System ordering not working

[fluffysquirrels]

I'm trying to run two systems one after the other, with the first system inserting a resource and the second system using the resource. This is failing with error: Requested resource does not exist: my_project::enemy::Sprites

Here's my code:

struct Sprites {
    idle: Handle<ColorMaterial>,
}

impl bevy::app::Plugin for Plugin {
    fn build(&self, app: &mut AppBuilder) {
        app
            .add_startup_system(load_resources.system().label("load"))
            .add_startup_system(spawn_current_enemy.system().after("load"));
    }
}

fn load_resources(
    mut commands: Commands,
    asset_server: Res<AssetServer>,
    mut materials: ResMut<Assets<ColorMaterial>>,
) {
    commands.insert_resource(Sprites {
        idle: materials.add(asset_server.load("x.png").into()),
    });
}

fn spawn_current_enemy(
    mut commands: Commands,
    asset_server: Res<AssetServer>,
    mut materials: ResMut<Assets<ColorMaterial>>,
    sprites: Res<Sprites>,
) {
    commands.spawn_bundle(SpriteBundle {
        material: sprites.idle.clone(),
        .. Default::default()
    });
}

Any ideas how to implement this correctly?

[alice-i-cecile]

Commands are not processed until the end of the current stage, so while the systems are running in the correct order the resource won't exist in time.

You need to split which stages these systems are in, by using add_system_to_stage and selecting one of the variants of the StartupStage enum.

[alice-i-cecile]

In 0.10, commands are only processed after apply_system_buffers, which is a special system. By default, there's a copy of this system inserted between base system sets. You can add more though, and either use (system_a, apply_system_buffers, system_b).chain() or just give a unique label to your copy of apply_system_buffers.

In 0.11, base system sets are no more, and this simply uses schedules. apply_system_buffers has been renamed to apply_deferred, and deferred system parameters (like Commands) are automatically applied at the end of each schedule. You can still insert your own copies of apply_deferred within a schedule if you'd like as well!

Note that inserting exclusive systems like apply_system_buffers / apply_deferred will create a choke point for your parallel system evaluation: all running systems must complete before the schedule can advance. Not a big deal, but something to be mindful of at times.

[MinerSebas]

.label() + .before()/.after() is only for ordering inside a Stage, but Commands are only applied at the end of a stage.

Solution:

		app
            .add_startup_system_to_stage(StartupStage::PreStartup, load_resources.system())
            .add_startup_system(spawn_current_enemy.system());

[alice-i-cecile]

0.10: app.add_startup_system(load_resources.in_base_set(StartupStage::PreStartup) (now a base set)

0.11: app.add_systems(PreStartup, load_resources) (just a schedule)

[Ewan88]

Can these answers please be updated to v.0.10?

[alice-i-cecile]

Sure, I'll follow up below my answer.