Application-wide Styling

[emann]

Goals

• Enable implementation of various design languages within iced that can be used without having to call .style() on every widget or use pre-made widgets with styling pre-applied.
• Allow developers using iced to select a design language at compile time
• Enable users to change the theming of an app at runtime

Proposal

the iced_style::Styling trait

pub trait Styling {
    type Theme;
}

This is bound as an associated type on an Application and dictates the default style sheets used for styling widgets as well as the Theme type that is passed as a parameter to the various stylesheet methods. Initially I had thought making the trait Theme with an associated type ColorPalette was the right way of going about it but realized that there's no reason why the struct passed to the various stylesheet methods should contain only color information - it could also contain things like button border radius etc. and allow users to completely change the look of the app at runtime.

Additionally, the Application trait (and others where required) would have a method to return an instance of the Theme type to be passed to styling methods:

/// Returns the theme used to render widgets according to the specified [`Styling`]
fn theme(&self) -> <Self::Styling as Styling>::Theme {
    Default::default()
}

The various StyleSheets then have use Theme as an associated type and their methods take a reference to the Theme as an argument:

/// A set of rules that dictate the style of a button.
pub trait StyleSheet {
    type Theme;

    fn active(&self, theme: &Self::Theme) -> Style;

    fn hovered(&self, theme: &Self::Theme) -> Style;

    fn pressed(&self, theme: &Self::Theme) -> Style;

    fn disabled(&self, theme: &Self::Theme) -> Style;
}

Any widgets then specify that the Renderer's styling implements its own StyleSheet trait:

impl<'a, Message, Renderer, Styling, Theme> Widget<Message, Renderer, Styling>
    for Button<'a, Message, Renderer, Styling, Theme>
where
    Message: Clone,
    Styling: iced_style::Styling<Theme = Theme> + StyleSheet,
    Renderer: crate::Renderer<Styling>,

so that their Widget::draw() implementation can take the Styling::Theme as a parameter to pass along to the style sheet methods:

 fn draw(
        &self,
        renderer: &mut Renderer,
        theme: &Theme,
        layout: Layout<'_>,
        cursor_position: Point,
        _viewport: &Rectangle,
    ) {

You can find a WIP implementation in my branch or in my draft PR