My use-case for resource loading in a system.

usecase.rs

pub struct DeSavegameSystem;

impl<'a> System<'a> for DeSavegameSystem {
    type SystemData = (
        Option<Write<'a, GameSessionData>>,
        Option<Read<'a, SavegamePaths>>,
    );

    fn run(&mut self,(
            session_data,
            paths,
        ): Self::SystemData,
    ) {
        match (session_data, paths) {
            (Some(mut session_data), Some(paths)) => {
                match GameSessionData::load(&paths){
                    Ok(data) => {
                        let mut buffer = session_data.deref_mut();
                        *buffer = data;
                    },
                    Err(e) => {
                        error!("Error loading savegame data at {:?}: {:?}", paths.savegame_file_path.clone(), e);
                    }
                }
            },
            (None, Some(paths)) => {
                match GameSessionData::load(&paths){
                    Ok(data) => {
                        /*
                        let mut buffer = session_data;
                        buffer = Some(data);
                        */
                        error!("Found savegame data at {:?}, but cannot add resource from inside system.", paths.savegame_file_path.clone());
                    },
                    Err(e) => {
                        error!("Error loading savegame data at {:?}: {:?}", paths.savegame_file_path.clone(), e);
                    }
                }
            },
            _ => {
                error!("| SavegamePaths not found.");
            }
        }
    }
}