Which API / CLASS triggers when user clicks SAVE button on item view?

Hello,

I wan to call a my_custom_function or class's method, when the moment user clicks the SAVE button on portfolio item screen/view, pls. let me know where should i keep my code for calling my_custom_function, i mean which API / Class / Method triggers as soon as when user clicks the SAVE button so that i willl put my code in that API. In this function i am gonna populate some fields on the views explicitly, hence my system/SAP should not over write my changes, my changes should remain/considered

Thank you