UndoManager

[Horusiath]

This PR is determined to provide a minimal implementation of UndoManager and expose it to the user API.

It's still work in progress.

/cc @embirico

[heckj]

I'm looking forward to seeing how you use the YUndoManager from the Swift side - what sequence calls need to be ordered, and how you wrap changes. All looks good here - mentioned Origin as a hard (opaque) type might be easier, but it totally works as an existential type as you've set it here too.

[embirico]

Great stuff. I think the API makes sense to me.

My only question as far as usage goes:
If we have a block-based editor with a few NSTextViews tracking a few root-level YTexts, would we be able to / would it make sense to create YUndoManagers on demand, each tracking a single YText?

Probably obvious, but would be great to also have a test like test_undoLocalChangeAfterRemoteEdits, as that's really the main use case here. In Multi we currently use a native Swift UndoManager in NSTextViews that are connected to YSwift. We leave the UndoManager enabled as long as the local edit is the most recent change, but we have to disable it as soon as any more recent remote edits come in.

Let us know how we can help @Horusiath!

[Horusiath]

@embirico Yes, it will be possible through use of transactions with origins, which can be used to describe the source of change. UndoManager can be narrowed to only track changes coming from specific origins, so that undo/redo don't operate on other people updates.

I'll add test cases that can serve as examples on how to use them.

[Horusiath]

@heckj current branch passes build and tests on my machine using YSWIFT_LOCAL=true, however CI build produces weird compilation errors. Could that be somehow related to some difference in Swift versions or something?

[heckj]

I'd suspect so - the complaints are about the code being compatible with an earlier version of swift. I'll take a look and see if I can get it updated.

That's got it 👍

Two commits when one would have done - walked it forward until it was using the latest version of Swift, which resolved the compilation complaints.