Draft virtual Button, Column, and Text

... as well as a very naive diffing strategy!

virtual/src/widget.rs

@@ -1,52 +1,73 @@
 mod button;
+mod column;
+mod element;
+mod text;
+mod tree;
 
 pub use button::Button;
+pub use column::Column;
+pub use element::Element;
+pub use text::Text;
+pub use tree::Tree;
 
+use iced_native::event::{self, Event};
 use iced_native::layout::{self, Layout};
+use iced_native::mouse;
 use iced_native::renderer;
-use iced_native::{Hasher, Length, Point, Rectangle};
+use iced_native::{Clipboard, Hasher, Length, Point, Rectangle, Shell};
 
-pub trait Widget<Message, Renderer> {}
+use std::any::{self, Any};
 
-pub(crate) enum Tree<Message, Renderer> {
-    Node {
-        widget: Box<dyn Widget<Message, Renderer>>,
-        children: Vec<Tree<Message, Renderer>>,
-    },
-    Leaf {
-        widget: Box<dyn Widget<Message, Renderer>>,
-    },
-}
+pub trait Widget<Message, Renderer> {
+    fn tag(&self) -> any::TypeId;
 
-impl<Message, Renderer> Tree<Message, Renderer> {
-    pub fn width(&self) -> Length {
-        unimplemented! {}
-    }
+    fn state(&self) -> Box<dyn Any>;
 
-    pub fn height(&self) -> Length {
-        unimplemented! {}
-    }
+    fn children(&self) -> &[Element<Message, Renderer>];
 
-    pub fn hash_layout(&self, state: &mut Hasher) {
-        unimplemented! {}
-    }
+    fn width(&self) -> Length;
 
-    pub fn layout(
+    fn height(&self) -> Length;
+
+    fn hash_layout(&self, state: &mut Hasher);
+
+    fn layout(
         &self,
         renderer: &Renderer,
         limits: &layout::Limits,
-    ) -> layout::Node {
-        unimplemented! {}
-    }
+    ) -> layout::Node;
 
-    pub fn draw(
+    fn draw(
         &self,
+        state: &Tree<Message, Renderer>,
         renderer: &mut Renderer,
         style: &renderer::Style,
         layout: Layout<'_>,
         cursor_position: Point,
         viewport: &Rectangle,
-    ) {
-        unimplemented! {}
+    );
+
+    fn mouse_interaction(
+        &self,
+        _state: &Tree<Message, Renderer>,
+        _layout: Layout<'_>,
+        _cursor_position: Point,
+        _viewport: &Rectangle,
+        _renderer: &Renderer,
+    ) -> mouse::Interaction {
+        mouse::Interaction::Idle
+    }
+
+    fn on_event(
+        &mut self,
+        _state: &mut Tree<Message, Renderer>,
+        _event: Event,
+        _layout: Layout<'_>,
+        _cursor_position: Point,
+        _renderer: &Renderer,
+        _clipboard: &mut dyn Clipboard,
+        _shell: &mut Shell<'_, Message>,
+    ) -> event::Status {
+        event::Status::Ignored
     }
 }