From 0d05ea56d3abbc58d2269369955f3fca18023706 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?H=C3=A9ctor=20Ram=C3=B3n=20Jim=C3=A9nez?= <hector@hecrj.dev>
Date: Sat, 6 Sep 2025 21:17:38 +0200
Subject: [PATCH] Introduce lightweight `draw` for `wgpu::Primitive`

---
 wgpu/src/lib.rs       | 60 ++++++++++++++++++++++++++---------------
 wgpu/src/primitive.rs | 62 ++++++++++++++++++++++++++++++++++++++-----
 2 files changed, 95 insertions(+), 27 deletions(-)

diff --git a/wgpu/src/lib.rs b/wgpu/src/lib.rs
index 0ff74524..db68748a 100644
--- a/wgpu/src/lib.rs
+++ b/wgpu/src/lib.rs
@@ -534,7 +534,6 @@ impl Renderer {
 
             if !layer.primitives.is_empty() {
                 let render_span = debug::render(debug::Primitive::Shader);
-                let _ = ManuallyDrop::into_inner(render_pass);
 
                 let primitive_storage = self
                     .engine
@@ -542,11 +541,30 @@ impl Renderer {
                     .read()
                     .expect("Read primitive storage");
 
+                let mut need_render = Vec::new();
+
                 for instance in &layer.primitives {
                     if let Some(clip_bounds) = (instance.bounds * scale)
                         .intersection(&physical_bounds)
                         .and_then(Rectangle::snap)
                     {
+                        let drawn = instance.primitive.draw(
+                            &primitive_storage,
+                            &mut render_pass,
+                            frame,
+                            &clip_bounds,
+                        );
+
+                        if !drawn {
+                            need_render.push((instance, clip_bounds));
+                        }
+                    }
+                }
+
+                if !need_render.is_empty() {
+                    let _ = ManuallyDrop::into_inner(render_pass);
+
+                    for (instance, clip_bounds) in need_render {
                         instance.primitive.render(
                             &primitive_storage,
                             encoder,
@@ -554,29 +572,29 @@ impl Renderer {
                             &clip_bounds,
                         );
                     }
+
+                    render_pass = ManuallyDrop::new(encoder.begin_render_pass(
+                        &wgpu::RenderPassDescriptor {
+                            label: Some("iced_wgpu render pass"),
+                            color_attachments: &[Some(
+                                wgpu::RenderPassColorAttachment {
+                                    view: frame,
+                                    depth_slice: None,
+                                    resolve_target: None,
+                                    ops: wgpu::Operations {
+                                        load: wgpu::LoadOp::Load,
+                                        store: wgpu::StoreOp::Store,
+                                    },
+                                },
+                            )],
+                            depth_stencil_attachment: None,
+                            timestamp_writes: None,
+                            occlusion_query_set: None,
+                        },
+                    ));
                 }
 
                 render_span.finish();
-
-                render_pass = ManuallyDrop::new(encoder.begin_render_pass(
-                    &wgpu::RenderPassDescriptor {
-                        label: Some("iced_wgpu render pass"),
-                        color_attachments: &[Some(
-                            wgpu::RenderPassColorAttachment {
-                                view: frame,
-                                depth_slice: None,
-                                resolve_target: None,
-                                ops: wgpu::Operations {
-                                    load: wgpu::LoadOp::Load,
-                                    store: wgpu::StoreOp::Store,
-                                },
-                            },
-                        )],
-                        depth_stencil_attachment: None,
-                        timestamp_writes: None,
-                        occlusion_query_set: None,
-                    },
-                ));
             }
 
             #[cfg(any(feature = "svg", feature = "image"))]
diff --git a/wgpu/src/primitive.rs b/wgpu/src/primitive.rs
index eef7cfe9..49081b60 100644
--- a/wgpu/src/primitive.rs
+++ b/wgpu/src/primitive.rs
@@ -42,14 +42,39 @@ pub trait Primitive: Debug + MaybeSend + MaybeSync + 'static {
         viewport: &Viewport,
     );
 
-    /// Renders the [`Primitive`].
+    /// Draws the [`Primitive`] in the given [`wgpu::RenderPass`].
+    ///
+    /// When possible, this should be implement over [`render`](Self::render)
+    /// since reusing the existing render pass should be considerably more
+    /// efficient than issuing a completely new one.
+    ///
+    /// If you have complex composition needs, then you can leverage
+    /// [`render`](Self::render) by returning `false` here.
+    ///
+    /// By default, it does nothing and returns `false`.
+    fn draw(
+        &self,
+        _renderer: &Self::Renderer,
+        _render_pass: &mut wgpu::RenderPass<'_>,
+        _target: &wgpu::TextureView,
+        _clip_bounds: &Rectangle<u32>,
+    ) -> bool {
+        false
+    }
+
+    /// Renders the [`Primitive`], using the given [`wgpu::CommandEncoder`].
+    ///
+    /// This will only be called if [`draw`](Self::draw) returns `false`.
+    ///
+    /// By default, it does nothing.
     fn render(
         &self,
-        renderer: &Self::Renderer,
-        encoder: &mut wgpu::CommandEncoder,
-        target: &wgpu::TextureView,
-        clip_bounds: &Rectangle<u32>,
-    );
+        _renderer: &Self::Renderer,
+        _encoder: &mut wgpu::CommandEncoder,
+        _target: &wgpu::TextureView,
+        _clip_bounds: &Rectangle<u32>,
+    ) {
+    }
 }
 
 pub(crate) trait Stored:
@@ -65,6 +90,14 @@ pub(crate) trait Stored:
         viewport: &Viewport,
     );
 
+    fn draw(
+        &self,
+        storage: &Storage,
+        render_pass: &mut wgpu::RenderPass<'_>,
+        target: &wgpu::TextureView,
+        clip_bounds: &Rectangle<u32>,
+    ) -> bool;
+
     fn render(
         &self,
         storage: &Storage,
@@ -105,6 +138,23 @@ impl<P: Primitive> Stored for BlackBox<P> {
             .prepare(renderer, device, queue, bounds, viewport);
     }
 
+    fn draw(
+        &self,
+        storage: &Storage,
+        render_pass: &mut wgpu::RenderPass<'_>,
+        target: &wgpu::TextureView,
+        clip_bounds: &Rectangle<u32>,
+    ) -> bool {
+        let renderer = storage
+            .get::<P>()
+            .expect("renderer should be initialized")
+            .downcast_ref::<P::Renderer>()
+            .expect("renderer should have the proper type");
+
+        self.primitive
+            .draw(renderer, render_pass, target, clip_bounds)
+    }
+
     fn render(
         &self,
         storage: &Storage,