Items of alias - On Open?

Question

It's pretty blatant below what I'd like to do. I wrote two different examples, where the 2nd one accomplishes the task. I just don't know how to get it to do the same using on open?? I'd like to drop a folder filled with contents onto the droplet. Then scan that folder and set a variable to the list of items.  I've never used on open and I find very little on the subject online. I tried multiple ways of writing it and haven't hit the jackpot yet. Thank you in advance!
on open ItemList -- items were dropped onto the application
  ShowChoices(ItemList) -- do something with them
end open
to ShowChoices(TheChoices) -- show what was dropped
          set listing to every item of TheChoices
end ShowChoices
--ItemList below matches what ItemList is above
tell application "Finder"
          set ItemList to folder "Screenshots" of desktop as alias
          set listing to every item of ItemList
end tell
--difference is the 2nd example lists the items inside the folder

Answer 1

No, you can arrange your existing handlers to process items from either, for example:
on run -- the application was double-clicked
  open (choose file with multiple selections allowed)
end run
on open theItems -- items were dropped onto the application
  repeat with anItem in theItems
    process(anItem)
  end repeat
end open
on process(theItem)
  # do whatever with theItem
end process