# Objective
- Alternative to and builds on top of #16284.
- Fixes#15849.
## Solution
- Rename component `StateScoped` to `DespawnOnExitState`.
- Rename system `clear_state_scoped_entities` to
`despawn_entities_on_exit_state`.
- Add `DespawnOnEnterState` and `despawn_entities_on_enter_state` which
is the `OnEnter` equivalent.
> [!NOTE]
> Compared to #16284, the main change is that I did the rename in such a
way as to keep the terms `OnExit` and `OnEnter` together. In my own
game, I was adding `VisibleOnEnterState` and `HiddenOnExitState` and
when naming those, I kept the `OnExit` and `OnEnter` together. When I
checked #16284 it stood out to me that the naming was a bit awkward.
Putting the `State` in the middle and breaking up `OnEnter` and `OnExit`
also breaks searching for those terms.
## Open questions
1. Should we split `enable_state_scoped_entities` into two functions,
one for the `OnEnter` and one for the `OnExit`? I personally have zero
need thus far for the `OnEnter` version, so I'd be interested in not
having this enabled unless I ask for it.
2. If yes to 1., should we follow my lead in my `Visibility` state
components (see below) and name these
`app.enable_despawn_entities_on_enter_state()` and
`app.enable_despawn_entities_on_exit_state()`, which IMO says what it
does on the tin?
## Testing
Ran all changed examples.
## Side note: `VisibleOnEnterState` and `HiddenOnExitState`
For reference to anyone else and to help with the open questions, I'm
including the code I wrote for controlling entity visibility when a
state is entered/exited.
<details>
<summary>visibility.rs</summary>
```rust
use bevy_app::prelude::*;
use bevy_ecs::prelude::*;
use bevy_reflect::prelude::*;
use bevy_render::prelude::*;
use bevy_state::{prelude::*, state::StateTransitionSteps};
use tracing::*;
pub trait AppExtStates {
fn enable_visible_entities_on_enter_state<S: States>(&mut self) -> &mut Self;
fn enable_hidden_entities_on_exit_state<S: States>(&mut self) -> &mut Self;
}
impl AppExtStates for App {
fn enable_visible_entities_on_enter_state<S: States>(&mut self) -> &mut Self {
self.main_mut()
.enable_visible_entities_on_enter_state::<S>();
self
}
fn enable_hidden_entities_on_exit_state<S: States>(&mut self) -> &mut Self {
self.main_mut().enable_hidden_entities_on_exit_state::<S>();
self
}
}
impl AppExtStates for SubApp {
fn enable_visible_entities_on_enter_state<S: States>(&mut self) -> &mut Self {
if !self
.world()
.contains_resource::<Events<StateTransitionEvent<S>>>()
{
let name = core::any::type_name::<S>();
warn!("Visible entities on enter state are enabled for state `{}`, but the state isn't installed in the app!", name);
}
// We work with [`StateTransition`] in set
// [`StateTransitionSteps::ExitSchedules`] as opposed to [`OnExit`],
// because [`OnExit`] only runs for one specific variant of the state.
self.add_systems(
StateTransition,
update_to_visible_on_enter_state::<S>.in_set(StateTransitionSteps::ExitSchedules),
)
}
fn enable_hidden_entities_on_exit_state<S: States>(&mut self) -> &mut Self {
if !self
.world()
.contains_resource::<Events<StateTransitionEvent<S>>>()
{
let name = core::any::type_name::<S>();
warn!("Hidden entities on exit state are enabled for state `{}`, but the state isn't installed in the app!", name);
}
// We work with [`StateTransition`] in set
// [`StateTransitionSteps::ExitSchedules`] as opposed to [`OnExit`],
// because [`OnExit`] only runs for one specific variant of the state.
self.add_systems(
StateTransition,
update_to_hidden_on_exit_state::<S>.in_set(StateTransitionSteps::ExitSchedules),
)
}
}
#[derive(Clone, Component, Debug, Reflect)]
#[reflect(Component, Debug)]
pub struct VisibleOnEnterState<S: States>(pub S);
#[derive(Clone, Component, Debug, Reflect)]
#[reflect(Component, Debug)]
pub struct HiddenOnExitState<S: States>(pub S);
/// Makes entities marked with [`VisibleOnEnterState<S>`] visible when the state
/// `S` is entered.
pub fn update_to_visible_on_enter_state<S: States>(
mut transitions: EventReader<StateTransitionEvent<S>>,
mut query: Query<(&VisibleOnEnterState<S>, &mut Visibility)>,
) {
// We use the latest event, because state machine internals generate at most
// 1 transition event (per type) each frame. No event means no change
// happened and we skip iterating all entities.
let Some(transition) = transitions.read().last() else {
return;
};
if transition.entered == transition.exited {
return;
}
let Some(entered) = &transition.entered else {
return;
};
for (binding, mut visibility) in query.iter_mut() {
if binding.0 == *entered {
visibility.set_if_neq(Visibility::Visible);
}
}
}
/// Makes entities marked with [`HiddenOnExitState<S>`] invisible when the state
/// `S` is exited.
pub fn update_to_hidden_on_exit_state<S: States>(
mut transitions: EventReader<StateTransitionEvent<S>>,
mut query: Query<(&HiddenOnExitState<S>, &mut Visibility)>,
) {
// We use the latest event, because state machine internals generate at most
// 1 transition event (per type) each frame. No event means no change
// happened and we skip iterating all entities.
let Some(transition) = transitions.read().last() else {
return;
};
if transition.entered == transition.exited {
return;
}
let Some(exited) = &transition.exited else {
return;
};
for (binding, mut visibility) in query.iter_mut() {
if binding.0 == *exited {
visibility.set_if_neq(Visibility::Hidden);
}
}
}
```
</details>
---------
Co-authored-by: Benjamin Brienen <Benjamin.Brienen@outlook.com>
Co-authored-by: Ben Frankel <ben.frankel7@gmail.com>
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