItems[i], img = item['img'], time = item['time'], top = getOffsetTop(img);
            if (top > 0 && top < sh) {
                fs = time > fs ? time : fs;
            }
        }
        return fs;
    }
    window.addEventListener('load', function() {
                            apm.firstScreen(firstscreen_time());
                        });
}