<!--
Please read the "Making a PR" section of
[`CONTRIBUTING.md`](https://github.com/emilk/egui/blob/master/CONTRIBUTING.md)
before opening a Pull Request!
* Keep your PR:s small and focused.
* The PR title is what ends up in the changelog, so make it descriptive!
* If applicable, add a screenshot or gif.
* If it is a non-trivial addition, consider adding a demo for it to
`egui_demo_lib`, or a new example.
* Do NOT open PR:s from your `master` branch, as that makes it hard for
maintainers to test and add commits to your PR.
* Remember to run `cargo fmt` and `cargo clippy`.
* Open the PR as a draft until you have self-reviewed it and run
`./scripts/check.sh`.
* When you have addressed a PR comment, mark it as resolved.
Please be patient! I will review your PR, but my time is limited!
-->
* Closes N/A, but this is part of
https://github.com/emilk/egui/issues/3378
* [x] I have followed the instructions in the PR template
Other text layout libraries in Rust--namely, Parley and Cosmic
Text--have one canonical text cursor type (Parley's is a byte index,
Cosmic Text's also stores the line index). To prepare for migrating egui
to one of those libraries, it should also have only one text cursor
type. I also think simplifying the API is a good idea in and of
itself--having three different cursor types that you have to convert
between (and a `Cursor` struct which contains all three at once) is
confusing.
After a bit of experimentation, I found that the best cursor type to
coalesce around is `CCursor`. In the few places where we need a
paragraph index or row/column position, we can calculate them as
necessary.
I've removed `CursorRange` and `PCursorRange` (the latter appears to
have never been used), merging the functionality with `CCursorRange`. To
preserve the cursor position when navigating row-by-row, `CCursorRange`
now stores the previous horizontal position of the cursor.
I've also removed `PCursor`, and renamed `RowCursor` to `LayoutCursor`
(since it includes not only the row but the column). I have not renamed
either `CCursorRange` or `CCursor` as those names are used in a lot of
places, and I don't want to clutter this PR with a bunch of renames.
I'll leave it for a later PR.
Finally, I've removed the deprecated methods from `TextEditState`--it
made the refactoring easier, and it should be pretty easy to migrate to
the equivalent `TextCursorState` methods.
I'm not sure how many breaking changes people will actually encounter. A
lot of these APIs were technically public, but I don't think many were
useful. The `TextBuffer` trait now takes `&CCursorRange` instead of
`&CursorRange` in a couple of methods, and I renamed
`CCursorRange::sorted` to `CCursorRange::sorted_cursors` to match
`CursorRange`.
I did encounter a couple of apparent minor bugs when testing out text
cursor behavior, but I checked them against the current version of egui
and they're all pre-existing.
267485976b
