Advance deployment SystemId

[MushineLament]

What problem does this solve or what need does it fill?

1.disentanglement.
2.enhance flexibility.
3.Reduce world exclusive time.

What solution would you like?

Such:

    let entity = world.spawn_empty();
    world.register_system_by_entity(entity,intosystem);

register_system_by_entity function:

    pub fn register_system_by_entity<I, O>(
        &mut self,
        entity: Entity,
        system: BoxedSystem<I, O>,
    ) -> SystemId<I, O>
    where
        I: SystemInput + 'static,
        O: 'static,
    {
        self.entity_mut(entity)
            .insert(RegisteredSystem::new(system));

        SystemId::from_entity(entity)
    }

What alternative(s) have you considered?

not

Additional context

The common method to obtain SystemId is through system exclusive world:

fn usually(world:&mut World){
    let systemid = world.register_system(system);
    ...
}

If the above method is supported,

fn support(
    mut commands: Commands, //Or ParallelCommands
    ...
) {
    let empty = commands.spawn_empty().id();
    // It is necessary to ensure that the type corresponds to the system
    let systemid: SystemId<(), ()> = SystemId::from_entity(empty);

    commands.queue(|world:&mut World|{
        let world_systemid = world.register_system_by_entity(empty,system);
        //You can do some checks
        assert_eq!(systemid,world_systemid);
    });

    // For example, certain components may run unique systems
    // However, the current system ID has not been fully successfully registered
    #[derive(Debug, Component)]
    struct Component1(SystemId<(), ()>);

    commands.spawn(Component1(systemid));
}

I conducted some tests on the SystemId of the current version (0.16.1)

    let mut world = World::new();

    let empty = world.spawn_empty().id();

    let systemid = SystemId::<(), ()>::from_entity(empty);

    let result = world.run_system(systemid);

    assert!(result.is_err());
    println!("{:?}", result);

println:
Err(Recursive(SystemId(0v1#4294967296)))

 let systemid: SystemId<(), bool> = world.register_system(|| -> bool { true });

    let result = world.run_system(systemid);

    assert!(result.is_ok());

    let entity = systemid.entity();

    // Modified type parameters
    let systemid = SystemId::<(), ()>::from_entity(entity);

    let result = world.run_system(systemid);

    assert!(result.is_err());

println:
Err(Recursive(SystemId(1v1#4294967297)))

World can identify which systemid are valid.

Overall, I believe that the text should be supported.

[hukasu]

RegisteredSystem is just a component, just put it into the spawn

The error message for the last example has been improved in #19011

[hukasu]

Commands also have register_system so you don't need World https://docs.rs/bevy/latest/bevy/prelude/struct.Commands.html#method.register_system

[MushineLament]

Commands也有，所以你不需要 https://docs.rs/bevy/latest/bevy/prelude/struct.Commands.html#method.register_system`register_system``World`

Thank you, I didn't notice that Commands can be registere_system