Make hot reload work (flutter/engine#37304)

engine/src/flutter/impeller/entity/contents/runtime_effect_contents.cc

@@ -52,6 +52,13 @@ bool RuntimeEffectContents::Render(const ContentContext& renderer,
   std::shared_ptr<const ShaderFunction> function = library->GetFunction(
       runtime_stage_->GetEntrypoint(), ShaderStage::kFragment);
 
+  if (function && runtime_stage_->IsDirty()) {
+    library->UnregisterFunction(runtime_stage_->GetEntrypoint(),
+                                ShaderStage::kFragment);
+
+    function = nullptr;
+  }
+
   if (!function) {
     std::promise<bool> promise;
     auto future = promise.get_future();
@@ -79,6 +86,8 @@ bool RuntimeEffectContents::Render(const ContentContext& renderer,
           << runtime_stage_->GetEntrypoint() << ")";
       return false;
     }
+
+    runtime_stage_->SetClean();
   }
 
   //--------------------------------------------------------------------------

engine/src/flutter/impeller/entity/entity_unittests.cc

@@ -2142,12 +2142,21 @@ TEST_P(EntityTest, RuntimeEffect) {
     GTEST_SKIP_("This test only has a Metal fixture at the moment.");
   }
 
+  auto runtime_stage =
+      OpenAssetAsRuntimeStage("runtime_stage_example.frag.iplr");
+  ASSERT_TRUE(runtime_stage->IsDirty());
+
+  bool first_frame = true;
   auto callback = [&](ContentContext& context, RenderPass& pass) -> bool {
+    if (first_frame) {
+      first_frame = false;
+    } else {
+      assert(runtime_stage->IsDirty() == false);
+    }
+
     auto contents = std::make_shared<RuntimeEffectContents>();
     contents->SetGeometry(Geometry::MakeCover());
 
-    auto runtime_stage =
-        OpenAssetAsRuntimeStage("runtime_stage_example.frag.iplr");
     contents->SetRuntimeStage(runtime_stage);
 
     struct FragUniforms {

engine/src/flutter/impeller/runtime_stage/runtime_stage.cc

@@ -144,4 +144,12 @@ RuntimeShaderStage RuntimeStage::GetShaderStage() const {
   return stage_;
 }
 
+bool RuntimeStage::IsDirty() const {
+  return is_dirty_;
+}
+
+void RuntimeStage::SetClean() {
+  is_dirty_ = false;
+}
+
 }  // namespace impeller

engine/src/flutter/impeller/runtime_stage/runtime_stage.h

@@ -36,6 +36,10 @@ class RuntimeStage {
 
   const std::shared_ptr<fml::Mapping>& GetSkSLMapping() const;
 
+  bool IsDirty() const;
+
+  void SetClean();
+
  private:
   RuntimeShaderStage stage_ = RuntimeShaderStage::kVertex;
   std::shared_ptr<fml::Mapping> payload_;
@@ -44,6 +48,7 @@ class RuntimeStage {
   std::shared_ptr<fml::Mapping> sksl_mapping_;
   std::vector<RuntimeUniformDescription> uniforms_;
   bool is_valid_ = false;
+  bool is_dirty_ = true;
 
   FML_DISALLOW_COPY_AND_ASSIGN(RuntimeStage);
 };