How NSFontManager addFontTrait changes the actual font in the user interface element

Question

I want to achieve the same as [[NSFontManager sharedFontManager] addFontTrait:(nullable id)sender] using only my code

Document Status

 This action method causes the receiver to send its action message up the responder chain. By default, the action message is changeFont:. When a responder replies by providing a font to convert in a convertFont: message, the receiver converts the font by adding the trait specified by sender. This trait is determined by sending a tag message to sender and interpreting it as a font trait mask for a convertFont:toHaveTrait: message. 

so i tried

 NSFontManager * fm = [NSFontManager sharedFontManager]; NSFont * font = [fm selectedFont]; NSFont * modifiedFont = [fm convertFont:font toHaveTrait:NSFontItalicTrait]; [fm setSelectedFont:modifiedFont isMultiple:YES]; [NSApp sendAction:@selector(changeFont:) to:fm.target from:fm]; 

or

 [[self.window firstResponder] tryToPerform:@selector(changeFont:) with:modifiedFont]; 

but it’s clear that I missed some point

How the actual implementation in NSFontManager changes the font to NSTextField and how it can be reproduced