Lib.rs

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_plugin(bevy::time::TimePlugin)
        .add_system(
            drag_gizmo
                .pipe(print_gizmo_error)
                .in_set(TransformGizmoSystem::Drag),
        )
        .add_system(
            delete_gizmo
                .pipe(|In(_)| {})
                .after(TransformGizmoSystem::Place))
        .add_system(
            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::macros::*;

use thiserror::Error;

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

fn main() {
    let mut app = App::new();
    app.add_plugin(bevy::time::TimePlugin)
        .add_system(drag_gizmo)
        .add_system(delete_gizmo.after(place_gizmo))
        .add_system(place_gizmo.after(drag_gizmo));
    app.update();
}

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

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

#[quick_sysfail]
fn delete_gizmo(time: Res<Time>) {
    println!("delete time is: {}", time.elapsed_seconds());
    let _ = None?;
    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.

The sysfail attribute can only be used on systems returning a type implementing the Failure trait. Failure is implemented for Result<(), impl FailureMode> and Option<()>. sysfail takes a single argument, it is one of the following:

• log: print the Err of the Result return value, prints a very generic "A none value" when the return type is Option. By default, most things are logged at Warn level, but it is possible to customize the log level based on the error value.
• log(level = "{silent,trace,debug,info,warn,error}"): This forces logging of errors at a certain level (make sure to add the quotes)
• ignore: This is like log(level="silent") but simplifies the generated code.

Note that with log, the macro generates a new system with additional parameters.

quick_sysfail attribute

quick_sysfail is like sysfail(ignore) but only works on Option<()>. This attribute, unlike sysfail allows you to elide the final Some(()) and the type signature of the system. It's for the maximally lazy, like me.

use bevy_mod_sysfail::macros::*;

#[sysfail(ignore)]
fn place_gizmo() -> Option<()> {
  // …
  Some(())
}
// equivalent to:
#[quick_sysfail]
fn quick_place_gizmo() {
  // …
}

Traits

How error is handled is not very customizable, but there is a few behaviors controllable by the user, always through traits.

Failure trait

Failure is implemented for Result<(), impl FailureMode> and Option<()>.

Systems marked with the sysfail attribute must return a type implementing Failure.

FailureMode trait

FailureMode defines how the failure is handled. By implementing the trait on your own error types, you can specify:

• What constitutes "distinct" error types.

• The log level of specific values.

• How long an error must not be produced in order to be displayed again.

• TODO: provide a derive macro that allows setting log level and cooldown.

FailureMode is implemented for Box<dyn Error>, anyhow::Error, () and &'static str.

LogLevelOverride trait

LogLevelOverride is an extension trait that allows you to override the log level of a failure. Use the warn, trace, debug, silent, error and info methods to specify the level of logging of a failure.

Change log

• 1.0.0: Update to bevy 0.9
• 1.1.0: Allow usage of mutable queries (oops)
• 2.0.0: Breaking: Update to bevy 0.10

Version Matrix

License

Copyright © 2022 Nicola Papale

This software is licensed under Apache 2.0.