Finish clock example

wgpu/src/widget/canvas/layer.rs

@@ -1,13 +1,28 @@
-use crate::canvas::Frame;
+use crate::{canvas::Frame, triangle};
 
-pub trait Layer: std::fmt::Debug {}
+use iced_native::Size;
+use std::cell::RefCell;
+use std::sync::Arc;
+
+pub trait Layer: std::fmt::Debug {
+    fn draw(&self, bounds: Size) -> Arc<triangle::Mesh2D>;
+}
 
 use std::marker::PhantomData;
-use std::sync::{Arc, Weak};
 
 #[derive(Debug)]
 pub struct Cached<T: Drawable> {
     input: PhantomData<T>,
+    cache: RefCell<Cache>,
+}
+
+#[derive(Debug)]
+enum Cache {
+    Empty,
+    Filled {
+        mesh: Arc<triangle::Mesh2D>,
+        bounds: Size,
+    },
 }
 
 impl<T> Cached<T>
@@ -15,14 +30,19 @@ where
     T: Drawable + std::fmt::Debug,
 {
     pub fn new() -> Self {
-        Cached { input: PhantomData }
+        Cached {
+            input: PhantomData,
+            cache: RefCell::new(Cache::Empty),
+        }
     }
 
-    pub fn clear(&mut self) {}
+    pub fn clear(&mut self) {
+        *self.cache.borrow_mut() = Cache::Empty;
+    }
 
     pub fn with<'a>(&'a self, input: &'a T) -> impl Layer + 'a {
         Bind {
-            cache: self,
+            layer: self,
             input: input,
         }
     }
@@ -30,11 +50,38 @@ where
 
 #[derive(Debug)]
 struct Bind<'a, T: Drawable> {
-    cache: &'a Cached<T>,
+    layer: &'a Cached<T>,
     input: &'a T,
 }
 
-impl<'a, T> Layer for Bind<'a, T> where T: Drawable + std::fmt::Debug {}
+impl<'a, T> Layer for Bind<'a, T>
+where
+    T: Drawable + std::fmt::Debug,
+{
+    fn draw(&self, current_bounds: Size) -> Arc<triangle::Mesh2D> {
+        use std::ops::Deref;
+
+        if let Cache::Filled { mesh, bounds } =
+            self.layer.cache.borrow().deref()
+        {
+            if *bounds == current_bounds {
+                return mesh.clone();
+            }
+        }
+
+        let mut frame = Frame::new(current_bounds.width, current_bounds.height);
+        self.input.draw(&mut frame);
+
+        let mesh = Arc::new(frame.into_mesh());
+
+        *self.layer.cache.borrow_mut() = Cache::Filled {
+            mesh: mesh.clone(),
+            bounds: current_bounds,
+        };
+
+        mesh
+    }
+}
 
 pub trait Drawable {
     fn draw(&self, frame: &mut Frame);