Draft `iced_virtual` subcrate

The idea here is to expose a set of "virtual widgets" that can be used with a
`Virtual` widget and its `virtual::State`.

A virtual widget is a widget that does not contain any state, but instead is a
"virtual" representation of the "real" widget. The real widgets are stored in
the `virtual::State`.

Every time a new virtual widget tree is created during `view`, it is compared to
the previous one and "real" widgets are added / removed to the `virtual::State`.

Effectively, this removes the need to keep track of local widget state in the
application state and allows `view` to take an immutable reference to `self`.

To summarize, using this crate should allow users to remove `State` structs
in their application state.

Eventually, the strategy used here may be adopted generally and, as a result,
all of the widgets in `iced_native` would be replaced!

6 files changed, 223 insertions, 0 deletions

diff --git a/Cargo.toml b/Cargo.toml
index 7c222fbb..7a4cecc8 100644
--- a/Cargo.toml
+++ b/Cargo.toml
@@ -58,6 +58,7 @@ members = [
     "lazy",
     "native",
     "style",
+    "virtual",
     "wgpu",
     "winit",
     "examples/bezier_tool",
diff --git a/virtual/Cargo.toml b/virtual/Cargo.toml
new file mode 100644
index 00000000..74960b80
--- /dev/null
+++ b/virtual/Cargo.toml
@@ -0,0 +1,8 @@
+[package]
+name = "iced_virtual"
+version = "0.1.0"
+edition = "2021"
+
+[dependencies]
+iced_native = { version = "0.4", path = "../native" }
+iced_style = { version = "0.3", path = "../style" }
diff --git a/virtual/src/element.rs b/virtual/src/element.rs
new file mode 100644
index 00000000..ecdb0677
--- /dev/null
+++ b/virtual/src/element.rs
@@ -0,0 +1,37 @@
+use crate::Widget;
+
+pub struct Element<Message, Renderer> {
+    widget: Box<dyn Descriptor<Message, Renderer>>,
+}
+
+impl<Message, Renderer> Element<Message, Renderer> {
+    pub fn new(widget: impl Descriptor<Message, Renderer> + 'static) -> Self {
+        Self {
+            widget: Box::new(widget),
+        }
+    }
+}
+
+pub trait Descriptor<Message, Renderer> {
+    fn tag(&self) -> std::any::TypeId;
+
+    fn build(&self) -> Box<dyn Widget<Message, Renderer>>;
+
+    fn children(&self) -> &[Element<Message, Renderer>];
+
+    fn clone(&self) -> Box<dyn Descriptor<Message, Renderer>>;
+}
+
+impl<Message, Renderer> Clone for Box<dyn Descriptor<Message, Renderer>> {
+    fn clone(&self) -> Self {
+        self.as_ref().clone()
+    }
+}
+
+impl<Message, Renderer> Clone for Element<Message, Renderer> {
+    fn clone(&self) -> Self {
+        Element {
+            widget: self.widget.clone(),
+        }
+    }
+}
diff --git a/virtual/src/lib.rs b/virtual/src/lib.rs
new file mode 100644
index 00000000..80953872
--- /dev/null
+++ b/virtual/src/lib.rs
@@ -0,0 +1,61 @@
+mod element;
+pub mod widget;
+
+pub use element::Element;
+pub use widget::Widget;
+
+use iced_native::layout::{self, Layout};
+use iced_native::renderer;
+use iced_native::{Hasher, Length, Point, Rectangle};
+
+pub struct Virtual<'a, Message, Renderer> {
+    state: &'a mut State<Message, Renderer>,
+}
+
+pub struct State<Message, Renderer> {
+    widget_tree: widget::Tree<Message, Renderer>,
+    last_element: Element<Message, Renderer>,
+}
+
+impl<'a, Message, Renderer> iced_native::Widget<Message, Renderer>
+    for Virtual<'a, Message, Renderer>
+where
+    Renderer: iced_native::Renderer,
+{
+    fn width(&self) -> Length {
+        self.state.widget_tree.width()
+    }
+
+    fn height(&self) -> Length {
+        self.state.widget_tree.height()
+    }
+
+    fn layout(
+        &self,
+        renderer: &Renderer,
+        limits: &layout::Limits,
+    ) -> layout::Node {
+        self.state.widget_tree.layout(renderer, limits)
+    }
+
+    fn hash_layout(&self, state: &mut Hasher) {
+        self.state.widget_tree.hash_layout(state)
+    }
+
+    fn draw(
+        &self,
+        renderer: &mut Renderer,
+        style: &renderer::Style,
+        layout: Layout<'_>,
+        cursor_position: Point,
+        viewport: &Rectangle,
+    ) {
+        self.state.widget_tree.draw(
+            renderer,
+            style,
+            layout,
+            cursor_position,
+            viewport,
+        )
+    }
+}
diff --git a/virtual/src/widget.rs b/virtual/src/widget.rs
new file mode 100644
index 00000000..f8697bc0
--- /dev/null
+++ b/virtual/src/widget.rs
@@ -0,0 +1,52 @@
+mod button;
+
+pub use button::Button;
+
+use iced_native::layout::{self, Layout};
+use iced_native::renderer;
+use iced_native::{Hasher, Length, Point, Rectangle};
+
+pub trait Widget<Message, Renderer> {}
+
+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>>,
+    },
+}
+
+impl<Message, Renderer> Tree<Message, Renderer> {
+    pub fn width(&self) -> Length {
+        unimplemented! {}
+    }
+
+    pub fn height(&self) -> Length {
+        unimplemented! {}
+    }
+
+    pub fn hash_layout(&self, state: &mut Hasher) {
+        unimplemented! {}
+    }
+
+    pub fn layout(
+        &self,
+        renderer: &Renderer,
+        limits: &layout::Limits,
+    ) -> layout::Node {
+        unimplemented! {}
+    }
+
+    pub fn draw(
+        &self,
+        renderer: &mut Renderer,
+        style: &renderer::Style,
+        layout: Layout<'_>,
+        cursor_position: Point,
+        viewport: &Rectangle,
+    ) {
+        unimplemented! {}
+    }
+}
diff --git a/virtual/src/widget/button.rs b/virtual/src/widget/button.rs
new file mode 100644
index 00000000..115be80b
--- /dev/null
+++ b/virtual/src/widget/button.rs
@@ -0,0 +1,64 @@
+use crate::element::{self, Element};
+use crate::Widget;
+
+pub struct Button<Message, Renderer> {
+    content: Element<Message, Renderer>,
+    on_press: Option<Message>,
+}
+
+impl<Message, Renderer> Button<Message, Renderer> {
+    pub fn new(
+        content: impl element::Descriptor<Message, Renderer> + 'static,
+    ) -> Self {
+        Button {
+            content: Element::new(content),
+            on_press: None,
+        }
+    }
+
+    pub fn on_press(mut self, on_press: Message) -> Self {
+        self.on_press = Some(on_press);
+        self
+    }
+}
+
+impl<Message, Renderer> element::Descriptor<Message, Renderer>
+    for Button<Message, Renderer>
+where
+    Message: 'static + Clone,
+    Renderer: 'static,
+{
+    fn tag(&self) -> std::any::TypeId {
+        std::any::TypeId::of::<Self>()
+    }
+
+    fn build(&self) -> Box<dyn Widget<Message, Renderer>> {
+        Box::new(State { is_pressed: false })
+    }
+
+    fn children(&self) -> &[Element<Message, Renderer>] {
+        std::slice::from_ref(&self.content)
+    }
+
+    fn clone(&self) -> Box<dyn element::Descriptor<Message, Renderer>> {
+        Box::new(Clone::clone(self))
+    }
+}
+
+impl<Message, Renderer> Clone for Button<Message, Renderer>
+where
+    Message: Clone,
+{
+    fn clone(&self) -> Self {
+        Self {
+            content: self.content.clone(),
+            on_press: self.on_press.clone(),
+        }
+    }
+}
+
+pub struct State {
+    is_pressed: bool,
+}
+
+impl<Message, Renderer> Widget<Message, Renderer> for State {}