Reintroduce generic Message type for canvas::Program

As it is useful to make the `Message` completely free in many
implementations.

examples/bezier_tool/src/main.rs

@@ -104,9 +104,7 @@ mod bezier {
         curves: &'a [Curve],
     }
 
-    impl<'a> canvas::Program for Bezier<'a> {
-        type Message = Curve;
-
+    impl<'a> canvas::Program<Curve> for Bezier<'a> {
         fn update(
             &mut self,
             event: Event,

examples/clock/src/main.rs

@@ -76,9 +76,7 @@ impl Application for Clock {
     }
 }
 
-impl canvas::Program for Clock {
-    type Message = Message;
-
+impl<Message> canvas::Program<Message> for Clock {
     fn draw(&self, bounds: Rectangle, _cursor: Cursor) -> Vec<Geometry> {
         let clock = self.clock.draw(bounds.size(), |frame| {
             let center = frame.center();

examples/color_palette/src/main.rs

@@ -235,9 +235,7 @@ impl Theme {
     }
 }
 
-impl canvas::Program for Theme {
-    type Message = Message;
-
+impl<Message> canvas::Program<Message> for Theme {
     fn draw(&self, bounds: Rectangle, _cursor: Cursor) -> Vec<Geometry> {
         let theme = self.canvas_cache.draw(bounds.size(), |frame| {
             self.draw(frame);

examples/game_of_life/src/main.rs

@@ -328,9 +328,7 @@ mod grid {
         }
     }
 
-    impl<'a> canvas::Program for Grid {
-        type Message = Message;
-
+    impl<'a> canvas::Program<Message> for Grid {
         fn update(
             &mut self,
             event: Event,

examples/pure/game_of_life/src/main.rs

@@ -394,8 +394,7 @@ mod grid {
         }
     }
 
-    impl canvas::Program for Grid {
-        type Message = Message;
+    impl canvas::Program<Message> for Grid {
         type State = Interaction;
 
         fn update(

examples/solar_system/src/main.rs

@@ -128,9 +128,7 @@ impl State {
     }
 }
 
-impl canvas::Program for State {
-    type Message = Message;
-
+impl<Message> canvas::Program<Message> for State {
     fn draw(
         &self,
         bounds: Rectangle,

graphics/src/widget/canvas.rs

@@ -6,13 +6,6 @@
 use crate::renderer::{self, Renderer};
 use crate::{Backend, Primitive};
 
-use iced_native::layout;
-use iced_native::mouse;
-use iced_native::{
-    Clipboard, Element, Layout, Length, Point, Rectangle, Shell, Size, Vector,
-    Widget,
-};
-
 pub mod event;
 pub mod path;
 
@@ -36,6 +29,15 @@ pub use program::Program;
 pub use stroke::{LineCap, LineDash, LineJoin, Stroke};
 pub use text::Text;
 
+use iced_native::layout;
+use iced_native::mouse;
+use iced_native::{
+    Clipboard, Element, Layout, Length, Point, Rectangle, Shell, Size, Vector,
+    Widget,
+};
+
+use std::marker::PhantomData;
+
 /// A widget capable of drawing 2D graphics.
 ///
 /// # Examples
@@ -72,9 +74,7 @@ pub use text::Text;
 /// }
 ///
 /// // Then, we implement the `Program` trait
-/// impl Program for Circle {
-///     type Message = ();
-///
+/// impl Program<()> for Circle {
 ///     fn draw(&self, bounds: Rectangle, _cursor: Cursor) -> Vec<Geometry>{
 ///         // We prepare a new `Frame`
 ///         let mut frame = Frame::new(bounds.size());
@@ -94,13 +94,14 @@ pub use text::Text;
 /// let canvas = Canvas::new(Circle { radius: 50.0 });
 /// ```
 #[derive(Debug)]
-pub struct Canvas<P: Program> {
+pub struct Canvas<Message, P: Program<Message>> {
     width: Length,
     height: Length,
     program: P,
+    message_: PhantomData<Message>,
 }
 
-impl<P: Program> Canvas<P> {
+impl<Message, P: Program<Message>> Canvas<Message, P> {
     const DEFAULT_SIZE: u16 = 100;
 
     /// Creates a new [`Canvas`].
@@ -109,6 +110,7 @@ impl<P: Program> Canvas<P> {
             width: Length::Units(Self::DEFAULT_SIZE),
             height: Length::Units(Self::DEFAULT_SIZE),
             program,
+            message_: PhantomData,
         }
     }
 
@@ -125,9 +127,9 @@ impl<P: Program> Canvas<P> {
     }
 }
 
-impl<P, B> Widget<P::Message, Renderer<B>> for Canvas<P>
+impl<Message, P, B> Widget<Message, Renderer<B>> for Canvas<Message, P>
 where
-    P: Program,
+    P: Program<Message>,
     B: Backend,
 {
     fn width(&self) -> Length {
@@ -156,7 +158,7 @@ where
         cursor_position: Point,
         _renderer: &Renderer<B>,
         _clipboard: &mut dyn Clipboard,
-        shell: &mut Shell<'_, P::Message>,
+        shell: &mut Shell<'_, Message>,
     ) -> event::Status {
         let bounds = layout.bounds();
 
@@ -231,13 +233,14 @@ where
     }
 }
 
-impl<'a, P, B> From<Canvas<P>> for Element<'a, P::Message, Renderer<B>>
+impl<'a, Message, P, B> From<Canvas<Message, P>>
+    for Element<'a, Message, Renderer<B>>
 where
-    P::Message: 'static,
-    P: Program + 'a,
+    Message: 'static,
+    P: Program<Message> + 'a,
     B: Backend,
 {
-    fn from(canvas: Canvas<P>) -> Element<'a, P::Message, Renderer<B>> {
+    fn from(canvas: Canvas<Message, P>) -> Element<'a, Message, Renderer<B>> {
         Element::new(canvas)
     }
 }

graphics/src/widget/canvas/program.rs

@@ -8,10 +8,7 @@ use iced_native::{mouse, Rectangle};
 /// application.
 ///
 /// [`Canvas`]: crate::widget::Canvas
-pub trait Program {
-    /// The [`Message`] produced by the [`Program`].
-    type Message;
-
+pub trait Program<Message> {
     /// Updates the state of the [`Program`].
     ///
     /// When a [`Program`] is used in a [`Canvas`], the runtime will call this
@@ -28,7 +25,7 @@ pub trait Program {
         _event: Event,
         _bounds: Rectangle,
         _cursor: Cursor,
-    ) -> (event::Status, Option<Self::Message>) {
+    ) -> (event::Status, Option<Message>) {
         (event::Status::Ignored, None)
     }
 
@@ -56,18 +53,16 @@ pub trait Program {
     }
 }
 
-impl<T> Program for &mut T
+impl<T, Message> Program<Message> for &mut T
 where
-    T: Program,
+    T: Program<Message>,
 {
-    type Message = T::Message;
-
     fn update(
         &mut self,
         event: Event,
         bounds: Rectangle,
         cursor: Cursor,
-    ) -> (event::Status, Option<Self::Message>) {
+    ) -> (event::Status, Option<Message>) {
         T::update(self, event, bounds, cursor)
     }
 

graphics/src/widget/pure/canvas.rs

@@ -18,6 +18,8 @@ use iced_native::{Clipboard, Length, Point, Rectangle, Shell, Size, Vector};
 use iced_pure::widget::tree::{self, Tree};
 use iced_pure::{Element, Widget};
 
+use std::marker::PhantomData;
+
 /// A widget capable of drawing 2D graphics.
 ///
 /// ## Drawing a simple circle
@@ -40,8 +42,7 @@ use iced_pure::{Element, Widget};
 /// }
 ///
 /// // Then, we implement the `Program` trait
-/// impl Program for Circle {
-///     type Message = ();
+/// impl Program<()> for Circle {
 ///     type State = ();
 ///
 ///     fn draw(&self, _state: &(), bounds: Rectangle, _cursor: Cursor) -> Vec<Geometry>{
@@ -63,18 +64,19 @@ use iced_pure::{Element, Widget};
 /// let canvas = Canvas::new(Circle { radius: 50.0 });
 /// ```
 #[derive(Debug)]
-pub struct Canvas<P>
+pub struct Canvas<Message, P>
 where
-    P: Program,
+    P: Program<Message>,
 {
     width: Length,
     height: Length,
     program: P,
+    message_: PhantomData<Message>,
 }
 
-impl<P> Canvas<P>
+impl<Message, P> Canvas<Message, P>
 where
-    P: Program,
+    P: Program<Message>,
 {
     const DEFAULT_SIZE: u16 = 100;
 
@@ -84,6 +86,7 @@ where
             width: Length::Units(Self::DEFAULT_SIZE),
             height: Length::Units(Self::DEFAULT_SIZE),
             program,
+            message_: PhantomData,
         }
     }
 
@@ -100,9 +103,9 @@ where
     }
 }
 
-impl<P, B> Widget<P::Message, Renderer<B>> for Canvas<P>
+impl<Message, P, B> Widget<Message, Renderer<B>> for Canvas<Message, P>
 where
-    P: Program,
+    P: Program<Message>,
     B: Backend,
 {
     fn tag(&self) -> tree::Tag {
@@ -140,7 +143,7 @@ where
         cursor_position: Point,
         _renderer: &Renderer<B>,
         _clipboard: &mut dyn Clipboard,
-        shell: &mut Shell<'_, P::Message>,
+        shell: &mut Shell<'_, Message>,
     ) -> event::Status {
         let bounds = layout.bounds();
 
@@ -221,13 +224,14 @@ where
     }
 }
 
-impl<'a, P, B> From<Canvas<P>> for Element<'a, P::Message, Renderer<B>>
+impl<'a, Message, P, B> From<Canvas<Message, P>>
+    for Element<'a, Message, Renderer<B>>
 where
-    P::Message: 'static,
-    P: Program + 'a,
+    Message: 'a,
+    P: Program<Message> + 'a,
     B: Backend,
 {
-    fn from(canvas: Canvas<P>) -> Element<'a, P::Message, Renderer<B>> {
+    fn from(canvas: Canvas<Message, P>) -> Element<'a, Message, Renderer<B>> {
         Element::new(canvas)
     }
 }

graphics/src/widget/pure/canvas/program.rs

@@ -9,10 +9,7 @@ use crate::Rectangle;
 /// application.
 ///
 /// [`Canvas`]: crate::widget::Canvas
-pub trait Program {
-    /// The [`Message`] produced by the [`Program`].
-    type Message;
-
+pub trait Program<Message> {
     /// The internal [`State`] mutated by the [`Program`].
     type State: Default + 'static;
 
@@ -33,7 +30,7 @@ pub trait Program {
         _event: Event,
         _bounds: Rectangle,
         _cursor: Cursor,
-    ) -> (event::Status, Option<Self::Message>) {
+    ) -> (event::Status, Option<Message>) {
         (event::Status::Ignored, None)
     }
 
@@ -67,11 +64,10 @@ pub trait Program {
     }
 }
 
-impl<T> Program for &T
+impl<Message, T> Program<Message> for &T
 where
-    T: Program,
+    T: Program<Message>,
 {
-    type Message = T::Message;
     type State = T::State;
 
     fn update(
@@ -80,7 +76,7 @@ where
         event: Event,
         bounds: Rectangle,
         cursor: Cursor,
-    ) -> (event::Status, Option<Self::Message>) {
+    ) -> (event::Status, Option<Message>) {
         T::update(self, state, event, bounds, cursor)
     }