Bevy system error handling

Decorate your bevy system with the sysfail macro attribute to handle failure.

Before

use bevy::prelude::*;
use bevy::utils::Duration;

use thiserror::Error;

#[derive(Error, Debug)]
enum GizmoError {
    #[error("A Gizmo error")]
    Error,
}

#[derive(Debug, PartialEq, Eq, Hash, SystemSet, Clone)]
enum TransformGizmoSystem { Drag, Place }

fn main() {
    let mut app = App::new();
    app.add_plugins(bevy::time::TimePlugin)
        .add_systems(Update, (
            drag_gizmo
                .pipe(print_gizmo_error)
                .in_set(TransformGizmoSystem::Drag),
            delete_gizmo
                .pipe(|In(_)| {})
                .after(TransformGizmoSystem::Place),
            place_gizmo
                .pipe(print_gizmo_error)
                .in_set(TransformGizmoSystem::Place)
                .after(TransformGizmoSystem::Drag),
        ));
    app.update();
}

fn print_gizmo_error(
    In(result): In<Result<(), Box<dyn std::error::Error>>>,
    mut last_error_occurence: Local<Option<Duration>>,
    time: Res<Time>,
) {
  // error boilerplate, may include
  // - avoiding printing multiple times the same error
  // - Formatting and chosing the log level
}

fn drag_gizmo(time: Res<Time>) -> Result<(), Box<dyn std::error::Error>> {
    println!("drag time is: {}", time.elapsed_seconds());
    let _ = Err(GizmoError::Error)?;
    println!("This will never print");
    Ok(())
}

fn place_gizmo() -> Result<(), Box<dyn std::error::Error>> {
    let () = Result::<(), &'static str>::Ok(())?;
    println!("this line should actually show up");
    let _ = Err("Ah, some creative use of info logging I see")?;
    Ok(())
}

fn delete_gizmo(time: Res<Time>) -> Option<()> {
    println!("delete time is: {}", time.elapsed_seconds());
    let _ = None?;
    println!("This will never print");
    Some(())
}

After

use bevy::prelude::*;
use bevy_mod_sysfail::prelude::*;

use thiserror::Error;

#[derive(Error, Debug)]
enum GizmoError {
    #[error("A Gizmo error")]
    Error,
}

fn main() {
    let mut app = App::new();
    app.add_plugins(bevy::time::TimePlugin)
        .add_systems(Update, (
            drag_gizmo,
            delete_gizmo.after(place_gizmo),
            place_gizmo.after(drag_gizmo)
        ));
    app.update();
}

#[sysfail]
fn drag_gizmo(time: Res<Time>) {
    println!("drag time is: {}", time.elapsed_seconds());
    let _ = Err(GizmoError::Error)?;
    println!("This will never print");
}

#[sysfail(Log<&'static str, Info>)]
fn place_gizmo() {
    let () = Result::<(), &'static str>::Ok(())?;
    println!("this line should actually show up");
    let _ = Err("Ah, some creative use of info logging I see")?;
}

#[sysfail(Ignore)]
fn delete_gizmo(time: Res<Time>) {
    println!("delete time is: {}", time.elapsed_seconds());
    let _ = Err(342_i32)?;
    println!("This will never print");
}

sysfail attribute

sysfail is an attribute macro you can slap on top of your systems to define the handling of errors. Unlike pipe, this is done directly at the definition site, and not when adding to the app. As a result, it's easy to see at a glance what kind of error handling is happening in the system, it also allows using the system name as a label in system dependency specification.

sysfail(E) systems return a value of type Result<(), E>. The return type is added by the macro, so do not add it yourself!

E is a type that implements the Failure trait. bevy_mod_sysfail exports several types that implement Failure:

• Log<Err, Lvl = Warn>: Will log Err to the tracing logger.
  • The first type parameter Err implements the Dedup trait. You can implement Dedup for your own types, but you can always use the anyhow::Error, Box<dyn std::error::Error> and &'static str types, as those already implement Dedup.
  • The second type parameter specifies the level of the log. It is optional and by default it is Warn
• LogSimply: Is similar to Log, but without deduplication.
• Emit<Ev>: Will emit the Ev bevy Event whenever the system returns an Err
• Ignore: Ignore errors, do as if nothing happened.

Example usages:

use bevy::prelude::*;
use bevy_mod_sysfail::prelude::*;
use thiserror::Error;

// -- Log a value --

#[derive(Error, Debug)]
enum MyCustomError {
    #[error("A Custom error")]
    Error,
}

// Equivalent to #[sysfail(Log<Box<dyn std::error::Error>>)]
#[sysfail]
fn generic_failure() { /* ... */ }

#[sysfail(Log<&'static str>)]
fn log_a_str_message() {
    let _ = Err("Yep, just like that")?;
}

#[sysfail(Log<anyhow::Error>)]
fn log_an_anyhow_error() {
    let _ = Err(MyCustomError::Error)?;
}

#[sysfail(LogSimply<MyCustomError, Trace>)]
fn log_trace_on_failure() { /* ... */ }

#[sysfail(LogSimply<MyCustomError, Error>)]
fn log_error_on_failure() { /* ... */ }

// -- Emit an event --
use bevy::app::AppExit;

#[derive(Event)]
enum ChangeMenu {
    Main,
    Tools,
}

#[sysfail(Emit<ChangeMenu>)]
fn change_menu() { /* ... */ }

#[sysfail(Emit<AppExit>)]
fn quit_app_on_error() { /* ... */ }

// -- Ignore all errors --

#[sysfail(Ignore)]
fn do_not_care_about_failure() { /* ... */ }

Exclusive systems

For exclusive systems, use the #[exclusive_sysfail] macro. Note that only Failure<Param = ()> work with exclusive systems. This excludes Log, so make sure to use LogSimply instead.

Custom handling

bevy_mod_sysfail is not limited to the predefined set of Failures, you can define your own by implementing it yourself. See the custom_failure example for sample code.

Change log

See the CHANGELOG.

Version Matrix

bevy	latest supporting version
0.13	7.0.0
0.12	6.0.0
0.11	4.3.0
0.10	2.0.0
0.9	1.1.0
0.8	0.1.0

License

Copyright © 2022 Nicola Papale

This software is licensed under Apache 2.0.