Despawn panic test

main.rs

use bevy::prelude::*;

pub struct Game;
struct Box(u8);

struct AddBoxEventTriggerState {
    event_timer: Timer,
}

impl Default for AddBoxEventTriggerState {
    fn default() -> Self {
        AddBoxEventTriggerState {
            event_timer: Timer::from_seconds(1.0),
        }
    }
}

struct AddBoxEvent();

#[derive(Default)]
struct AddBoxEventListenerState {
    event_reader: EventReader<AddBoxEvent>,
}

// sends MyEvent every second
fn add_box_event_trigger(
    time: Res<Time>,
    mut state: ResMut<AddBoxEventTriggerState>,
    mut my_events: ResMut<Events<AddBoxEvent>>,
) {
    state.event_timer.tick(time.delta_seconds);
    if state.event_timer.finished {
        my_events.send(AddBoxEvent());
        state.event_timer.reset();
    }
}

fn setup(mut commands: Commands) {
    commands
        .spawn(LightComponents {
            translation: Translation::new(4.0, 8.0, 4.0),
            ..Default::default()
        })
        .spawn(Camera3dComponents {
            transform: Transform::new_sync_disabled(Mat4::face_toward(
                Vec3::new(0.0, 20.0, 5.0),
                Vec3::new(0.0, 10.0, 0.0),
                Vec3::new(0.0, 1.0, 0.0),
            )),
            ..Default::default()
        });
}

fn add_box(
    mut commands: Commands,
    mut meshes: ResMut<Assets<Mesh>>,
    mut materials: ResMut<Assets<StandardMaterial>>,
    mut state: ResMut<AddBoxEventListenerState>,
    events: Res<Events<AddBoxEvent>>,
) {
    for _event in state.event_reader.iter(&events) {
        commands
            .spawn(PbrComponents {
                mesh: meshes.add(Mesh::from(shape::Cube { size: 1.0 })),
                material: materials.add(Color::rgb(0.9, 0.4, 0.3).into()),
                translation: Translation(Vec3::new(0.0, 0.0, -20.0)),
                ..Default::default()
            })
            .with(Box(0));
        println!("Spawn Box");
    }
}

fn box_movement(
    mut commands: Commands,
    time: Res<Time>,
    mut query: Query<(Entity, &Box, &mut Translation)>,
) {
    for (entity, _box, mut translation) in &mut query.iter() {
        if *translation.0.z_mut() > 1.0 {
            println!("Despawn Box");
            commands.despawn(entity);
        } else {
            *translation.0.z_mut() += time.delta_seconds * 10.0;
        }
    }
}

impl Plugin for Game {
    fn build(&self, app: &mut AppBuilder) {
        app.add_startup_system(setup.system())
            .add_system(box_movement.system())
            .add_event::<AddBoxEvent>()
            .init_resource::<AddBoxEventTriggerState>()
            .init_resource::<AddBoxEventListenerState>()
            .add_system(add_box_event_trigger.system())
            .add_system(add_box.system());
    }
}

fn main() {
    App::build()
        .add_resource(Msaa { samples: 4 })
        .add_default_plugins()
        .add_plugin(Game)
        .run();
}