Add Widget and AsyncWidget traits

src/widget.rs

@@ -0,0 +1,28 @@
+use async_trait::async_trait;
+
+use crate::{Frame, Size};
+
+// TODO Feature-gate these traits
+
+pub trait Widget<E> {
+    fn size(
+        &self,
+        frame: &mut Frame,
+        max_width: Option<u16>,
+        max_height: Option<u16>,
+    ) -> Result<Size, E>;
+
+    fn draw(self, frame: &mut Frame) -> Result<(), E>;
+}
+
+#[async_trait]
+pub trait AsyncWidget<E> {
+    async fn size(
+        &self,
+        frame: &mut Frame,
+        max_width: Option<u16>,
+        max_height: Option<u16>,
+    ) -> Result<Size, E>;
+
+    async fn draw(self, frame: &mut Frame) -> Result<(), E>;
+}