The goal is to write an input method that allows entering characters by typing their Unicode hexadecimal code.

You'll need to:

• Write an input method skeleton (you can check the unfinished ones in Haiku and the working Canna source code)
• Parse the input:
  • check that each typed character is a hexadecimal digit
  • append it to a string and parse it as hexadecimal
• On Enter and/or after a timeout, validate the proposed Unicode (not all hexadecimal codes are allowed, you'll probably need to use an ICU API for this, possibly u_charType())
• Send the resulting character to the application using the input method protocol

Handling the input method activation via a shortcut can be left out for now as input_server currently only supports enabling the selected input method but not an arbitrary one on a specific shortcut yet.

References:

https://en.wikipedia.org/wiki/Unicode_input#Hexadecimal_code_input

https://www.haiku-os.org/legacy-docs/bebook/BInputServerMethod_Overview.html

http://cgit.haiku-os.org/haiku/tree/src/add-ons/input_server/methods

https://github.com/mt819/CannaIM

http://icu-project.org/apiref/icu4c/uchar_8h.html