Remove single window assumption from SceneBuilder (flutter#41559)

This PR fixes flutter#112202. `Scene`
is no longer tied to views, but receives method arguments for view
properties, clearing the path to multiview Flutter.

`Scene` no longer creates a `LayerTree` on construction, but only part
of the config. When a output method is called (`toImage`, `toImageSync`,
or `takeLayerTree`), the `LayerTree` is constructed on the spot. In this
way, all `LayerTree`s return to being `unique_ptr`s instead of
`shared_ptr`s, reverting part of
flutter/engine#35608 . And `Scene` no longer
needs to disfunction after one `takeLayerTree`, since the layer tree
config is never really taken away.

A `device_pixel_ratio` is now added to the parameter list of `Engine::Render` and `Animator::Render`.

## Pre-launch Checklist

- [x] I read the [Contributor Guide] and followed the process outlined
there for submitting PRs.
- [x] I read the [Tree Hygiene] wiki page, which explains my
responsibilities.
- [x] I read and followed the [Flutter Style Guide] and the [C++,
Objective-C, Java style guides].
- [x] I listed at least one issue that this PR fixes in the description
above.
- [ ] I added new tests to check the change I am making or feature I am
adding, or Hixie said the PR is test-exempt. See [testing the engine]
for instructions on writing and running engine tests.
- [x] I updated/added relevant documentation (doc comments with `///`).
- [x] I signed the [CLA].
- [x] All existing and new tests are passing.

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on [Discord].

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Author: dkwingsmt

flow/layers/layer_tree.cc

@@ -20,14 +20,13 @@
 
 namespace flutter {
 
-LayerTree::LayerTree(const SkISize& frame_size, float device_pixel_ratio)
-    : frame_size_(frame_size),
-      device_pixel_ratio_(device_pixel_ratio),
-      rasterizer_tracing_threshold_(0),
-      checkerboard_raster_cache_images_(false),
-      checkerboard_offscreen_layers_(false) {
-  FML_CHECK(device_pixel_ratio_ != 0.0f);
-}
+LayerTree::LayerTree(const Config& config, const SkISize& frame_size)
+    : root_layer_(config.root_layer),
+      frame_size_(frame_size),
+      rasterizer_tracing_threshold_(config.rasterizer_tracing_threshold),
+      checkerboard_raster_cache_images_(
+          config.checkerboard_raster_cache_images),
+      checkerboard_offscreen_layers_(config.checkerboard_offscreen_layers) {}
 
 inline SkColorSpace* GetColorSpace(DlCanvas* canvas) {
   return canvas ? canvas->GetImageInfo().colorSpace() : nullptr;

flow/layers/layer_tree.h

@@ -23,7 +23,14 @@ namespace flutter {
 
 class LayerTree {
  public:
-  LayerTree(const SkISize& frame_size, float device_pixel_ratio);
+  struct Config {
+    std::shared_ptr<Layer> root_layer;
+    uint32_t rasterizer_tracing_threshold = 0;
+    bool checkerboard_raster_cache_images = false;
+    bool checkerboard_offscreen_layers = false;
+  };
+
+  LayerTree(const Config& config, const SkISize& frame_size);
 
   // Perform a preroll pass on the tree and return information about
   // the tree that affects rendering this frame.
@@ -50,36 +57,18 @@ class LayerTree {
       GrDirectContext* gr_context = nullptr);
 
   Layer* root_layer() const { return root_layer_.get(); }
-
-  void set_root_layer(std::shared_ptr<Layer> root_layer) {
-    root_layer_ = std::move(root_layer);
-  }
-
   const SkISize& frame_size() const { return frame_size_; }
-  float device_pixel_ratio() const { return device_pixel_ratio_; }
 
   const PaintRegionMap& paint_region_map() const { return paint_region_map_; }
   PaintRegionMap& paint_region_map() { return paint_region_map_; }
 
   // The number of frame intervals missed after which the compositor must
-  // trace the rasterized picture to a trace file. Specify 0 to disable all
-  // tracing
-  void set_rasterizer_tracing_threshold(uint32_t interval) {
-    rasterizer_tracing_threshold_ = interval;
-  }
-
+  // trace the rasterized picture to a trace file. 0 stands for disabling all
+  // tracing.
   uint32_t rasterizer_tracing_threshold() const {
     return rasterizer_tracing_threshold_;
   }
 
-  void set_checkerboard_raster_cache_images(bool checkerboard) {
-    checkerboard_raster_cache_images_ = checkerboard;
-  }
-
-  void set_checkerboard_offscreen_layers(bool checkerboard) {
-    checkerboard_offscreen_layers_ = checkerboard;
-  }
-
   /// When `Paint` is called, if leaf layer tracing is enabled, additional
   /// metadata around raterization of leaf layers is collected.
   ///
@@ -95,7 +84,6 @@ class LayerTree {
  private:
   std::shared_ptr<Layer> root_layer_;
   SkISize frame_size_ = SkISize::MakeEmpty();  // Physical pixels.
-  const float device_pixel_ratio_;  // Logical / Physical pixels ratio.
   uint32_t rasterizer_tracing_threshold_;
   bool checkerboard_raster_cache_images_;
   bool checkerboard_offscreen_layers_;

flow/layers/layer_tree_unittests.cc

@@ -19,8 +19,7 @@ namespace testing {
 class LayerTreeTest : public CanvasTest {
  public:
   LayerTreeTest()
-      : layer_tree_(SkISize::Make(64, 64), 1.0f),
-        root_transform_(SkMatrix::Translate(1.0f, 1.0f)),
+      : root_transform_(SkMatrix::Translate(1.0f, 1.0f)),
         scoped_frame_(compositor_context_.AcquireFrame(nullptr,
                                                        &mock_canvas(),
                                                        nullptr,
@@ -31,12 +30,14 @@ class LayerTreeTest : public CanvasTest {
                                                        nullptr,
                                                        nullptr)) {}
 
-  LayerTree& layer_tree() { return layer_tree_; }
   CompositorContext::ScopedFrame& frame() { return *scoped_frame_.get(); }
   const SkMatrix& root_transform() { return root_transform_; }
 
+  std::unique_ptr<LayerTree> BuildLayerTree(const LayerTree::Config& config) {
+    return std::make_unique<LayerTree>(config, SkISize::Make(64, 64));
+  }
+
  private:
-  LayerTree layer_tree_;
   CompositorContext compositor_context_;
   SkMatrix root_transform_;
   std::unique_ptr<CompositorContext::ScopedFrame> scoped_frame_;
@@ -45,12 +46,14 @@ class LayerTreeTest : public CanvasTest {
 TEST_F(LayerTreeTest, PaintingEmptyLayerDies) {
   auto layer = std::make_shared<ContainerLayer>();
 
-  layer_tree().set_root_layer(layer);
-  layer_tree().Preroll(frame());
+  auto layer_tree = BuildLayerTree(LayerTree::Config{
+      .root_layer = layer,
+  });
+  layer_tree->Preroll(frame());
   EXPECT_EQ(layer->paint_bounds(), SkRect::MakeEmpty());
   EXPECT_TRUE(layer->is_empty());
 
-  layer_tree().Paint(frame());
+  layer_tree->Paint(frame());
 }
 
 TEST_F(LayerTreeTest, PaintBeforePrerollDies) {
@@ -61,13 +64,15 @@ TEST_F(LayerTreeTest, PaintBeforePrerollDies) {
   auto layer = std::make_shared<ContainerLayer>();
   layer->Add(mock_layer);
 
-  layer_tree().set_root_layer(layer);
+  auto layer_tree = BuildLayerTree(LayerTree::Config{
+      .root_layer = layer,
+  });
   EXPECT_EQ(mock_layer->paint_bounds(), kEmptyRect);
   EXPECT_EQ(layer->paint_bounds(), kEmptyRect);
   EXPECT_TRUE(mock_layer->is_empty());
   EXPECT_TRUE(layer->is_empty());
 
-  layer_tree().Paint(frame());
+  layer_tree->Paint(frame());
   EXPECT_EQ(mock_canvas().draw_calls(), std::vector<MockCanvas::DrawCall>());
 }
 
@@ -79,15 +84,17 @@ TEST_F(LayerTreeTest, Simple) {
   auto layer = std::make_shared<ContainerLayer>();
   layer->Add(mock_layer);
 
-  layer_tree().set_root_layer(layer);
-  layer_tree().Preroll(frame());
+  auto layer_tree = BuildLayerTree(LayerTree::Config{
+      .root_layer = layer,
+  });
+  layer_tree->Preroll(frame());
   EXPECT_EQ(mock_layer->paint_bounds(), child_bounds);
   EXPECT_EQ(layer->paint_bounds(), mock_layer->paint_bounds());
   EXPECT_FALSE(mock_layer->is_empty());
   EXPECT_FALSE(layer->is_empty());
   EXPECT_EQ(mock_layer->parent_matrix(), root_transform());
 
-  layer_tree().Paint(frame());
+  layer_tree->Paint(frame());
   EXPECT_EQ(mock_canvas().draw_calls(),
             std::vector({MockCanvas::DrawCall{
                 0, MockCanvas::DrawPathData{child_path, child_paint}}}));
@@ -107,8 +114,10 @@ TEST_F(LayerTreeTest, Multiple) {
 
   SkRect expected_total_bounds = child_path1.getBounds();
   expected_total_bounds.join(child_path2.getBounds());
-  layer_tree().set_root_layer(layer);
-  layer_tree().Preroll(frame());
+  auto layer_tree = BuildLayerTree(LayerTree::Config{
+      .root_layer = layer,
+  });
+  layer_tree->Preroll(frame());
   EXPECT_EQ(mock_layer1->paint_bounds(), child_path1.getBounds());
   EXPECT_EQ(mock_layer2->paint_bounds(), child_path2.getBounds());
   EXPECT_EQ(layer->paint_bounds(), expected_total_bounds);
@@ -121,7 +130,7 @@ TEST_F(LayerTreeTest, Multiple) {
   EXPECT_EQ(mock_layer2->parent_cull_rect(),
             kGiantRect);  // Siblings are independent
 
-  layer_tree().Paint(frame());
+  layer_tree->Paint(frame());
   EXPECT_EQ(
       mock_canvas().draw_calls(),
       std::vector({MockCanvas::DrawCall{
@@ -140,8 +149,10 @@ TEST_F(LayerTreeTest, MultipleWithEmpty) {
   layer->Add(mock_layer1);
   layer->Add(mock_layer2);
 
-  layer_tree().set_root_layer(layer);
-  layer_tree().Preroll(frame());
+  auto layer_tree = BuildLayerTree(LayerTree::Config{
+      .root_layer = layer,
+  });
+  layer_tree->Preroll(frame());
   EXPECT_EQ(mock_layer1->paint_bounds(), child_path1.getBounds());
   EXPECT_EQ(mock_layer2->paint_bounds(), SkPath().getBounds());
   EXPECT_EQ(layer->paint_bounds(), child_path1.getBounds());
@@ -153,7 +164,7 @@ TEST_F(LayerTreeTest, MultipleWithEmpty) {
   EXPECT_EQ(mock_layer1->parent_cull_rect(), kGiantRect);
   EXPECT_EQ(mock_layer2->parent_cull_rect(), kGiantRect);
 
-  layer_tree().Paint(frame());
+  layer_tree->Paint(frame());
   EXPECT_EQ(mock_canvas().draw_calls(),
             std::vector({MockCanvas::DrawCall{
                 0, MockCanvas::DrawPathData{child_path1, child_paint1}}}));
@@ -172,8 +183,10 @@ TEST_F(LayerTreeTest, NeedsSystemComposite) {
 
   SkRect expected_total_bounds = child_path1.getBounds();
   expected_total_bounds.join(child_path2.getBounds());
-  layer_tree().set_root_layer(layer);
-  layer_tree().Preroll(frame());
+  auto layer_tree = BuildLayerTree(LayerTree::Config{
+      .root_layer = layer,
+  });
+  layer_tree->Preroll(frame());
   EXPECT_EQ(mock_layer1->paint_bounds(), child_path1.getBounds());
   EXPECT_EQ(mock_layer2->paint_bounds(), child_path2.getBounds());
   EXPECT_EQ(layer->paint_bounds(), expected_total_bounds);
@@ -185,7 +198,7 @@ TEST_F(LayerTreeTest, NeedsSystemComposite) {
   EXPECT_EQ(mock_layer1->parent_cull_rect(), kGiantRect);
   EXPECT_EQ(mock_layer2->parent_cull_rect(), kGiantRect);
 
-  layer_tree().Paint(frame());
+  layer_tree->Paint(frame());
   EXPECT_EQ(
       mock_canvas().draw_calls(),
       std::vector({MockCanvas::DrawCall{

lib/ui/compositing/scene.cc

@@ -40,27 +40,22 @@ Scene::Scene(std::shared_ptr<flutter::Layer> rootLayer,
              uint32_t rasterizerTracingThreshold,
              bool checkerboardRasterCacheImages,
              bool checkerboardOffscreenLayers) {
-  // Currently only supports a single window.
-  auto viewport_metrics = UIDartState::Current()
-                              ->platform_configuration()
-                              ->get_window(0)
-                              ->viewport_metrics();
-
-  layer_tree_ = std::make_shared<LayerTree>(
-      SkISize::Make(viewport_metrics.physical_width,
-                    viewport_metrics.physical_height),
-      static_cast<float>(viewport_metrics.device_pixel_ratio));
-  layer_tree_->set_root_layer(std::move(rootLayer));
-  layer_tree_->set_rasterizer_tracing_threshold(rasterizerTracingThreshold);
-  layer_tree_->set_checkerboard_raster_cache_images(
-      checkerboardRasterCacheImages);
-  layer_tree_->set_checkerboard_offscreen_layers(checkerboardOffscreenLayers);
+  layer_tree_config_.root_layer = std::move(rootLayer);
+  layer_tree_config_.rasterizer_tracing_threshold = rasterizerTracingThreshold;
+  layer_tree_config_.checkerboard_raster_cache_images =
+      checkerboardRasterCacheImages;
+  layer_tree_config_.checkerboard_offscreen_layers =
+      checkerboardOffscreenLayers;
 }
 
 Scene::~Scene() {}
 
+bool Scene::valid() {
+  return layer_tree_config_.root_layer != nullptr;
+}
+
 void Scene::dispose() {
-  layer_tree_.reset();
+  layer_tree_config_.root_layer.reset();
   ClearDartWrapper();
 }
 
@@ -69,8 +64,8 @@ Dart_Handle Scene::toImageSync(uint32_t width,
                                Dart_Handle raw_image_handle) {
   TRACE_EVENT0("flutter", "Scene::toImageSync");
 
-  if (!layer_tree_) {
-    return tonic::ToDart("Scene did not contain a layer tree.");
+  if (!valid()) {
+    return tonic::ToDart("Scene has been disposed.");
   }
 
   Scene::RasterizeToImage(width, height, raw_image_handle);
@@ -82,32 +77,32 @@ Dart_Handle Scene::toImage(uint32_t width,
                            Dart_Handle raw_image_callback) {
   TRACE_EVENT0("flutter", "Scene::toImage");
 
-  if (!layer_tree_) {
-    return tonic::ToDart("Scene did not contain a layer tree.");
+  if (!valid()) {
+    return tonic::ToDart("Scene has been disposed.");
   }
 
-  return Picture::RasterizeLayerTreeToImage(std::move(layer_tree_), width,
-                                            height, raw_image_callback);
+  return Picture::RasterizeLayerTreeToImage(BuildLayerTree(width, height),
+                                            raw_image_callback);
 }
 
 static sk_sp<DlImage> CreateDeferredImage(
     bool impeller,
-    std::shared_ptr<LayerTree> layer_tree,
-    uint32_t width,
-    uint32_t height,
+    std::unique_ptr<LayerTree> layer_tree,
     fml::TaskRunnerAffineWeakPtr<SnapshotDelegate> snapshot_delegate,
     fml::RefPtr<fml::TaskRunner> raster_task_runner,
     fml::RefPtr<SkiaUnrefQueue> unref_queue) {
 #if IMPELLER_SUPPORTS_RENDERING
   if (impeller) {
-    return DlDeferredImageGPUImpeller::Make(
-        std::move(layer_tree), SkISize::Make(width, height),
-        std::move(snapshot_delegate), std::move(raster_task_runner));
+    return DlDeferredImageGPUImpeller::Make(std::move(layer_tree),
+                                            std::move(snapshot_delegate),
+                                            std::move(raster_task_runner));
   }
 #endif  // IMPELLER_SUPPORTS_RENDERING
 
-  const SkImageInfo image_info = SkImageInfo::Make(
-      width, height, kRGBA_8888_SkColorType, kPremul_SkAlphaType);
+  const auto& frame_size = layer_tree->frame_size();
+  const SkImageInfo image_info =
+      SkImageInfo::Make(frame_size.width(), frame_size.height(),
+                        kRGBA_8888_SkColorType, kPremul_SkAlphaType);
   return DlDeferredImageGPUSkia::MakeFromLayerTree(
       image_info, std::move(layer_tree), std::move(snapshot_delegate),
       raster_task_runner, std::move(unref_queue));
@@ -126,15 +121,25 @@ void Scene::RasterizeToImage(uint32_t width,
 
   auto image = CanvasImage::Create();
   auto dl_image = CreateDeferredImage(
-      dart_state->IsImpellerEnabled(), layer_tree_, width, height,
+      dart_state->IsImpellerEnabled(), BuildLayerTree(width, height),
       std::move(snapshot_delegate), std::move(raster_task_runner),
       std::move(unref_queue));
   image->set_image(dl_image);
   image->AssociateWithDartWrapper(raw_image_handle);
 }
 
-std::shared_ptr<flutter::LayerTree> Scene::takeLayerTree() {
-  return std::move(layer_tree_);
+std::unique_ptr<flutter::LayerTree> Scene::takeLayerTree(uint64_t width,
+                                                         uint64_t height) {
+  return BuildLayerTree(width, height);
+}
+
+std::unique_ptr<LayerTree> Scene::BuildLayerTree(uint32_t width,
+                                                 uint32_t height) {
+  if (!valid()) {
+    return nullptr;
+  }
+  return std::make_unique<LayerTree>(layer_tree_config_,
+                                     SkISize::Make(width, height));
 }
 
 }  // namespace flutter

lib/ui/compositing/scene.h

@@ -26,7 +26,8 @@ class Scene : public RefCountedDartWrappable<Scene> {
                      bool checkerboardRasterCacheImages,
                      bool checkerboardOffscreenLayers);
 
-  std::shared_ptr<flutter::LayerTree> takeLayerTree();
+  std::unique_ptr<flutter::LayerTree> takeLayerTree(uint64_t width,
+                                                    uint64_t height);
 
   Dart_Handle toImageSync(uint32_t width,
                           uint32_t height,
@@ -44,16 +45,16 @@ class Scene : public RefCountedDartWrappable<Scene> {
         bool checkerboardRasterCacheImages,
         bool checkerboardOffscreenLayers);
 
+  // Returns true if `dispose()` has not been called.
+  bool valid();
+
   void RasterizeToImage(uint32_t width,
                         uint32_t height,
                         Dart_Handle raw_image_handle);
 
-  // This is a shared_ptr to support flattening the layer tree from the UI
-  // thread onto the raster thread - allowing access to the texture registry
-  // required to render TextureLayers.
-  //
-  // No longer valid after calling `takeLayerTree`.
-  std::shared_ptr<flutter::LayerTree> layer_tree_;
+  std::unique_ptr<LayerTree> BuildLayerTree(uint32_t width, uint32_t height);
+
+  flutter::LayerTree::Config layer_tree_config_;
 };
 
 }  // namespace flutter