Implement composable, type-safe renderer fallback

core/src/renderer.rs

@@ -11,17 +11,41 @@ use crate::{
 
 /// A component that can be used by widgets to draw themselves on a screen.
 pub trait Renderer {
+    /// Starts recording a new layer.
+    fn start_layer(&mut self);
+
+    /// Ends recording a new layer.
+    ///
+    /// The new layer will clip its contents to the provided `bounds`.
+    fn end_layer(&mut self, bounds: Rectangle);
+
     /// Draws the primitives recorded in the given closure in a new layer.
     ///
     /// The layer will clip its contents to the provided `bounds`.
-    fn with_layer(&mut self, bounds: Rectangle, f: impl FnOnce(&mut Self));
+    fn with_layer(&mut self, bounds: Rectangle, f: impl FnOnce(&mut Self)) {
+        self.start_layer();
+        f(self);
+        self.end_layer(bounds);
+    }
+
+    /// Starts recording with a new [`Transformation`].
+    fn start_transformation(&mut self);
+
+    /// Ends recording a new layer.
+    ///
+    /// The new layer will clip its contents to the provided `bounds`.
+    fn end_transformation(&mut self, transformation: Transformation);
 
     /// Applies a [`Transformation`] to the primitives recorded in the given closure.
     fn with_transformation(
         &mut self,
         transformation: Transformation,
         f: impl FnOnce(&mut Self),
-    );
+    ) {
+        self.start_transformation();
+        f(self);
+        self.end_transformation(transformation);
+    }
 
     /// Applies a translation to the primitives recorded in the given closure.
     fn with_translation(

core/src/renderer/null.rs

@@ -21,14 +21,13 @@ impl Null {
 }
 
 impl Renderer for Null {
-    fn with_layer(&mut self, _bounds: Rectangle, _f: impl FnOnce(&mut Self)) {}
+    fn start_layer(&mut self) {}
 
-    fn with_transformation(
-        &mut self,
-        _transformation: Transformation,
-        _f: impl FnOnce(&mut Self),
-    ) {
-    }
+    fn end_layer(&mut self, _bounds: Rectangle) {}
+
+    fn start_transformation(&mut self) {}
+
+    fn end_transformation(&mut self, _transformation: Transformation) {}
 
     fn clear(&mut self) {}