As this page is outdated, please refer to Bevy's official migration guides while reading, to cover the differences: 0.12 to 0.13.

I apologize for the inconvenience. I will update the page as soon as I find the time.

Logging, Console Messages

Relevant official examples: logs.

You may have noticed how, when you run your Bevy project, you get messages in your console window. For example:

2022-06-12T13:28:25.445644Z  WARN wgpu_hal::vulkan::instance: Unable to find layer: VK_LAYER_KHRONOS_validation
2022-06-12T13:28:25.565795Z  INFO bevy_render::renderer: AdapterInfo { name: "AMD Radeon RX 6600 XT", vendor: 4098, device: 29695, device_type: DiscreteGpu, backend: Vulkan }
2022-06-12T13:28:25.565795Z  INFO mygame: Entered new map area.

Log messages like this can come from Bevy, dependencies (like wgpu), and also from your own code.

Bevy offers a logging framework that is much more advanced than simply using println/eprintln from Rust. Log messages can have metadata, like the level, timestamp, and Rust module where it came from. You can see that this metadata is printed alongside the contents of the message.

This is set up by Bevy's LogPlugin. It is part of the DefaultPlugins plugin group, so most Bevy users will have it automatically in every typical Bevy project.

Levels

Levels determine how important a message is, and allow messages to be filtered.

The available levels are: off, error, warn, info, debug, trace.

A rough guideline for when to use each level, could be:

• off: disable all log messages
• error: something happened that prevents things from working correctly
• warn: something unusual happened, but things can continue to work
• info: general informational messages
• debug: for development, messages about what your code is doing
• trace: for very verbose debug data, like dumping values

Printing your own log messages

To display a message, just use the macro named after the level of the message. The syntax is exactly the same as with Rust's println. See the std::fmt documentation for more details.

#![allow(unused)]
fn main() {
error!("Unknown condition!");
warn!("Something unusual happened!");
info!("Entered game level: {}", level_id);
debug!("x: {}, state: {:?}", x, state);
trace!("entity transform: {:?}", transform);
}

Filtering messages

To control what messages you would like to see, you can configure Bevy's LogPlugin:

#![allow(unused)]
fn main() {
use bevy::log::LogPlugin;

app.add_plugins(DefaultPlugins.set(LogPlugin {
    filter: "info,wgpu_core=warn,wgpu_hal=warn,mygame=debug".into(),
    level: bevy::log::Level::DEBUG,
}));
}

The filter field is a string specifying a list of rules for what level to enable for different Rust modules/crates. In the example above, the string means: show up to info by default, limit wgpu_core and wgpu_hal to warn level, for mygame show debug.

All levels higher than the one specified are also enabled. All levels lower than the one specified are disabled, and those messages will not be displayed.

The level filter is a global limit on the lowest level to use. Messages below that level will be ignored and most of the performance overhead avoided.

Environment Variable

You can override the filter string when running your app, using the RUST_LOG environment variable.

RUST_LOG="warn,mygame=debug" ./mygame

Note that other Rust projects, such as cargo, also use the same environment variable to control their logging. This can lead to unexpected consequences. For example, doing:

RUST_LOG="debug" cargo run

will cause your console to also be filled with debug messages from cargo.

Different settings for debug and release builds

If you want to do different things in your Rust code for debug/release builds, an easy way to achieve it is using conditional compilation on "debug assertions".

#![allow(unused)]
fn main() {
use bevy::log::LogPlugin;

// this code is compiled only if debug assertions are enabled (debug mode)
#[cfg(debug_assertions)]
app.add_plugins(DefaultPlugins.set(LogPlugin {
    level: bevy::log::Level::DEBUG,
    filter: "debug,wgpu_core=warn,wgpu_hal=warn,mygame=debug".into(),
}));

// this code is compiled only if debug assertions are disabled (release mode)
#[cfg(not(debug_assertions))]
app.add_plugins(DefaultPlugins.set(LogPlugin {
    level: bevy::log::Level::INFO,
    filter: "info,wgpu_core=warn,wgpu_hal=warn".into(),
}));
}

This is a good reason why you should not use release mode during development just for performance reasons.

On Microsoft Windows, your game EXE will also launch with a console window for displaying log messages by default. You might not want that in release builds. See here.

Performance Implications

Printing messages to the console is a relatively slow operation.

However, if you are not printing a large volume of messages, don't worry about it. Just avoid spamming lots of messages from performance-sensitive parts of your code like inner loops.

You can disable log levels like trace and debug in release builds.