[Solved] How to select a subitem by clicking a button

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Hi all ,
I have an issue related with treeView.
Currently , my tree view is using "Load On Demand"

And I can select a item by clicking a button with this below code:

var items = $("> ul > li ", treeView().element);

and using the loop to select the item I want.

But I don't know how to do with sub item.
I try to expand parent Item first and then my code is below:
  var sub_items = $("> ul > li > ul > li ", treeView().element);

It cannot work.

I know my code is bad for deeper subitem.

Anyone can help me to resolve?
Thanks so much.