specs-system-macro/src/lib.rs

use specs::join::Join;

struct Pos {
    x: usize,
}
impl specs::Component for Pos {
    type Storage = specs::VecStorage<Pos>;
}
struct Mov;
impl specs::Component for Mov {
    type Storage = specs::VecStorage<Mov>;
}

macro_rules! system {
    ($name:ident $pat:tt => $block:block ) => {
        struct $name;
        impl<'a> specs::System<'a> for $name {
            type SystemData = args_to_systemdata!($pat);
            fn run(&mut self, args_to_pat!($pat): Self::SystemData) {
                for args_to_pat!($pat) in (args_to_join!($pat)).join() {
                    $block
                }
            }
        }
    };
}

macro_rules! args_to_systemdata {
    ( ( $( $name:ident : $ty: ty ),* ) ) => {
        ( $( specs::WriteStorage<'a, $ty> ),* )
    }
}

macro_rules! args_to_pat {
    ( ( $( $name:ident : $ty: ty ),* ) ) => {
        ( $( mut $name ),* )
    }
}

macro_rules! args_to_join {
    ( ( $( $name:ident : $ty: ty ),* ) ) => {
        ( $( &mut $name ),* )
    }
}

system!{ Foo (_x: Mov, y: Pos) => {
    y.x += 1;
}
}

#[cfg(test)]
mod tests {
    #[test]
    fn it_works() {
        assert_eq!(2 + 2, 4);
    }
}