1 files changed, 108 insertions, 0 deletions
diff --git a/widget/src/canvas/program.rs b/widget/src/canvas/program.rs
new file mode 100644
index 00000000..b3f6175e
--- /dev/null
+++ b/widget/src/canvas/program.rs
@@ -0,0 +1,108 @@
+use crate::canvas::event::{self, Event};
+use crate::canvas::mouse;
+use crate::core::Rectangle;
+use crate::graphics::geometry;
+
+/// The state and logic of a [`Canvas`].
+///
+/// A [`Program`] can mutate internal state and produce messages for an
+/// application.
+///
+/// [`Canvas`]: crate::widget::Canvas
+pub trait Program<Message, Renderer = crate::Renderer>
+where
+    Renderer: geometry::Renderer,
+{
+    /// 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`].
+    ///
+    /// This method can optionally return a `Message` to notify an application
+    /// of any meaningful interactions.
+    ///
+    /// By default, this method does and returns nothing.
+    ///
+    /// [`Canvas`]: crate::widget::Canvas
+    fn update(
+        &self,
+        _state: &mut Self::State,
+        _event: Event,
+        _bounds: Rectangle,
+        _cursor: mouse::Cursor,
+    ) -> (event::Status, Option<Message>) {
+        (event::Status::Ignored, None)
+    }
+
+    /// Draws the state of the [`Program`], producing a bunch of [`Geometry`].
+    ///
+    /// [`Geometry`] can be easily generated with a [`Frame`] or stored in a
+    /// [`Cache`].
+    ///
+    /// [`Frame`]: crate::widget::canvas::Frame
+    /// [`Cache`]: crate::widget::canvas::Cache
+    fn draw(
+        &self,
+        state: &Self::State,
+        renderer: &Renderer,
+        theme: &Renderer::Theme,
+        bounds: Rectangle,
+        cursor: mouse::Cursor,
+    ) -> Vec<Renderer::Geometry>;
+
+    /// Returns the current mouse interaction of the [`Program`].
+    ///
+    /// The interaction returned will be in effect even if the cursor position
+    /// is out of bounds of the program's [`Canvas`].
+    ///
+    /// [`Canvas`]: crate::widget::Canvas
+    fn mouse_interaction(
+        &self,
+        _state: &Self::State,
+        _bounds: Rectangle,
+        _cursor: mouse::Cursor,
+    ) -> mouse::Interaction {
+        mouse::Interaction::default()
+    }
+}
+
+impl<Message, Renderer, T> Program<Message, Renderer> for &T
+where
+    Renderer: geometry::Renderer,
+    T: Program<Message, Renderer>,
+{
+    type State = T::State;
+
+    fn update(
+        &self,
+        state: &mut Self::State,
+        event: Event,
+        bounds: Rectangle,
+        cursor: mouse::Cursor,
+    ) -> (event::Status, Option<Message>) {
+        T::update(self, state, event, bounds, cursor)
+    }
+
+    fn draw(
+        &self,
+        state: &Self::State,
+        renderer: &Renderer,
+        theme: &Renderer::Theme,
+        bounds: Rectangle,
+        cursor: mouse::Cursor,
+    ) -> Vec<Renderer::Geometry> {
+        T::draw(self, state, renderer, theme, bounds, cursor)
+    }
+
+    fn mouse_interaction(
+        &self,
+        state: &Self::State,
+        bounds: Rectangle,
+        cursor: mouse::Cursor,
+    ) -> mouse::Interaction {
+        T::mouse_interaction(self, state, bounds, cursor)
+    }
+}