Hierarchical states (substates) through system ordering

[MiniaczQ]

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

Bevy's state API is good for flat states, but it's hard to work with nested/hierarchical states.

What solution would you like?

Add API for adding substates and system ordering for propagating it same tick.
This also requires substates to be inactive when their parent isn't in the correct state.

Prototype, tests double as examples.

What alternative(s) have you considered?

• State pattern matching

• Propagate it in multiple ticks

[SIGSTACKFAULT]

previous work: #9942

[MiniaczQ]

previous work: #9942

Appears to be the source of #10088

[lee-orr]

@MiniaczQ - I'd consider this more of a compliment to #10088. That PR doesn't actually provide built in hierarchical states, instead focusing on allowing a wider array of pattern matching options for the currently supported state types (which can be structs or enums, but currently only have a built-in way to handle full equality for matches).

Overall I do like a lot of this approach, though I only had a cursory glance here. The text history around #9957 might give some context for how I ended up with #10088, and the removal of Conditional States (which were my attempt at handling hierarchy). - figured that older PR might be useful as a reference for prior work as well...

[MiniaczQ]

Yeah, the more I work on a solution the less it collides with the state matching PR.
While access to lower level states in a simple way is important, I run into a lot of cases where partial access, both for matching and updating, would be nice.
Pattern matching is certainly not something that can be done with schedules.

A complementary to matching would be partial update through NextState, which I'm considering the necessity of

[lee-orr]

@MiniaczQ - With the addition of SubStates to #11426 - do you think that would resolve this issue? Or would more work be necessary on that front?

[MiniaczQ]

I think this covers the use case :)