I'll answer if you like:
Yes you can filter which nodes you show. In your tree instantiation the nodepath attribute is usually: nodepath="*". This means it'll create treenodes for every node in the dataset. You can change the nodepath to be ="level1" and that should work.
You can also have an expander, in my code I have one defined as such on the treenode:
<view name="exphouse" width="10" height="10">
<view name="expander" resource="tree_toggle_rsc" initstage="early" visible="${this.parent.parent.haschildren}">
<attribute name="open" value="${this.parent.parent.open}" />
<handler name="onopen" args="theval">
if(theval){
this.setResourceNumber(2);
}
else{
this.setResourceNumber(1);
}
</handler>
<handler name="onclick">
this.parent.parent.setOpen(!this.parent.parent.ope n );
</handler>
</view>
</view>
The outer view is only there for spacing for me, so don't worry about that if you don't want it. The haschildren attribute is stored on the dataset and is set by the business objects that the data represents, but you can set it by using the various data nethods to ascertain if a given node has children.