Where to insert a Resource needed for a Plugin when expansive computation is required?

[IceSentry]

If my plugin adds a Resource that needs to do a few somewhat intensive things before it's ready to be added, should I insert it in a startup system or directly in the build function of a Plugin?

[will-hart]

I'm not sure what the "official" approach is, but I would add the resource in an "empty" state in the build function. I would then have the expensive setup process work in a separate, earlier game state to the system(s) that consume it. Depending on the expensiveness of your expensive things you may also want to consider running the expensive things using bevy's async features.

For example a sync version might look something like this:

// in plugin builder
app.insert_resource(ExpensiveComputationForThis::default())
    .add_system_set(SystemSet::on_enter(MyGameState::DoExpensiveThings).with_system(do_expensive_things.system())
    .add_system_set(SystemSet::on_update(MyGameState:AfterDoingExpensiveThings).with_system(use_expensive_things.system());

// in the `do_expensive_things` system
fn do_expensive_things(mut expensive: ResMut<ExpensiveComputationForThis>, mut state: ResMut<State<MyGameState>>) {
    // do something expensive
    expensive.gym_membership = exercise_expensively();

    // move to the next game state
    state.set(MyGameState::AfterDoingExpensiveThings).unwrap();
}

[IceSentry]

I'm essentially doing something like that right now, but I only insert the Resource in the system once it's ready. Any reason why you insert it then mutate in the system it instead of just generating it in the system?

[will-hart]

No real reason, just for consistency - i.e. I add all my resources at the start of the plugin it belongs to, which I think is easier and for me less likely to break something than having resources inserted in various parts of the code base in various systems / stages etc.

[ManevilleF]

You can use the state system and make your plugin struct generic:

struct MyPlugin<T> {
     pub start_state: T
}

impl<T: 'static + Debug + Clone + Eq + PartialEq + Hash + Send + Sync> Plugin for MyPlugin<T> {
    fn build(&self, app: &mut AppBuilder) {
        app.add_system_set(
            SystemSet::on_enter(self.start_state.clone())
                .with_system(Self::my_expensive_system.system()),
        )
    }
}

[IceSentry]

I'm not sure I understand the point of making this generic. I'm also not sure where should I make the .insert_resouce() call.

[ManevilleF]

You can avoid using startup systems with states