Since both roScreen and roImageCanvas are relatively free form - you will have to generate the focus effect yourself - for example, you could draw a box around the item you want to represent as "selected".
If you want your screen to behave like a standard roku component, then read up on anonymous functions, there is some documentation in the brightscript manual, and a few good examples in the forum here. That way you can do something like:
MyGui=newgui()
Mygui.drawMyCustomUI()
Mygui.selectelement(1)
And any other methods you want to invent.
-Joel
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