extern crate shred;

use shred::{DispatcherBuilder, Read, Resource, Resources, System, Write};

#[derive(Debug, Default)]
struct ResA;

#[derive(Debug, Default)]
struct ResB;

struct PrintSystem;

// Systems should be generic over the
// context if possible, so it's easy
// to introduce one.
impl<'a> System<'a> for PrintSystem {
    type SystemData = (Read<'a, ResA>, Write<'a, ResB>);

    fn run(&mut self, data: Self::SystemData) {
        let (a, mut b) = data;

        println!("{:?}", &*a);
        println!("{:?}", &*b);

        *b = ResB; // We can mutate ResB here
        // because it's `FetchMut`.
    }
}

fn main() {
    let mut resources = Resources::new();
    let mut dispatcher = DispatcherBuilder::new()
        .with(PrintSystem, "print", &[]) // Adds a system "print" without dependencies
        .build();
    //resources.add(ResA); (We don't need to add `ResA`, a default value will be instantiated)
    resources.insert(ResB);

    dispatcher.dispatch(&mut resources);
}