获取Extjs中的TreePanel中所有的被checked的叶子节点的id

tree.on("checkchange", function(node){
     var cc = "";
    root.cascade( function(node1){    
         if(node1.leaf== true&&node1.attributes.checked=='true'){
            cc+=node1.id+",";
        }
    }); 
     if(cc!=""){
        cc = cc.substring(0,cc.length-1)
    }
})