[tqh]

Haiku has it own set of API's, that is a bit different than other platforms (message passing, multi-threaded). The UI API's is defined in the Interface Kit. See https://www.haiku-os.org/docs/api/

Here is the code for the Magnify App in Haiku: https://github.com/haiku/haiku/blob/master/src/apps/magnify/...