log_layers/
log_layers.rs

1//! This example illustrates how to add custom log layers in bevy.
2
3use bevy::{
4    log::{
5        tracing::{self, Subscriber},
6        tracing_subscriber::{field::MakeExt, Layer},
7        BoxedFmtLayer, BoxedLayer,
8    },
9    prelude::*,
10};
11
12struct CustomLayer;
13
14impl<S: Subscriber> Layer<S> for CustomLayer {
15    fn on_event(
16        &self,
17        event: &tracing::Event<'_>,
18        _ctx: bevy::log::tracing_subscriber::layer::Context<'_, S>,
19    ) {
20        println!("Got event!");
21        println!("  level={}", event.metadata().level());
22        println!("  target={}", event.metadata().target());
23        println!("  name={}", event.metadata().name());
24    }
25}
26
27// We don't need App for this example, as we are just printing log information.
28// For an example that uses App, see log_layers_ecs.
29fn custom_layer(_app: &mut App) -> Option<BoxedLayer> {
30    // You can provide multiple layers like this, since Vec<Layer> is also a layer:
31    Some(Box::new(vec![
32        bevy::log::tracing_subscriber::fmt::layer()
33            .with_file(true)
34            .boxed(),
35        CustomLayer.boxed(),
36    ]))
37}
38
39// While `custom_layer` allows you to add _additional_ layers, it won't allow you to override the
40// default `tracing_subscriber::fmt::Layer` added by `LogPlugin`. To do that, you can use the
41// `fmt_layer` option.
42//
43// In this example, we're disabling the timestamp in the log output and enabling the alternative debugging format.
44// This formatting inserts newlines into logs that use the debug sigil (`?`) like `info!(foo=?bar)`
45fn fmt_layer(_app: &mut App) -> Option<BoxedFmtLayer> {
46    Some(Box::new(
47        bevy::log::tracing_subscriber::fmt::Layer::default()
48            .without_time()
49            .map_fmt_fields(MakeExt::debug_alt)
50            .with_writer(std::io::stderr),
51    ))
52}
53
54fn main() {
55    App::new()
56        .add_plugins(DefaultPlugins.set(bevy::log::LogPlugin {
57            custom_layer,
58            fmt_layer,
59
60            ..default()
61        }))
62        .add_systems(Update, log_system)
63        .run();
64}
65
66fn log_system() {
67    // here is how you write new logs at each "log level" (in "most important" to
68    // "least important" order)
69    error!("something failed");
70    warn!("something bad happened that isn't a failure, but thats worth calling out");
71    info!("helpful information that is worth printing by default");
72    let secret_message = "Bevy";
73    info!(?secret_message, "Here's a log that uses the debug sigil");
74    debug!("helpful for debugging");
75    trace!("very noisy");
76}
log_layers/log_layers.rs

log_layers/
log_layers.rs
