define different font depending on the compiler

Question

I am writing a class file and want to use fontspec and unicode-math if the compiler is luatex or xetex. If the compiler is pdftex I would like to define default fonts like cmr and phv instead. I am trying to use \sys_if_engine* in the cls-file but getting the error: sys not defined or somethins like that. Can I get around this?

Answer 1

I realised that these packages did not use the \ProvidesPackage but \ProvidesExplPackage so I researched that and found this solution:

\newif \if@luatex
\newif \if@xetex
@luatexfalse
@xetexfalse
\ExplSyntaxOn
    \sys_if_engine_luatex:T {@luatextrue}
    \sys_if_engine_xetex:T  {@xetextrue}
\ExplSyntaxOff
...
\ifthenelse{\boolean{@luatex} \OR \boolean{@xetex}}{
    \RequirePackage{unicode-math}
    \RequirePackage{fontspec}
    \setmainfont{Times New Roman}
    \newfontfamily \arial {Arial}
    \setmathfont{Cambria Math}
}{
    % Have not solved this part yet.
}