Add app.add_state(inital_state) convenience method

[honungsburk]

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

I want to initialize a state to an arbitrary state, not just default.

Right now I am using app.add_state::<MyState>()

What solution would you like?

Add a convenience method app.add_state(MyState::MyArbitraryState) to App

What alternative(s) have you considered?

I read through the motivation for the new state system but it doesn't mention why only the default trait is used for initialization. This seems like a clear win to me so I would be surprised at any objections.

[honungsburk]

I tried out this idea in my own game and it works great:

use bevy::ecs::schedule::common_conditions::run_once as run_once_condition;
use bevy::ecs::schedule::run_enter_schedule;
use bevy::prelude::*;

pub trait AppExtension {
    fn init_state<S: States>(&mut self, initial_state: S) -> &mut Self;
}

impl AppExtension for App {
    // Remove when [add_state is added](https://github.com/bevyengine/bevy/issues/10731)
    fn init_state<S: States>(&mut self, initial_state: S) -> &mut Self {
        self.insert_resource::<State<S>>(State::new(initial_state))
            .init_resource::<NextState<S>>()
            .add_systems(
                StateTransition,
                (
                    run_enter_schedule::<S>.run_if(run_once_condition()),
                    apply_state_transition::<S>,
                )
                    .chain(),
            );

        // The OnEnter, OnExit, and OnTransition schedules are lazily initialized
        // (i.e. when the first system is added to them), and World::try_run_schedule is used to fail
        // gracefully if they aren't present.
        self
    }
}

[honungsburk]

If someone gives the greenlight I'll happily create PR 😄

[roboteng]

I've done something a little different in my projects before. I'm not sure if its any better, but its another option.

pub struct BasePlugin(GameState);
impl Plugin for BasePlugin {
    fn build(&self, app: &mut App) {
        app.add_state::<GameState>();

        app.world
            .get_resource_mut::<NextState<GameState>>()
            .unwrap()
            .set(self.0);
   }
}

I just copied this from my project, but I don't think it'd be hard to change this to a method on App just like add_state

Is this any better/worse?

[doonv]

How about we split the add_state method into two methods: init_state<T: State>() and insert_state<T: State>(state: T). init_state would work like add_state, and insert_state will initialize the state to the state you pass in.

The names are inspired by the init_resource and insert_resource methods.