Allow canvas::State to produce messages

wgpu/src/widget/canvas/cache.rs

@@ -80,7 +80,10 @@ impl Cache {
         Geometry::from_primitive(Primitive::Cached { cache: primitive })
     }
 
-    pub fn with<'a, T>(&'a self, input: T) -> impl crate::canvas::State + 'a
+    pub fn with<'a, T, Message>(
+        &'a self,
+        input: T,
+    ) -> impl crate::canvas::State<Message> + 'a
     where
         T: Drawable + std::fmt::Debug + 'a,
     {
@@ -93,7 +96,7 @@ struct Bind<'a, T> {
     input: T,
 }
 
-impl<'a, T> crate::canvas::State for Bind<'a, T>
+impl<'a, T, Message> crate::canvas::State<Message> for Bind<'a, T>
 where
     T: Drawable + std::fmt::Debug + 'a,
 {

wgpu/src/widget/canvas/state.rs

@@ -1,17 +1,19 @@
 use crate::canvas::{Event, Geometry, Size};
 
-pub trait State {
-    fn update(&mut self, _event: Event, _bounds: Size) {}
+pub trait State<Message> {
+    fn update(&mut self, _event: Event, _bounds: Size) -> Option<Message> {
+        None
+    }
 
     fn draw(&self, bounds: Size) -> Vec<Geometry>;
 }
 
-impl<T> State for &mut T
+impl<T, Message> State<Message> for &mut T
 where
-    T: State,
+    T: State<Message>,
 {
-    fn update(&mut self, event: Event, bounds: Size) {
-        T::update(self, event, bounds);
+    fn update(&mut self, event: Event, bounds: Size) -> Option<Message> {
+        T::update(self, event, bounds)
     }
 
     fn draw(&self, bounds: Size) -> Vec<Geometry> {