Add parallel query example

Cargo.toml

@@ -171,6 +171,10 @@ path = "examples/ecs/startup_system.rs"
 name = "ecs_guide"
 path = "examples/ecs/ecs_guide.rs"
 
+[[example]]
+name = "parallel_query"
+path = "examples/ecs/parallel_query.rs"
+
 [[example]]
 name = "breakout"
 path = "examples/game/breakout.rs"

examples/ecs/parallel_query.rs

@@ -0,0 +1,44 @@
+use bevy::{prelude::*, tasks::prelude::*};
+use std::{
+    sync::{atomic, atomic::AtomicUsize},
+    thread,
+    time::{Duration, Instant},
+};
+
+fn spawn_system(mut commands: Commands) {
+    for i in 0..16usize {
+        commands.spawn((i,));
+    }
+}
+
+fn square_system(pool: Res<ComputeTaskPool>, mut nums: Query<&mut usize>) {
+    let i = AtomicUsize::new(0);
+    nums.iter().iter_batched(1).for_each(&pool, |mut n| {
+        println!(
+            "Processing entity {}",
+            i.fetch_add(1, atomic::Ordering::Relaxed)
+        );
+        thread::sleep(Duration::from_secs(1));
+        *n = *n * *n;
+    });
+}
+
+fn print_threads_system(pool: Res<ComputeTaskPool>) {
+    println!("Using {} threads in compute pool", pool.thread_num());
+}
+
+fn print_system(num: &usize) {
+    print!("{} ", num);
+}
+
+fn main() {
+    let t0 = Instant::now();
+    App::build()
+        .add_startup_system(spawn_system.system())
+        .add_startup_system(print_threads_system.system())
+        .add_system(square_system.system())
+        .add_system(print_system.system())
+        .run();
+    let t1 = Instant::now();
+    println!("\nTook {:.3}s", (t1 - t0).as_secs_f32());
+}