Draft Canvas types and clock example

wgpu/src/widget/canvas/layer.rs

@@ -0,0 +1,41 @@
+use crate::canvas::Frame;
+
+pub trait Layer: std::fmt::Debug {}
+
+use std::marker::PhantomData;
+use std::sync::{Arc, Weak};
+
+#[derive(Debug)]
+pub struct Cached<T: Drawable> {
+    input: PhantomData<T>,
+}
+
+impl<T> Cached<T>
+where
+    T: Drawable + std::fmt::Debug,
+{
+    pub fn new() -> Self {
+        Cached { input: PhantomData }
+    }
+
+    pub fn clear(&mut self) {}
+
+    pub fn with<'a>(&'a self, input: &'a T) -> impl Layer + 'a {
+        Bind {
+            cache: self,
+            input: input,
+        }
+    }
+}
+
+#[derive(Debug)]
+struct Bind<'a, T: Drawable> {
+    cache: &'a Cached<T>,
+    input: &'a T,
+}
+
+impl<'a, T> Layer for Bind<'a, T> where T: Drawable + std::fmt::Debug {}
+
+pub trait Drawable {
+    fn draw(&self, frame: &mut Frame);
+}