Add an example

Cargo.toml

@@ -78,6 +78,9 @@ log = "0.4"
 rand = "0.7.3"
 ron = "0.6"
 serde = {version = "1", features = ["derive"]}
+tracing = "0.1.21"
+tracing-chrome = "0.2.0"
+tracing-subscriber = "0.2.15"
 
 # bevy (Android)
 [target.'cfg(target_os = "android")'.dependencies]
@@ -168,6 +171,10 @@ path = "examples/app/return_after_run.rs"
 name = "thread_pool_resources"
 path = "examples/app/thread_pool_resources.rs"
 
+[[example]]
+name = "tracing"
+path = "examples/app/tracing.rs"
+
 [[example]]
 name = "hot_asset_reloading"
 path = "examples/asset/hot_asset_reloading.rs"

examples/README.md

@@ -42,6 +42,7 @@ Example | File | Description
 `headless` | [`app/headless.rs`](./app/headless.rs) | An application that runs without default plugins
 `plugin` | [`app/plugin.rs`](./app/plugin.rs) | Demonstrates the creation and registration of a custom plugin
 `thread_pool_resources` | [`app/thread_pool_resources.rs`](./app/thread_pool_resources.rs) | Creates and customizes the internal thread pool
+`tracing` | [`app/tracing.rs`](./app/tracing.rs) | Demonstrates `trace` feature output. Run with `RUST_LOG=info cargo run --example tracing --features trace` and then open the `trace-1234.json` file (where 1234 is a time since the beginning of the epoch) in `chrome://tracing` in Chrome.
 
 ## Assets
 

examples/app/tracing.rs

@@ -0,0 +1,49 @@
+use bevy::{input::system::exit_on_esc_system, prelude::*};
+use std::{thread, time};
+use tracing::info;
+use tracing_chrome::ChromeLayerBuilder;
+use tracing_subscriber::{fmt, prelude::*, registry::Registry, EnvFilter};
+
+pub fn setup_global_subscriber() -> impl Drop {
+    let fmt_layer = fmt::Layer::default();
+    let filter_layer = EnvFilter::try_from_default_env()
+        .or_else(|_| EnvFilter::try_new("info,wgpu=warn"))
+        .unwrap();
+
+    let (chrome_layer, _guard) = ChromeLayerBuilder::new().build();
+
+    let subscriber = Registry::default()
+        .with(filter_layer)
+        .with(fmt_layer)
+        .with(chrome_layer);
+
+    tracing::subscriber::set_global_default(subscriber).expect("Could not set global default");
+    _guard
+}
+
+fn main() {
+    let _guard = setup_global_subscriber();
+
+    App::build()
+        .add_plugins(DefaultPlugins)
+        .add_startup_system(a_system.system())
+        .add_system(foo_bar_baz.system())
+        .add_system(exit_on_esc_system.system())
+        .run();
+}
+
+fn a_system(commands: &mut Commands) {
+    let ten_millis = time::Duration::from_millis(10);
+    thread::sleep(ten_millis);
+
+    commands.spawn((GlobalTransform::default(), Transform::default()));
+}
+
+fn foo_bar_baz(query: Query<&Transform>) {
+    for transform in query.iter() {
+        let five_millis = time::Duration::from_millis(5);
+        thread::sleep(five_millis);
+
+        info!(?transform);
+    }
+}