[Impeller] allowing enabling Impeller on macOS.

shell/platform/darwin/macos/framework/Source/FlutterDartProject.mm

@@ -59,6 +59,15 @@ - (instancetype)initWithAssetsPath:(NSString*)assets ICUDataPath:(NSString*)icuP
   return self;
 }
 
+- (BOOL)enableImpeller {
+  NSNumber* enableImpeller =
+      [[NSBundle mainBundle] objectForInfoDictionaryKey:@"FLTEnableImpeller"];
+  if (enableImpeller != nil) {
+    return enableImpeller.boolValue;
+  }
+  return NO;
+}
+
 - (NSString*)assetsPath {
   if (_assetsPath) {
     return _assetsPath;

shell/platform/darwin/macos/framework/Source/FlutterDartProject_Internal.h

@@ -31,6 +31,11 @@
  */
 @property(nonatomic, nullable) void (*rootIsolateCreateCallback)(void* _Nullable);
 
+/**
+ * Whether the Impeller rendering backend is enabled
+ */
+@property(nonatomic, readonly) BOOL enableImpeller;
+
 /**
  * Instead of looking up the assets and ICU data path in the application bundle, this initializer
  * allows callers to create a Dart project with custom locations specified for the both.

shell/platform/darwin/macos/framework/Source/FlutterEngine.mm

@@ -481,6 +481,13 @@ - (BOOL)runWithEntrypoint:(NSString*)entrypoint {
   // The first argument of argv is required to be the executable name.
   std::vector<const char*> argv = {[self.executableName UTF8String]};
   std::vector<std::string> switches = self.switches;
+
+  // Enable Impeller only if specifically asked for from the project or cmdline arguments.
+  if (_project.enableImpeller ||
+      std::find(switches.begin(), switches.end(), "--enable-impeller=true") != switches.end()) {
+    switches.push_back("--enable-impeller=true");
+  }
+
   std::transform(switches.begin(), switches.end(), std::back_inserter(argv),
                  [](const std::string& arg) -> const char* { return arg.c_str(); });
 

shell/platform/embedder/embedder_surface_metal_impeller.h

@@ -62,6 +62,9 @@ class EmbedderSurfaceMetalImpeller final : public EmbedderSurface,
   // |GPUSurfaceMetalDelegate|
   bool PresentTexture(GPUMTLTextureInfo texture) const override;
 
+  // |EmbedderSurface|
+  std::shared_ptr<impeller::Context> CreateImpellerContext() const override;
+
   FML_DISALLOW_COPY_AND_ASSIGN(EmbedderSurfaceMetalImpeller);
 };
 

shell/platform/embedder/embedder_surface_metal_impeller.mm

@@ -49,6 +49,7 @@
       std::make_shared<fml::SyncSwitch>(false),  // is_gpu_disabled_sync_switch
       "Impeller Library"                         // library_label
   );
+  FML_LOG(ERROR) << "Using the Impeller rendering backend (Metal).";
 
   valid_ = !!context_;
 }
@@ -75,6 +76,10 @@
   return surface;
 }
 
+std::shared_ptr<impeller::Context> EmbedderSurfaceMetalImpeller::CreateImpellerContext() const {
+  return context_;
+}
+
 GPUCAMetalLayerHandle EmbedderSurfaceMetalImpeller::GetCAMetalLayer(
     const SkISize& frame_info) const {
   FML_CHECK(false) << "Only rendering to MTLTexture is supported.";