to be busy doing sth :
They were in the middle of dinner when I called.
I'm in the middle of writing a difficult letter.
to be busy doing sth :
They were in the middle of dinner when I called.
I'm in the middle of writing a difficult letter.
Oxford Advanced Learner's English Dictionary. Оксфордский английский словарь для изучающик язык на продвинутом уровне. 2005