Args
ARG(1) is a 1 if one of your windows is being activated (because the user switched from some window in another program), or a 0 if one of your windows is being deactivated (because the user is switching to some other program's window).
ARG(2) a thread identifier. If ARG(1) = 1, then ARG(2_ is the identifier of the thread that owns the window being deactivated. If ARG(1) = 0, then ARG(2) is the identifier of the thread that owns the window being activated.
Notes
See also MOUSEACTIVATE, NCACTIVATE, ACTIVATE.