add example of creating systems via closures

[cab]

Prompted by this discord discussion.

This example shows how to pass an arbitrary argument to a closure, and how to then use that closure as a bevy_ecs::system::System.

[mockersf]

could you also add the example to https://github.com/bevyengine/bevy/blob/main/examples/README.md?

[mockersf]

To pass parameters to your systems, you can use a Local parameter and config it rather than using move to capture arg:

Box::new(|mut cmd: Commands, arg: Local<String>| {
    info!("this system uses the moved arg: {:?}", arg);
    let id = cmd.spawn().id();
    info!("also it spawned an entity {:?}", id);
})
.system()
.config(|config| config.1 = Some("hello".to_string()))

[cab]

Thanks for the review!

To pass parameters to your systems, you can use a Local parameter and config it rather than using move to capture arg:

Box::new(|mut cmd: Commands, arg: Local<String>| {
    info!("this system uses the moved arg: {:?}", arg);
    let id = cmd.spawn().id();
    info!("also it spawned an entity {:?}", id);
})
.system()
.config(|config| config.1 = Some("hello".to_string()))

Interesting, I didn't know about this. Do you think this PR is still useful as an example? (cc @alice-i-cecile)

[mockersf]

Do you think this PR is still useful as an example?

Yes! We don't show how to pass closures as system, so that's valuable. And it would be even more with the Local/config way, as we don't have an example on that yet

[NathanSWard]

Suggested change

	}).system().config(|config| config.1 = Some("hello".to_string())))
	}).system())

Unless the .config() is completely necessary here, I feel like it's just adding unnecessary noise.

[cab]

I think it is necessary -- that's what sets the Local<String>.

[alice-i-cecile]

The content itself looks solid, it but could use a bit more motivating text at the start, explaining why you might want to do this.

[cab]

The content itself looks solid, it but could use a bit more motivating text at the start, explaining why you might want to do this.

Just to make sure I'm understanding -- should I add a comment/text to the example, or do you mean add more explanation to the PR description?

[alice-i-cecile]

The former please; it's much more important that end users can understand the motivation. And the reviewers can check the comments in the code ;)

[cab]

sorry for dropping this! will try to wrap it up by the end of this week.

[alice-i-cecile]

FYI, this has been substantially improved by #2398 and #2403. Check those out and try again using the new impl!

[mockersf]

as seen on Discord, .system() removal may not yet work with .config(), we should do a full pass on Bevy first

[Ratysz]

#2422 should help with that.

[alice-i-cecile]

Hey @cab I'm interested in merging this. Are you interested in refreshing this?

If so, please comment in #2373 and refresh this to main :)

[alice-i-cecile]

Closing due to no response on relicensing.

[alice-i-cecile]

Closing to let someone else pick this work up :) Please credit the author of this PR if you build off it.