logs/
logs.rs

1//! This example illustrates how to use logs in bevy.
2
3use bevy::{log::once, prelude::*};
4
5fn main() {
6    App::new()
7        .add_plugins(DefaultPlugins.set(bevy::log::LogPlugin {
8            // Uncomment this to override the default log settings:
9            // level: bevy::log::Level::TRACE,
10            // filter: "wgpu=warn,bevy_ecs=info".to_string(),
11            ..default()
12        }))
13        .add_systems(Startup, setup)
14        .add_systems(Update, log_system)
15        .add_systems(Update, log_once_system)
16        .add_systems(Update, panic_on_p)
17        .run();
18}
19
20fn setup(mut commands: Commands) {
21    commands.spawn(Camera2d);
22    commands.spawn((
23        Text::new("Press P to panic"),
24        Node {
25            position_type: PositionType::Absolute,
26            top: px(12),
27            left: px(12),
28            ..default()
29        },
30    ));
31}
32
33fn panic_on_p(keys: Res<ButtonInput<KeyCode>>) {
34    if keys.just_pressed(KeyCode::KeyP) {
35        panic!("P pressed, panicking");
36    }
37}
38
39fn log_system() {
40    // here is how you write new logs at each "log level" (in "least important" to "most important"
41    // order)
42    trace!("very noisy");
43    debug!("helpful for debugging");
44    info!("helpful information that is worth printing by default");
45    warn!("something bad happened that isn't a failure, but thats worth calling out");
46    error!("something failed");
47
48    // by default, trace and debug logs are ignored because they are "noisy"
49    // you can control what level is logged by setting up the LogPlugin
50    // alternatively you can set the log level via the RUST_LOG=LEVEL environment variable
51    // ex: RUST_LOG=trace, RUST_LOG=info,bevy_ecs=warn
52    // the format used here is super flexible. check out this documentation for more info:
53    // https://docs.rs/tracing-subscriber/*/tracing_subscriber/filter/struct.EnvFilter.html
54}
55
56fn log_once_system() {
57    // The 'once' variants of each log level are useful when a system is called every frame,
58    // but we still wish to inform the user only once. In other words, use these to prevent spam :)
59
60    trace_once!("one time noisy message");
61    debug_once!("one time debug message");
62    info_once!("some info which is printed only once");
63    warn_once!("some warning we wish to call out only once");
64    error_once!("some error we wish to report only once");
65
66    for i in 0..10 {
67        info_once!("logs once per call site, so this works just fine: {}", i);
68    }
69
70    // you can also use the `once!` macro directly,
71    // in situations where you want to do something expensive only once
72    // within the context of a continuous system.
73    once!({
74        info!("doing expensive things");
75        let mut a: u64 = 0;
76        for i in 0..100000000 {
77            a += i;
78        }
79        info!("result of some expensive one time calculation: {}", a);
80    });
81}
logs/logs.rs

logs/
logs.rs
