learn how workspaces + proc macros work & restructure everything

core/src/layout/widget/mod.rs

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+mod handle;
+mod like;
+mod tag;
+mod widgets;
+
+pub use handle::*;
+pub use like::*;
+pub use tag::*;
+pub use widgets::*;
+
+use crate::layout::{Len, Painter, SizeCtx, Ui};
+
+pub trait Widget: 'static {
+    fn draw(&mut self, painter: &mut Painter);
+    fn desired_width(&mut self, ctx: &mut SizeCtx) -> Len;
+    fn desired_height(&mut self, ctx: &mut SizeCtx) -> Len;
+}
+
+impl Widget for () {
+    fn draw(&mut self, _: &mut Painter) {}
+    fn desired_width(&mut self, _: &mut SizeCtx) -> Len {
+        Len::ZERO
+    }
+    fn desired_height(&mut self, _: &mut SizeCtx) -> Len {
+        Len::ZERO
+    }
+}
+
+/// A function that returns a widget given a UI.
+/// Useful for defining trait functions on widgets that create a parent widget so that the children
+/// don't need to be IDs yet
+pub trait WidgetFn<W: Widget + ?Sized>: FnOnce(&mut Ui) -> W {}
+impl<W: Widget, F: FnOnce(&mut Ui) -> W> WidgetFn<W> for F {}
+
+impl<W: Widget, F: FnOnce(&mut Ui) -> W> WidgetLike<FnTag> for F {
+    type Widget = W;
+    fn add(self, ui: &mut Ui) -> WidgetRef<W> {
+        self(ui).add(ui)
+    }
+}
+
+impl<W: Widget> WidgetLike<WidgetTag> for W {
+    type Widget = W;
+    fn add(self, ui: &mut Ui) -> WidgetRef<W> {
+        ui.add_widget(self)
+    }
+}
+
+pub trait WidgetOption {
+    fn get(self, ui: &mut Ui) -> Option<WidgetRef>;
+}
+
+impl WidgetOption for () {
+    fn get(self, _: &mut Ui) -> Option<WidgetRef> {
+        None
+    }
+}
+
+impl<F: FnOnce(&mut Ui) -> Option<WidgetRef>> WidgetOption for F {
+    fn get(self, ui: &mut Ui) -> Option<WidgetRef> {
+        self(ui)
+    }
+}