# Objective
- Fix issue where `SubStates` depending on multiple source states would
only react when _all_ source states changed simultaneously.
- SubStates should be created/destroyed whenever _any_ of their source
states transitions, not only when all change together.
# Solution
- Changed the "did parent change" detection logic from AND to OR. We
need to check if _any_ of the event readers changed, not if _all_ of
them changed.
- See
https://github.com/bevyengine/bevy/actions/runs/15610159742/job/43968937544?pr=19595
for failing test proof before I pushed the fix.
- The generated code we want needs `||`s not `&&`s like this:
```rust
fn register_sub_state_systems_in_schedule<T: SubStates<SourceStates = Self>>(schedule: &mut Schedule) {
let apply_state_transition = |(mut ereader0, mut ereader1, mut ereader2): (
EventReader<StateTransitionEvent<S0::RawState>>,
EventReader<StateTransitionEvent<S1::RawState>>,
EventReader<StateTransitionEvent<S2::RawState>>,
),
event: EventWriter<StateTransitionEvent<T>>,
commands: Commands,
current_state_res: Option<ResMut<State<T>>>,
next_state_res: Option<ResMut<NextState<T>>>,
(s0, s1, s2): (
Option<Res<State<S0::RawState>>>,
Option<Res<State<S1::RawState>>>,
Option<Res<State<S2::RawState>>>,
)| {
// With `||` we can correctly count parent changed if any of the sources changed.
let parent_changed = (ereader0.read().last().is_some()
|| ereader1.read().last().is_some()
|| ereader2.read().last().is_some());
let next_state = take_next_state(next_state_res);
if !parent_changed && next_state.is_none() {
return;
}
// ...
}
}
```
# Testing
- Add new test.
- Check the fix worked in my game.
1f2fd3d29d