Remove Layer trait and simplify Canvas

wgpu/src/widget/canvas/cache.rs

@@ -0,0 +1,110 @@
+use crate::{
+    canvas::{Drawable, Frame, Geometry},
+    Primitive,
+};
+
+use iced_native::Size;
+use std::{cell::RefCell, sync::Arc};
+
+enum State {
+    Empty,
+    Filled {
+        bounds: Size,
+        primitive: Arc<Primitive>,
+    },
+}
+
+impl Default for State {
+    fn default() -> Self {
+        State::Empty
+    }
+}
+/// A simple cache that stores generated geometry to avoid recomputation.
+///
+/// A [`Cache`] will not redraw its geometry unless the dimensions of its layer
+/// change or it is explicitly cleared.
+///
+/// [`Layer`]: ../trait.Layer.html
+/// [`Cache`]: struct.Cache.html
+#[derive(Debug, Default)]
+pub struct Cache {
+    state: RefCell<State>,
+}
+
+impl Cache {
+    /// Creates a new empty [`Cache`].
+    ///
+    /// [`Cache`]: struct.Cache.html
+    pub fn new() -> Self {
+        Cache {
+            state: Default::default(),
+        }
+    }
+
+    /// Clears the cache, forcing a redraw the next time it is used.
+    ///
+    /// [`Cached`]: struct.Cached.html
+    pub fn clear(&mut self) {
+        *self.state.borrow_mut() = State::Empty;
+    }
+
+    pub fn draw<T>(&self, new_bounds: Size, input: T) -> Geometry
+    where
+        T: Drawable + std::fmt::Debug,
+    {
+        use std::ops::Deref;
+
+        if let State::Filled { bounds, primitive } = self.state.borrow().deref()
+        {
+            if *bounds == new_bounds {
+                return Geometry::from_primitive(primitive.clone());
+            }
+        }
+
+        let mut frame = Frame::new(new_bounds.width, new_bounds.height);
+        input.draw(&mut frame);
+
+        let primitive = Arc::new(frame.into_primitive());
+
+        *self.state.borrow_mut() = State::Filled {
+            bounds: new_bounds,
+            primitive: primitive.clone(),
+        };
+
+        Geometry::from_primitive(primitive)
+    }
+
+    pub fn with<'a, T>(&'a self, input: T) -> impl crate::canvas::State + 'a
+    where
+        T: Drawable + std::fmt::Debug + 'a,
+    {
+        Bind { cache: self, input }
+    }
+}
+
+struct Bind<'a, T> {
+    cache: &'a Cache,
+    input: T,
+}
+
+impl<'a, T> crate::canvas::State for Bind<'a, T>
+where
+    T: Drawable + std::fmt::Debug + 'a,
+{
+    fn draw(&self, bounds: Size) -> Vec<Geometry> {
+        vec![self.cache.draw(bounds, &self.input)]
+    }
+}
+
+impl std::fmt::Debug for State {
+    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
+        match self {
+            State::Empty => write!(f, "Empty"),
+            State::Filled { primitive, bounds } => f
+                .debug_struct("Filled")
+                .field("primitive", primitive)
+                .field("bounds", bounds)
+                .finish(),
+        }
+    }
+}