参考：类似于仿途牛：/UI2/demo/tuniu/index_main.w
onload事件中实现的效果！

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//页面初始化
    Model.prototype.modelLoad = function(event){
        //页面下拉时页头显示背景颜色
        var _this=this;
        $(this.comp("homeContent").domNode).scroll(function() {
            if($(this).scrollTop()>100){        
                $(".x-titlebar", _this.getRootNode()).css({"background-color":"rgba(0,0,0,0.8)"});
            } else {
                $(".x-titlebar", _this.getRootNode()).css({"background-color":"rgba(0,0,0,0)"});
            }
        });
    };