Executing an action when the monster target a new entity

Hello, I making a corruption mod and I wanted to add a corrupted endermen. I succed in adding him in the game and I want to make him more reactive.

I want him to scream once at his target when he spot his victim. I also want him to scream again if he change target ( like another entity hit him ).

I tried to use local variables like : "previous_target" and "new_target", I also tried to use global variables like :"can_scream" (logic variable) but it doesn't work.

{The mob reference is the warden when he target an entity that making sound, he scream and execute pursuit}