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!

1 files changed, 37 insertions, 0 deletions

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(),
+        }
+    }
+}