Add Theme Switch Widget

[bash]

I got a juust a bit sidetracked while working on #4490 because I realized that the global dark/light mode toggle would be missing a "follow system" option.

My solution is a custom switch widget with three options: follow system, dark and light.

(follow system is disabled for now but is supported by the widget)

The icons are hand-drawn because:

1. I couldn't for the life of me figure out how to properly center the icons from the icon font.
2. The sun, moon and gear icons in the built-in icon font don't look tooo nice on small sizes.
3. I can smoothly animate them between different sizes
4. Drawing the icons by hand is fun?

I tested the widget on my Mac with VoiceOver on to ensure that the widget is announced as Theme, radio group and each radio button is announced as <label>, <n> out of 3.

The widget and the accompanying ThemePreference type are both pub(crate) for now, but I intend to make them public in #4744.

• I have followed the instructions in the PR template

[bash]

I forgot to mention that I'd be happy to split this up into smaller PRs if needed :)

Some individual parts that could be extracted:

• Return value from with_accessibility_parent
• arc drawing could be moved to epaint and exposed as public API
• general shape rotation would remove the need for my rotated_rect workaround.

[AurevoirXavier]

Just curious, how did you obtain the current system theme in egui?

[bash]

@AurevoirXavier egui itself currently doesn't know about the system theme. It relies on eframe (or your integration of choice) to set the Visuals to match the system theme (controlled by follow_system_theme).

The way that the switch widget currently recognizes dark vs. light mode is by checking Visuals.dark_mode.

If you want to know the system theme and are using eframe then you can get system_theme via frame.info().

[emilk]

This is a very pretty theme selector, but I worry it is a bit overkill for the default one in egui for several reasons:

A) it adds a lot of code to maintain
B) it adds a lot of code => increase .wasm size
C) the style of the buttons don't fit in with the rest of egui's default theme (they are too pretty)

I suggest you create this as a 3rd party crate instead, and for egui make a much simpler drop-down menu.

[bash]

Thank you for the kind words ☺️

I totally understand the reasoning behind not wanting this in egui itself. Time for egui-theme-switch 😀

Would you be interested in upstreaming some of the following things from this PR (as individual PRs of course):

• Add a return value to with_accessibility_parent
• WidgetType::ButtonGroup
• Arc drawing

[AurevoirXavier]

This is a very pretty theme selector, but I worry it is a bit overkill for the default one in egui for several reasons:

A) it adds a lot of code to maintain B) it adds a lot of code => increase .wasm size C) the style of the buttons don't fit in with the rest of egui's default theme (they are too pretty)

I suggest you create this as a 3rd party crate instead, and for egui make a much simpler drop-down menu.

I agree with this. To be honest, I ask that question because I want one for my app, but in a combobox. This one feels a bit overkill for my app. (just as @emilk mentioned, it's too pretty and will look out of place in my poor UI)

[bash]

I'm closing this as I have published a crate for this and extracted the "salvageable" parts into two separate PRs.