layer load event,Layer shows a set

[liuwei]

flex viewer for flex3
I have MapManager.mxml file

case "dynamic":{
var dynlayer:ArcGISDynamicMapServiceLayer = new ArcGISDynamicMapServiceLayer(url);
dynlayer.id = label;
dynlayer.visible = visible; 
dynlayer.alpha = alpha;
dynlayer.addEventListener(com.esri.ags.events.LayerEvent.LOAD,view_tieluJianting);
map.addLayer(dynlayer);
break;
}

private function view_tieluJianting(event:LayerEvent):void{
    var vislays:ArcGISDynamicMapServiceLayer=ArcGISDynamicMapServiceLayer(event.layer);
    var visable_lays:ArrayCollection=vislays.visibleLayers;
    var alllays:Array=ArcGISDynamicMapServiceLayer(event.layer).layerInfos;
    var temp_lay:LayerInfo;
    for(var i:Number=0;i<alllays.length;i++){
     temp_lay=alllays;
     if(temp_lay.name=="railroad"){
      visable_lays.addItem(temp_lay.id);
     }
    }
    
   }

but,After this code plus "visable_lays.addItem(temp_lay.id)";Map shows no.if Notes "visable_lays.addItem(temp_lay.id);",it is good.
why ?
i am chinese,My English is not very good.