1 files changed, 22 insertions, 12 deletions

diff --git a/graphics/src/widget/canvas/program.rs b/graphics/src/widget/canvas/program.rs
index dddc387d..656dbfa6 100644
--- a/graphics/src/widget/canvas/program.rs
+++ b/graphics/src/widget/canvas/program.rs
@@ -1,8 +1,7 @@
-use crate::canvas::event::{self, Event};
-use crate::canvas::{Cursor, Geometry};
-
-use iced_native::mouse;
-use iced_native::Rectangle;
+use crate::widget::canvas::event::{self, Event};
+use crate::widget::canvas::mouse;
+use crate::widget::canvas::{Cursor, Geometry};
+use crate::Rectangle;
 
 /// The state and logic of a [`Canvas`].
 ///
@@ -11,7 +10,10 @@ use iced_native::Rectangle;
 ///
 /// [`Canvas`]: crate::widget::Canvas
 pub trait Program<Message, Theme = iced_native::Theme> {
-    /// Updates the state of the [`Program`].
+    /// The internal state mutated by the [`Program`].
+    type State: Default + 'static;
+
+    /// Updates the [`State`](Self::State) of the [`Program`].
     ///
     /// When a [`Program`] is used in a [`Canvas`], the runtime will call this
     /// method for each [`Event`].
@@ -23,7 +25,8 @@ pub trait Program<Message, Theme = iced_native::Theme> {
     ///
     /// [`Canvas`]: crate::widget::Canvas
     fn update(
-        &mut self,
+        &self,
+        _state: &mut Self::State,
         _event: Event,
         _bounds: Rectangle,
         _cursor: Cursor,
@@ -40,6 +43,7 @@ pub trait Program<Message, Theme = iced_native::Theme> {
     /// [`Cache`]: crate::widget::canvas::Cache
     fn draw(
         &self,
+        state: &Self::State,
         theme: &Theme,
         bounds: Rectangle,
         cursor: Cursor,
@@ -53,6 +57,7 @@ pub trait Program<Message, Theme = iced_native::Theme> {
     /// [`Canvas`]: crate::widget::Canvas
     fn mouse_interaction(
         &self,
+        _state: &Self::State,
         _bounds: Rectangle,
         _cursor: Cursor,
     ) -> mouse::Interaction {
@@ -60,33 +65,38 @@ pub trait Program<Message, Theme = iced_native::Theme> {
     }
 }
 
-impl<T, Message, Theme> Program<Message, Theme> for &mut T
+impl<Message, Theme, T> Program<Message, Theme> for &T
 where
     T: Program<Message, Theme>,
 {
+    type State = T::State;
+
     fn update(
-        &mut self,
+        &self,
+        state: &mut Self::State,
         event: Event,
         bounds: Rectangle,
         cursor: Cursor,
     ) -> (event::Status, Option<Message>) {
-        T::update(self, event, bounds, cursor)
+        T::update(self, state, event, bounds, cursor)
     }
 
     fn draw(
         &self,
+        state: &Self::State,
         theme: &Theme,
         bounds: Rectangle,
         cursor: Cursor,
     ) -> Vec<Geometry> {
-        T::draw(self, theme, bounds, cursor)
+        T::draw(self, state, theme, bounds, cursor)
     }
 
     fn mouse_interaction(
         &self,
+        state: &Self::State,
         bounds: Rectangle,
         cursor: Cursor,
     ) -> mouse::Interaction {
-        T::mouse_interaction(self, bounds, cursor)
+        T::mouse_interaction(self, state, bounds, cursor)
     }
 }