Integrate external code in to TouchGFX engine

[BGuth.1]

I am looking to call a function from TouchGFX engine. This function resides in external code. Ideally, the function should be called from TouchGFX before handleTickEvent() from each screen is called. I would like to know both the ways i.e. call the function before and after handleTickEvent(). Please let me know.

[MM..1]

For allscreen use model.cpp tick