[Merged by Bors] - Fix panic in multiple_windows example

[rparrett]

Fixes #1736

There seemed to be an obvious transcription error where setup_pipeline was being called during on_update of AppState::CreateWindow instead of AppState::Setup.

However, the example still panicked after fixing that.

I'm not sure if there's some intended or unintended change in event processing during state transitions or something, but this PR make the example work again.

Cleaned up an unused Stage label while I was in there.

Please feel free to close this in favor of another approach.

[alice-i-cecile]

@TheRawMeatball you might want to take a look at this; I can't tell if it's reflective of an underlying State bug.

[TheRawMeatball]

Why the extra Done state? You should change the on_updates to on_enters instead.

[mockersf]

the setup_pipeline system is executed several times, until it gets a window_id that's not None

[jakobhellermann]

Also fixes #1709. I looking into fixing that and added a Done state as well.

[rparrett]

Yeah, 1709 (oops, missed that) explains a little better how I came to add the new Done state.

The issue is that state transitions seem to be behaving slightly differently since #1424 and the window no longer exists when CreateWindow's on_enter happens. Hence the on_update and checking for the existence of the new window and the new state.

It's not clear to me if this new behavior is expected, though.

Although setup_window could definitely be on_enter. (But it wasn't prior to or after 1424)

[cart]

I think the previous impl worked with two states because we made a guarantee that only one "on_update" gets called per frame. This ensured that the stage exited after on_update and allowed the queued window to be created on the next frame.

The new state impl allows for multiple on_update calls per frame if a state change is queued up in on_update. This allows the entire state graph traversal (and associated events) to execute within the same frame.

I think the changes in this pr are the right call. The only way to improve this would be to somehow allow mid-frame window creation.

[cart]

bors r+

[bors]

Pull request successfully merged into main.

Build succeeded:

• build-android
• build (nightly, ubuntu-latest)
• build (stable, macos-latest)
• build (stable, ubuntu-latest)
• build (stable, windows-latest)
• build-wasm (nightly, ubuntu-latest)
• build-wasm (stable, ubuntu-latest)
• markdownlint