How to use lifetime in event?

[PhanDungTri]

Hi, I'm a beginner in Rust and Bevy. I can basically understand the concept of 'lifetime' but still can't actually use it in practice.

Here is my situation, I have defined a struct that has a property referencing to another struct.

pub struct JobProcessingEvent<'a> {
  pub job: &'a Job,
  pub effort: f32,
}

I want to use it in a system but don't know how to specify those lifetimes.

pub fn process_job_system(
  job_completed_event: EventWriter<JobProcessingEvent>,
  mut commands: Commands,
  peasant_query: Query<(Entity, &Job)>,
) {
  //...
}

Any guidance is very appreciated. Thank you.

[james7132]

Events must be 'static.. They cannot have non-'static fields. You'll likely need to find other ways to refer to the Job (i.e. a JobId or Entity) instead of a reference.

[nicopap]

You can't pass a reference to a component through an event. This is because the lifetime of &Job in process_job_system is limited to that system. This is because at any time when process_job_system is not running, another system can update the value pointed-to by the reference, and this violates rust's aliasing rules. If you add a reference to Job in an event, it would require that the lifetime of &Job goes beyond process_job_system, which we already said is not possible.

This needs a different solution. How does your Job struct looks like? Most likely, you want your JobProcessingEvent to own a value of Job, or maybe an ID for it. If you decide to own the Job, you need to add #[derive(Clone)] to your Job declaration and use job.clone() to copy it into the event.