Javascript OpenLayers http刷新回调函数

这是我的代码的精简版本，每2秒重新加载一个KML文件。每次更新时，我都希望相机平移到图层的功能范围，但我不知道在哪里设置回调

//set up ajax    
var protocol = new OpenLayers.Protocol.HTTP({
    url: "feed.kml",
    format: new OpenLayers.Format.KML(),
    callback: function(){
        alert("here"); //never called
    }
});

//create refresher
var refresh = new OpenLayers.Strategy.Refresh({force: true, active: true});

//create layer for kml
var trackLayer = new OpenLayers.Layer.Vector("KML", {
    strategies: [new OpenLayers.Strategy.Fixed(), refresh],
    protocol: protocol,
});

//add to map
map.addLayer(trackLayer);

//refresh the kml every 2 seconds
setInterval(function(){
    refresh.refresh();      
    //pan to extent
    map.panTo(trackLayer.getDataExtent().getCenterLonLat());
},2000);

问题是这一行：

map.panTo（trackLayer.getDataExtent（）.getCenterLonLat（））

以下是文件：





我明白了。我觉得自己很愚蠢，因为我没有更快地弄明白这一点

//zoom callback
trackLayer.events.register("featuresadded",trackLayer,function(){
    var bounds = trackLayer.getDataExtent();
    if(bounds){ map.panTo(bounds.getCenterLonLat()); }
});