Allow setting a default easing function in Style

[FireFragment]

Is your feature request related to a problem? Please describe.

Developer may prefer using certain easing function because of branding, coherence with easing used by rest of the system or personal preference. But now, the built-in animations such as SidePanel::show_animated or CollapsingHeader use the default, non-customizable easing.

Describe the solution you'd like

I would add new field to Style, something like easing being a struct containing three easing functions: ease_out, ease_in and ease_in_out. The built-in widgets would use these easing functions by default (eg. animate_bool_responsive would use ease_out).

Additionally, widgets built by the community could also use these default easings, making animations customizable while keeping them consistent.

Describe alternatives you've considered

• Allowing to specify easing functions on the built-in widgets individually
• Letting user specify only the ease_out function in the style, because AFAIK ease-in isn't used anywhere in egui right now. This would also eliminate the need for something like an Easings struct.

[lucasmerlin]

I would add new field to Style, something like easing being a struct containing three easing functions: ease_out, ease_in and ease_in_out.

I would also like this, but the problem with this solution would be that Style is De/Serializable and there is no way to to serialize a function :/ we could of course just set the default easings when deserializing but that would suck.

An alternative approach would be to add an Easing enum that has all the predefined easings, an an Easing::Custom(fn(...)), that would mean at least the build in easings would be serializable.

[FireFragment]

@lucasmerlin

we could of course just set the default easings when deserializing but that would suck.

I thought that was okay, because Style::number_formatter already does this for the exact same reason:

egui/crates/egui/src/style.rs

Lines 240 to 241 in 27a5803

	#[cfg_attr(feature = "serde", serde(skip))]
	pub number_formatter: NumberFormatter,

egui/crates/egui/src/style.rs

Lines 19 to 21 in 27a5803

	pub struct NumberFormatter(
	Arc<dyn 'static + Sync + Send + Fn(f64, RangeInclusive<usize>) -> String>,
	);

But I personally like the enum idea :).

[lucasmerlin]

Ah, good point, also I think serde support here isn't super important, since the biggest usecase is probably for exporting / importing ui themes and I don't think a lot of egui Apps do that.