How to use a startup system in a unit-test?

[beerhond]

The test example documentation shows how to create tests for your systems that are called every time the stage is being run.

How would you add a startup system to the stage, that is just called the first time the stage is being run?

[bjorn3]

Startup systems are added to a separate startup stage I believe. You could do the same in your tests. Just run this separate stage once before the regular stage.

[beerhond]

Right, that works! I mixed up the concepts of stages and schedules.
Thanks for your answer @bjorn3

For anyone else confused, these tests both effectively do the same:

#[cfg(test)]
mod test {
    use bevy::ecs::schedule::RunOnce;
    use bevy::prelude::*;

    fn setup(mut _commands: Commands) {
        println!("setup!");
    }

    fn update() {
        println!("update!");
    }

    #[test]
    fn test_with_separate_stages() {
        let mut startup_stage = SystemStage::parallel();
        startup_stage.add_system(setup);

        let mut update_stage = SystemStage::parallel();
        update_stage.add_system(update);

        let mut world = World::default();
        startup_stage.run(&mut world);
        update_stage.run(&mut world);
        update_stage.run(&mut world);
    }

    #[test]
    fn test_with_schedule() {
        let mut schedule = Schedule::default();
        schedule
            .add_stage(
                CoreStage::Startup,
                SystemStage::parallel().with_run_criteria(RunOnce::default()),
            )
            .add_stage(CoreStage::Update, SystemStage::parallel());

        schedule.add_system_to_stage(CoreStage::Startup, setup);
        schedule.add_system_to_stage(CoreStage::Update, update);

        let mut world = World::default();
        schedule.run(&mut world);
        schedule.run(&mut world);
    }
}