1 files changed, 85 insertions, 0 deletions
diff --git a/wgpu/src/widget/canvas/program.rs b/wgpu/src/widget/canvas/program.rs
new file mode 100644
index 00000000..725d9d72
--- /dev/null
+++ b/wgpu/src/widget/canvas/program.rs
@@ -0,0 +1,85 @@
+use crate::canvas::{Cursor, Event, Geometry};
+use iced_native::{mouse, Rectangle};
+
+/// The state and logic of a [`Canvas`].
+///
+/// A [`Program`] can mutate internal state and produce messages for an
+/// application.
+///
+/// [`Canvas`]: struct.Canvas.html
+/// [`Program`]: trait.Program.html
+pub trait Program<Message> {
+    /// Updates the 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.
+    ///
+    /// [`Program`]: trait.Program.html
+    /// [`Canvas`]: struct.Canvas.html
+    /// [`Event`]: enum.Event.html
+    fn update(
+        &mut self,
+        _event: Event,
+        _bounds: Rectangle,
+        _cursor: Cursor,
+    ) -> Option<Message> {
+        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`].
+    ///
+    /// [`Program`]: trait.Program.html
+    /// [`Geometry`]: struct.Geometry.html
+    /// [`Frame`]: struct.Frame.html
+    /// [`Cache`]: struct.Cache.html
+    fn draw(&self, bounds: Rectangle, cursor: Cursor) -> Vec<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`].
+    ///
+    /// [`Program`]: trait.Program.html
+    /// [`Canvas`]: struct.Canvas.html
+    fn mouse_interaction(
+        &self,
+        _bounds: Rectangle,
+        _cursor: Cursor,
+    ) -> mouse::Interaction {
+        mouse::Interaction::default()
+    }
+}
+
+impl<T, Message> Program<Message> for &mut T
+where
+    T: Program<Message>,
+{
+    fn update(
+        &mut self,
+        event: Event,
+        bounds: Rectangle,
+        cursor: Cursor,
+    ) -> Option<Message> {
+        T::update(self, event, bounds, cursor)
+    }
+
+    fn draw(&self, bounds: Rectangle, cursor: Cursor) -> Vec<Geometry> {
+        T::draw(self, bounds, cursor)
+    }
+
+    fn mouse_interaction(
+        &self,
+        bounds: Rectangle,
+        cursor: Cursor,
+    ) -> mouse::Interaction {
+        T::mouse_interaction(self, bounds, cursor)
+    }
+}