If you're talking about Objective-C and TWAIN on a Mac, there is a DMG on this page on twain.org that you can which contains an example TWAINClientCocoa project.
This is from 2002 or something, so downloading it and building it requires a little trick (rename the .pbproj folder to the .xcode folder so that it opens in Xcode and then performs an “update” when you are in Xcode), but it works (just tried this last week) and should give you a good starting point. It builds on the TWAIN source concept providing it in the GUI, but something you can change.
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