add examples on how to have a data source running in another thread / in a task pool thread (#2915)

Add two examples on how to communicate with a task that is running either in another thread or in a thread from `AsyncComputeTaskPool`.

Loosely based on https://github.com/bevyengine/bevy/discussions/1150

Cargo.toml

@@ -105,6 +105,7 @@ serde = { version = "1", features = ["derive"] }
 bytemuck = "1.7"
 # Needed to poll Task examples
 futures-lite = "1.11.3"
+crossbeam-channel = "0.5.0"
 
 [[example]]
 name = "hello_world"
@@ -287,6 +288,10 @@ path = "examples/asset/hot_asset_reloading.rs"
 name = "async_compute"
 path = "examples/async_tasks/async_compute.rs"
 
+[[example]]
+name = "external_source_external_thread"
+path = "examples/async_tasks/external_source_external_thread.rs"
+
 # Audio
 [[example]]
 name = "audio"

examples/README.md

@@ -145,6 +145,7 @@ Example | File | Description
 Example | File | Description
 --- | --- | ---
 `async_compute` | [`async_tasks/async_compute.rs`](async_tasks/async_compute.rs) | How to use `AsyncComputeTaskPool` to complete longer running tasks
+`external_source_external_thread` | [`async_tasks/external_source_external_thread.rs`](async_tasks/external_source_external_thread.rs) | How to use an external thread to run an infinite task and communicate with a channel
 
 ## Audio
 

examples/async_tasks/external_source_external_thread.rs

@@ -0,0 +1,89 @@
+use bevy::prelude::*;
+// Using crossbeam_channel instead of std as std `Receiver` is `!Sync`
+use crossbeam_channel::{bounded, Receiver};
+use rand::Rng;
+use std::time::{Duration, Instant};
+
+fn main() {
+    App::new()
+        .add_event::<StreamEvent>()
+        .add_plugins(DefaultPlugins)
+        .add_startup_system(setup)
+        .add_system(read_stream)
+        .add_system(spawn_text)
+        .add_system(move_text)
+        .run();
+}
+
+struct StreamReceiver(Receiver<u32>);
+struct StreamEvent(u32);
+
+struct LoadedFont(Handle<Font>);
+
+fn setup(mut commands: Commands, asset_server: Res<AssetServer>) {
+    commands.spawn_bundle(OrthographicCameraBundle::new_2d());
+
+    let (tx, rx) = bounded::<u32>(10);
+    std::thread::spawn(move || loop {
+        // Everything here happens in another thread
+        // This is where you could connect to an external data source
+        let mut rng = rand::thread_rng();
+        let start_time = Instant::now();
+        let duration = Duration::from_secs_f32(rng.gen_range(0.0..0.2));
+        while Instant::now() - start_time < duration {
+            // Spinning for 'duration', simulating doing hard work!
+        }
+
+        tx.send(rng.gen_range(0..2000)).unwrap();
+    });
+
+    commands.insert_resource(StreamReceiver(rx));
+    commands.insert_resource(LoadedFont(asset_server.load("fonts/FiraSans-Bold.ttf")));
+}
+
+// This system reads from the receiver and sends events to Bevy
+fn read_stream(receiver: ResMut<StreamReceiver>, mut events: EventWriter<StreamEvent>) {
+    for from_stream in receiver.0.try_iter() {
+        events.send(StreamEvent(from_stream))
+    }
+}
+
+fn spawn_text(
+    mut commands: Commands,
+    mut reader: EventReader<StreamEvent>,
+    loaded_font: Res<LoadedFont>,
+) {
+    let text_style = TextStyle {
+        font: loaded_font.0.clone(),
+        font_size: 20.0,
+        color: Color::WHITE,
+    };
+    let text_alignment = TextAlignment {
+        vertical: VerticalAlign::Center,
+        horizontal: HorizontalAlign::Center,
+    };
+    for (per_frame, event) in reader.iter().enumerate() {
+        commands.spawn_bundle(Text2dBundle {
+            text: Text::with_section(format!("{}", event.0), text_style.clone(), text_alignment),
+            transform: Transform::from_xyz(
+                per_frame as f32 * 100.0 + rand::thread_rng().gen_range(-40.0..40.0),
+                300.0,
+                0.0,
+            ),
+            ..Default::default()
+        });
+    }
+}
+
+fn move_text(
+    mut commands: Commands,
+    mut texts: Query<(Entity, &mut Transform), With<Text>>,
+    time: Res<Time>,
+) {
+    for (entity, mut position) in texts.iter_mut() {
+        position.translation -= Vec3::new(0.0, 100.0 * time.delta_seconds(), 0.0);
+        if position.translation.y < -300.0 {
+            commands.entity(entity).despawn();
+        }
+    }
+}