theme: Use boxed functions instead fn pointers (#99)

This is more general, and necessary if the custom theming is dynamically
generated.

Iced's builtin theme also uses `Box`ed `Fn`, or `Rc` where clone is
required, so this seems reasonable.

Only `Text` is left using `fn`, since it needs to be `Copy`. Hopefully
that can be changed in Iced at some point.

`::custom` methods are added to make these variants a little more
convenient to construct. This replaces a couple `From` implementations,
which are potentially problematic with a generic.

examples/cosmic/src/window.rs

@@ -277,7 +277,7 @@ impl Window {
                 .spacing(16),
             )
             .padding([20, 24])
-            .style(theme::Container::Custom(list::column::style)),
+            .style(theme::Container::custom(list::column::style)),
         )
         .padding(0)
         .style(theme::Button::Transparent)

src/applet/mod.rs

@@ -19,16 +19,18 @@ pub use cosmic_panel_config;
 
 const APPLET_PADDING: u32 = 8;
 
-pub const APPLET_BUTTON_THEME: Button = Button::Custom {
-    active: |t| iced_style::button::Appearance {
-        border_radius: BorderRadius::from(0.0),
-        ..t.active(&Button::Text)
-    },
-    hover: |t| iced_style::button::Appearance {
-        border_radius: BorderRadius::from(0.0),
-        ..t.hovered(&Button::Text)
-    },
-};
+pub fn applet_button_theme() -> Button {
+    Button::Custom {
+        active: Box::new(|t| iced_style::button::Appearance {
+            border_radius: BorderRadius::from(0.0),
+            ..t.active(&Button::Text)
+        }),
+        hover: Box::new(|t| iced_style::button::Appearance {
+            border_radius: BorderRadius::from(0.0),
+            ..t.hovered(&Button::Text)
+        }),
+    }
+}
 
 #[derive(Debug, Clone)]
 pub struct CosmicAppletHelper {
@@ -130,7 +132,7 @@ impl CosmicAppletHelper {
         };
 
         Container::<Message, Renderer>::new(Container::<Message, Renderer>::new(content).style(
-            crate::theme::Container::Custom(|theme| Appearance {
+            crate::theme::Container::custom(|theme| Appearance {
                 text_color: Some(theme.cosmic().background.on.into()),
                 background: Some(Color::from(theme.cosmic().background.base).into()),
                 border_radius: 12.0,

src/theme/expander.rs

@@ -52,7 +52,7 @@ impl std::default::Default for Appearance {
 
 /// A set of rules that dictate the [`Appearance`] of a container.
 pub trait StyleSheet {
-    type Style: Default + Copy;
+    type Style: Default;
 
     /// Produces the [`Appearance`] of a container.
     fn appearance(&self, style: Self::Style) -> Appearance;

src/theme/mod.rs

@@ -6,6 +6,7 @@ mod segmented_button;
 
 use std::hash::Hash;
 use std::hash::Hasher;
+use std::rc::Rc;
 
 pub use self::segmented_button::SegmentedButton;
 
@@ -132,15 +133,16 @@ impl LayeredTheme for Theme {
     }
 }
 
-#[derive(Clone, Copy)]
+#[derive(Default)]
 pub enum Application {
+    #[default]
     Default,
-    Custom(fn(&Theme) -> application::Appearance),
+    Custom(Box<dyn Fn(&Theme) -> application::Appearance>),
 }
 
-impl Default for Application {
-    fn default() -> Self {
-        Self::Default
+impl Application {
+    pub fn custom<F: Fn(&Theme) -> application::Appearance + 'static>(f: F) -> Self {
+        Self::Custom(Box::new(f))
     }
 }
 
@@ -163,7 +165,6 @@ impl application::StyleSheet for Theme {
 /*
  * TODO: Button
  */
-#[derive(Clone, Copy)]
 pub enum Button {
     Deactivated,
     Destructive,
@@ -175,8 +176,8 @@ pub enum Button {
     LinkActive,
     Transparent,
     Custom {
-        active: fn(&Theme) -> button::Appearance,
-        hover: fn(&Theme) -> button::Appearance,
+        active: Box<dyn Fn(&Theme) -> button::Appearance>,
+        hover: Box<dyn Fn(&Theme) -> button::Appearance>,
     },
 }
 
@@ -439,21 +440,16 @@ impl checkbox::StyleSheet for Theme {
     }
 }
 
-#[derive(Clone, Copy)]
+#[derive(Default)]
 pub enum Expander {
+    #[default]
     Default,
-    Custom(fn(&Theme) -> expander::Appearance),
+    Custom(Box<dyn Fn(&Theme) -> expander::Appearance>),
 }
 
-impl Default for Expander {
-    fn default() -> Self {
-        Self::Default
-    }
-}
-
-impl From<fn(&Theme) -> expander::Appearance> for Expander {
-    fn from(f: fn(&Theme) -> expander::Appearance) -> Self {
-        Self::Custom(f)
+impl Expander {
+    pub fn custom<F: Fn(&Theme) -> expander::Appearance + 'static>(f: F) -> Self {
+        Self::Custom(Box::new(f))
     }
 }
 
@@ -471,24 +467,19 @@ impl expander::StyleSheet for Theme {
 /*
  * TODO: Container
  */
-#[derive(Clone, Copy)]
+#[derive(Default)]
 pub enum Container {
     Background,
     Primary,
     Secondary,
+    #[default]
     Transparent,
-    Custom(fn(&Theme) -> container::Appearance),
+    Custom(Box<dyn Fn(&Theme) -> container::Appearance>),
 }
 
-impl Default for Container {
-    fn default() -> Self {
-        Self::Transparent
-    }
-}
-
-impl From<fn(&Theme) -> container::Appearance> for Container {
-    fn from(_: fn(&Theme) -> container::Appearance) -> Self {
-        Self::default()
+impl Container {
+    pub fn custom<F: Fn(&Theme) -> container::Appearance + 'static>(f: F) -> Self {
+        Self::Custom(Box::new(f))
     }
 }
 
@@ -754,17 +745,18 @@ impl pane_grid::StyleSheet for Theme {
 /*
  * TODO: Progress Bar
  */
-#[derive(Clone, Copy)]
+#[derive(Default)]
 pub enum ProgressBar {
+    #[default]
     Primary,
     Success,
     Danger,
-    Custom(fn(&Theme) -> progress_bar::Appearance),
+    Custom(Box<dyn Fn(&Theme) -> progress_bar::Appearance>),
 }
 
-impl Default for ProgressBar {
-    fn default() -> Self {
-        Self::Primary
+impl ProgressBar {
+    pub fn custom<F: Fn(&Theme) -> progress_bar::Appearance + 'static>(f: F) -> Self {
+        Self::Custom(Box::new(f))
     }
 }
 
@@ -798,17 +790,18 @@ impl progress_bar::StyleSheet for Theme {
 /*
  * TODO: Rule
  */
-#[derive(Clone, Copy)]
+#[derive(Default)]
 pub enum Rule {
+    #[default]
     Default,
     LightDivider,
     HeavyDivider,
-    Custom(fn(&Theme) -> rule::Appearance),
+    Custom(Box<dyn Fn(&Theme) -> rule::Appearance>),
 }
 
-impl Default for Rule {
-    fn default() -> Self {
-        Self::Default
+impl Rule {
+    pub fn custom<F: Fn(&Theme) -> rule::Appearance + 'static>(f: F) -> Self {
+        Self::Custom(Box::new(f))
     }
 }
 
@@ -883,10 +876,10 @@ impl scrollable::StyleSheet for Theme {
     }
 }
 
-#[derive(Default, Clone, Copy)]
+#[derive(Clone, Default)]
 pub enum Svg {
     /// Apply a custom appearance filter
-    Custom(fn(&Theme) -> svg::Appearance),
+    Custom(Rc<dyn Fn(&Theme) -> svg::Appearance>),
     /// No filtering is applied
     #[default]
     Default,
@@ -915,6 +908,12 @@ impl Hash for Svg {
     }
 }
 
+impl Svg {
+    pub fn custom<F: Fn(&Theme) -> svg::Appearance + 'static>(f: F) -> Self {
+        Self::Custom(Rc::new(f))
+    }
+}
+
 impl svg::StyleSheet for Theme {
     type Style = Svg;
 
@@ -948,6 +947,7 @@ pub enum Text {
     #[default]
     Default,
     Color(Color),
+    // TODO: Can't use dyn Fn since this must be copy
     Custom(fn(&Theme) -> text::Appearance),
 }
 

src/theme/segmented_button.rs

@@ -6,7 +6,7 @@ use crate::{theme::Theme, widget::segmented_button::ItemStatusAppearance};
 use iced_core::{Background, BorderRadius};
 use palette::{rgb::Rgb, Alpha};
 
-#[derive(Clone, Copy, Default)]
+#[derive(Default)]
 pub enum SegmentedButton {
     /// A tabbed widget for switching between views in an interface.
     #[default]
@@ -14,7 +14,7 @@ pub enum SegmentedButton {
     /// A widget for multiple choice selection.
     Selection,
     /// Or implement any custom theme of your liking.
-    Custom(fn(&Theme) -> Appearance),
+    Custom(Box<dyn Fn(&Theme) -> Appearance>),
 }
 
 impl StyleSheet for Theme {

src/widget/icon.rs

@@ -207,7 +207,7 @@ impl<'a> Icon<'a> {
 
     fn svg_element<Message: 'static>(&self, handle: svg::Handle) -> Element<'static, Message> {
         svg::Svg::<Renderer>::new(handle)
-            .style(self.style)
+            .style(self.style.clone())
             .width(self.width.unwrap_or(Length::Units(self.size)))
             .height(self.height.unwrap_or(Length::Units(self.size)))
             .content_fit(self.content_fit)

src/widget/list/column.rs

@@ -45,7 +45,7 @@ impl<'a, Message: 'static> ListColumn<'a, Message> {
             .spacing(12)
             .apply(iced::widget::container)
             .padding([16, 6])
-            .style(theme::Container::Custom(style))
+            .style(theme::Container::custom(style))
             .into()
     }
 }

src/widget/nav_bar.rs

@@ -36,7 +36,7 @@ where
         .apply(container)
         .height(Length::Fill)
         .padding(11)
-        .style(theme::Container::Custom(nav_bar_style))
+        .style(theme::Container::custom(nav_bar_style))
 }
 
 #[must_use]

src/widget/search/field.rs

@@ -58,7 +58,7 @@ impl<'a, Message: 'static + Clone> Field<'a, Message> {
         .spacing(8)
         .align_items(iced::Alignment::Center)
         .apply(container)
-        .style(crate::theme::Container::Custom(active_style))
+        .style(crate::theme::Container::custom(active_style))
         .into()
     }
 }

src/widget/spin_button/mod.rs

@@ -83,7 +83,7 @@ impl<'a, Message: 'static> SpinButton<'a, Message> {
         .align_y(Vertical::Center)
         .width(Length::Units(95))
         .height(Length::Units(32))
-        .style(theme::Container::Custom(container_style))
+        .style(theme::Container::custom(container_style))
         .apply(Element::from)
         .map(on_change)
     }

src/widget/warning.rs

@@ -51,7 +51,7 @@ impl<'a, Message: 'static + Clone> Warning<'a, Message> {
             ])
             .align_items(Alignment::Center),
         )
-        .style(theme::Container::Custom(warning_container))
+        .style(theme::Container::custom(warning_container))
         .padding(10)
         .align_y(alignment::Vertical::Center)
         .width(Length::Fill)