Add docs for how to test systems

[tuzz]

Description

Firstly, thanks for maintaining this project.

I've just started using specs on a project and I'm unsure how to test my systems. What I'd really like to do is set up some input state, run one of the systems, then verify the resulting state is what I expect. I'm not sure how to do this?

I could try and call the run function of a system directly, but this doesn't have an output, it mutates the WriteStorage, so it's not so straightforward to test this way. I think a page of documentation about how to test system boundaries would be incredibly useful.

Thanks

[azriel91]

Whoever picks this up, maybe create a lightweight version of amethyst_test (source).

struct MyComponent(pub i32);

impl Component for MyComponent {
    type Storage = DenseVecStorage<Self>;
}

#[derive(Debug)]
struct MySystem;
impl<'s> System<'s> for MySystem {
    type SystemData = WriteStorage<'s, MyComponent>;
    fn run(&mut self, mut my_component_storage: Self::SystemData) {
        for mut my_component in (&mut my_component_storage).join() {
            my_component.0 += 1
        }
    }
}

// #[test]
fn system_increases_component_value_by_one() -> Result<(), Error> {
    AmethystApplication::blank()
        .with_system(MySystem, "my_system", &[])
        .with_effect(|world| {
            let entity = world.create_entity().with(MyComponent(0)).build();
            world.add_resource(EffectReturn(entity));
        })
        .with_assertion(|world| {
            let entity = world.read_resource::<EffectReturn<Entity>>().0.clone();

            let my_component_storage = world.read_storage::<MyComponent>();
            let my_component = my_component_storage
                .get(entity)
                .expect("Entity should have a `MyComponent` component.");

            // If the system ran, the value in the `MyComponent` should be 1.
            assert_eq!(1, my_component.0);
        })
        .run()
}

[stale]

This issue has been automatically marked as stale because it has not had recent activity. It will be closed in 7 days if no further activity occurs. Feel free to comment if this is still relevant.

[felipellrocha]

Any updates on this? It's ok that you can do this via amethyst, but specs really should be able to handle this on its own

[azriel91]

Something like this should work:

use specs::{World, WorldExt};

let mut world = World::new();

// setup
world.insert(resource_a);
let entity = world.create_entity()
    .with(Position { x: 0, y: 0 })
    .with(Velocity { x: 1, y: 2 })
    .build();

// execute
let system_a = SystemA::new();
system_a.run_now(&world);

// assert
let positions = world.read_storage::<Position>();
let position_actual = positions.get(entity).unwrap();
assert_eq!(Position { x: 1, y: 2 }, position_actual);