observers example doesn't follow standards (#13884)

# Objective

- Observers example is using an unseeded random, prints text to console
and doesn't display text as other examples

## Solution

- use seeded random
- log instead of printing
- use common settings for UI text

examples/ecs/observers.rs

@@ -4,6 +4,8 @@ use bevy::{
  prelude::*,
  utils::{HashMap, HashSet},
 };
+use rand::{Rng, SeedableRng};
+use rand_chacha::ChaCha8Rng;
 
 fn main() {
  App::new()
@@ -47,13 +49,13 @@ struct Mine {
 }
 
 impl Mine {
- fn random() -> Self {
+ fn random(rand: &mut ChaCha8Rng) -> Self {
  Mine {
  pos: Vec2::new(
- (rand::random::<f32>() - 0.5) * 1200.0,
- (rand::random::<f32>() - 0.5) * 600.0,
+ (rand.gen::<f32>() - 0.5) * 1200.0,
+ (rand.gen::<f32>() - 0.5) * 600.0,
  ),
- size: 4.0 + rand::random::<f32>() * 16.0,
+ size: 4.0 + rand.gen::<f32>() * 16.0,
  }
  }
 }
@@ -67,20 +69,29 @@ struct ExplodeMines {
 #[derive(Event)]
 struct Explode;
 
-fn setup(mut commands: Commands, asset_server: Res<AssetServer>) {
+fn setup(mut commands: Commands) {
  commands.spawn(Camera2dBundle::default());
- commands.spawn(TextBundle::from_section(
- "Click on a \"Mine\" to trigger it.\n\
+ commands.spawn(
+ TextBundle::from_section(
+ "Click on a \"Mine\" to trigger it.\n\
  When it explodes it will trigger all overlapping mines.",
- TextStyle {
- font: asset_server.load("fonts/FiraMono-Medium.ttf"),
- font_size: 24.,
- color: Color::WHITE,
- },
- ));
+ TextStyle {
+ color: Color::WHITE,
+ ..default()
+ },
+ )
+ .with_style(Style {
+ position_type: PositionType::Absolute,
+ top: Val::Px(12.),
+ left: Val::Px(12.),
+ ..default()
+ }),
+ );
+
+ let mut rng = ChaCha8Rng::seed_from_u64(19878367467713);
 
  commands
- .spawn(Mine::random())
+ .spawn(Mine::random(&mut rng))
  // Observers can watch for events targeting a specific entity.
  // This will create a new observer that runs whenever the Explode event
  // is triggered for this spawned entity.
@@ -97,7 +108,7 @@ fn setup(mut commands: Commands, asset_server: Res<AssetServer>) {
 
  // As we spawn entities, we can make this observer watch each of them:
  for _ in 0..1000 {
- let entity = commands.spawn(Mine::random()).id();
+ let entity = commands.spawn(Mine::random(&mut rng)).id();
  observer.watch_entity(entity);
  }
 
@@ -140,7 +151,7 @@ fn explode_mine(trigger: Trigger<Explode>, query: Query<&Mine>, mut commands: Co
  let Some(mut entity) = commands.get_entity(id) else {
  return;
  };
- println!("Boom! {:?} exploded.", id.index());
+ info!("Boom! {:?} exploded.", id.index());
  entity.despawn();
  let mine = query.get(id).unwrap();
  // Trigger another explosion cascade.