Adds metal background to engine dart tests (#180700)

Dart tests now also run against the metal backend on platforms where
that is supported.

test exempt: it is a test

issue: https://github.com/flutter/flutter/issues/180601

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we expect every widget to implement].
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above.
- [x] I updated/added relevant documentation (doc comments with `///`).
- [x] I added new tests to check the change I am making, or this PR is
[test-exempt].
- [x] I followed the [breaking change policy] and added [Data Driven
Fixes] where supported.
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engine/src/flutter/shell/testing/BUILD.gn

@@ -13,7 +13,7 @@ shell_gpu_configuration("tester_gpu_configuration") {
   enable_software = true
   enable_gl = true
   enable_vulkan = true
-  enable_metal = false
+  enable_metal = shell_enable_metal
 }
 
 executable("testing") {
@@ -26,6 +26,7 @@ executable("testing") {
 
   sources = [
     "tester_context.h",
+    "tester_context_mtl_factory.h",
     "tester_context_vk_factory.h",
     "tester_main.cc",
   ]
@@ -61,6 +62,9 @@ executable("testing") {
       "//flutter/third_party/swiftshader/src/Vulkan:swiftshader_libvulkan_static",
     ]
     sources += [ "tester_context_vk_factory.cc" ]
+    if (shell_enable_metal) {
+      sources += [ "tester_context_mtl_factory.mm" ]
+    }
   }
 
   metadata = {

engine/src/flutter/shell/testing/tester_context_mtl_factory.h

@@ -0,0 +1,24 @@
+// Copyright 2013 The Flutter Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+#ifndef FLUTTER_SHELL_TESTING_TESTER_CONTEXT_MTL_FACTORY_H_
+#define FLUTTER_SHELL_TESTING_TESTER_CONTEXT_MTL_FACTORY_H_
+
+#include <memory>
+#include "flutter/shell/testing/tester_context.h"
+
+// Impeller should only be enabled if the Metal backend is enabled.
+#define TESTER_ENABLE_METAL \
+  (IMPELLER_SUPPORTS_RENDERING && IMPELLER_ENABLE_METAL)
+
+namespace flutter {
+
+class TesterContextMTLFactory {
+ public:
+  static std::unique_ptr<TesterContext> Create();
+};
+
+}  // namespace flutter
+
+#endif  // FLUTTER_SHELL_TESTING_TESTER_CONTEXT_MTL_FACTORY_H_

engine/src/flutter/shell/testing/tester_context_mtl_factory.mm

@@ -0,0 +1,133 @@
+// Copyright 2013 The Flutter Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+#define FML_USED_ON_EMBEDDER
+
+#include "flutter/shell/testing/tester_context_mtl_factory.h"
+
+#include <Foundation/Foundation.h>
+#include <QuartzCore/QuartzCore.h>
+#include <deque>
+#include <optional>
+#include <type_traits>
+#include <vector>
+
+#include "flutter/fml/mapping.h"
+#include "flutter/fml/synchronization/sync_switch.h"
+#include "flutter/shell/gpu/gpu_surface_metal_impeller.h"
+#include "impeller/base/validation.h"
+#include "impeller/display_list/aiks_context.h"
+#include "impeller/entity/mtl/entity_shaders.h"
+#include "impeller/entity/mtl/framebuffer_blend_shaders.h"
+#include "impeller/entity/mtl/modern_shaders.h"
+#include "impeller/renderer/backend/metal/context_mtl.h"
+#include "impeller/typographer/backends/skia/typographer_context_skia.h"
+#include "impeller/typographer/typographer_context.h"
+
+namespace flutter {
+
+namespace {
+
+class TesterGPUSurfaceMetalDelegate : public GPUSurfaceMetalDelegate {
+ public:
+  explicit TesterGPUSurfaceMetalDelegate(id<MTLDevice> device)
+      : GPUSurfaceMetalDelegate(MTLRenderTargetType::kCAMetalLayer) {
+    layer_ = [[CAMetalLayer alloc] init];
+    layer_.device = device;
+    layer_.pixelFormat = MTLPixelFormatBGRA8Unorm;
+  }
+
+  ~TesterGPUSurfaceMetalDelegate() = default;
+
+  GPUCAMetalLayerHandle GetCAMetalLayer(const DlISize& frame_info) const override {
+    layer_.drawableSize = CGSizeMake(frame_info.width, frame_info.height);
+    return (__bridge GPUCAMetalLayerHandle)(layer_);
+  }
+
+  bool PresentDrawable(GrMTLHandle drawable) const override { return true; }
+
+  GPUMTLTextureInfo GetMTLTexture(const DlISize& frame_info) const override { return {}; }
+
+  bool PresentTexture(GPUMTLTextureInfo texture) const override { return true; }
+
+  bool AllowsDrawingWhenGpuDisabled() const override { return true; }
+
+ private:
+  CAMetalLayer* layer_ = nil;
+};
+
+std::vector<std::shared_ptr<fml::Mapping>> ShaderLibraryMappings() {
+  return {
+      std::make_shared<fml::NonOwnedMapping>(impeller_entity_shaders_data,
+                                             impeller_entity_shaders_length),
+      std::make_shared<fml::NonOwnedMapping>(impeller_modern_shaders_data,
+                                             impeller_modern_shaders_length),
+      std::make_shared<fml::NonOwnedMapping>(impeller_framebuffer_blend_shaders_data,
+                                             impeller_framebuffer_blend_shaders_length),
+  };
+}
+
+}  // namespace
+
+class TesterContextMTL : public TesterContext {
+ public:
+  TesterContextMTL() = default;
+
+  ~TesterContextMTL() override {
+    if (context_) {
+      context_->Shutdown();
+    }
+  }
+
+  bool Initialize() {
+    std::vector<std::shared_ptr<fml::Mapping>> shader_mappings = ShaderLibraryMappings();
+    auto sync_switch = std::make_shared<fml::SyncSwitch>(false);
+    context_ = impeller::ContextMTL::Create(impeller::Flags{}, shader_mappings, sync_switch,
+                                            "Impeller Library");
+
+    if (!context_ || !context_->IsValid()) {
+      VALIDATION_LOG << "Could not create Metal context.";
+      return false;
+    }
+
+    auto device = context_->GetMTLDevice();
+    if (!device) {
+      VALIDATION_LOG << "Could not get Metal device.";
+      return false;
+    }
+
+    delegate_ = std::make_unique<TesterGPUSurfaceMetalDelegate>(device);
+    aiks_context_ = std::make_shared<impeller::AiksContext>(
+        context_, /*typographer_context=*/impeller::TypographerContextSkia::Make());
+
+    return true;
+  }
+
+  // |TesterContext|
+  std::shared_ptr<impeller::Context> GetImpellerContext() const override { return context_; }
+
+  // |TesterContext|
+  std::unique_ptr<Surface> CreateRenderingSurface() override {
+    auto surface = std::make_unique<GPUSurfaceMetalImpeller>(delegate_.get(), aiks_context_);
+    if (!surface->IsValid()) {
+      return nullptr;
+    }
+    return surface;
+  }
+
+ private:
+  std::shared_ptr<impeller::ContextMTL> context_;
+  std::unique_ptr<TesterGPUSurfaceMetalDelegate> delegate_;
+  std::shared_ptr<impeller::AiksContext> aiks_context_;
+};
+
+std::unique_ptr<TesterContext> TesterContextMTLFactory::Create() {
+  auto context = std::make_unique<TesterContextMTL>();
+  if (!context->Initialize()) {
+    return nullptr;
+  }
+  return context;
+}
+
+}  // namespace flutter

engine/src/flutter/shell/testing/tester_main.cc

@@ -31,6 +31,7 @@
 #include "third_party/skia/include/core/SkSurface.h"
 
 #include "flutter/shell/testing/tester_context.h"
+#include "flutter/shell/testing/tester_context_mtl_factory.h"
 #include "flutter/shell/testing/tester_context_vk_factory.h"
 
 #if defined(FML_OS_WIN)
@@ -48,8 +49,14 @@ static absl::NoDestructor<std::unique_ptr<Shell>> g_shell;
 namespace {
 std::unique_ptr<TesterContext> CreateTesterContext(const Settings& settings) {
   std::unique_ptr<TesterContext> tester_context;
+#if TESTER_ENABLE_METAL
+  if (settings.enable_impeller &&
+      settings.requested_rendering_backend == "metal") {
+    tester_context = TesterContextMTLFactory::Create();
+  }
+#endif
 #if TESTER_ENABLE_VULKAN
-  if (settings.enable_impeller) {
+  if (settings.enable_impeller && !tester_context) {
     tester_context =
         TesterContextVKFactory::Create(settings.enable_vulkan_validation);
   }

engine/src/flutter/testing/run_tests.py

@@ -651,13 +651,13 @@ class FlutterTesterOptions():
   def __init__( # pylint: disable=too-many-arguments
       self,
       multithreaded: bool = False,
-      enable_impeller: bool = False,
+      impeller_backend: str = '',
       enable_vm_service: bool = False,
       enable_microtask_profiling: bool = False,
       expect_failure: bool = False
   ) -> None:
     self.multithreaded = multithreaded
-    self.enable_impeller = enable_impeller
+    self.impeller_backend = impeller_backend
     self.enable_vm_service = enable_vm_service
     self.enable_microtask_profiling = enable_microtask_profiling
     self.expect_failure = expect_failure
@@ -666,8 +666,10 @@ class FlutterTesterOptions():
     if not self.enable_vm_service:
       command_args.append('--disable-vm-service')
 
-    if self.enable_impeller:
-      command_args += ['--enable-impeller', '--enable-flutter-gpu']
+    if self.impeller_backend != '':
+      command_args += [
+          '--enable-impeller', '--enable-flutter-gpu', f'--impeller-backend={self.impeller_backend}'
+      ]
     else:
       command_args += ['--no-enable-impeller']
 
@@ -683,8 +685,8 @@ class FlutterTesterOptions():
     return 'single-threaded'
 
   def impeller_enabled(self) -> str:
-    if self.enable_impeller:
-      return 'impeller swiftshader'
+    if self.impeller_backend != '':
+      return f'impeller {self.impeller_backend}'
     return 'skia software'
 
 
@@ -935,12 +937,12 @@ def gather_dart_tests(
       else:
         _logger.info("Gathering dart test '%s' with VM service enabled", dart_test_file)
         for multithreaded in [False, True]:
-          for enable_impeller in [False, True]:
+          for impeller in ['', 'vulkan', 'metal']:
             yield gather_dart_test(
                 build_dir, dart_test_file,
                 FlutterTesterOptions(
                     multithreaded=multithreaded,
-                    enable_impeller=enable_impeller,
+                    impeller_backend=impeller,
                     enable_vm_service=True,
                     enable_microtask_profiling=dart_test_basename ==
                     'microtask_profiling_test.dart',
@@ -953,10 +955,10 @@ def gather_dart_tests(
     else:
       _logger.info("Gathering dart test '%s'", dart_test_file)
       for multithreaded in [False, True]:
-        for enable_impeller in [False, True]:
+        for impeller in ['', 'vulkan', 'metal']:
           yield gather_dart_test(
               build_dir, dart_test_file,
-              FlutterTesterOptions(multithreaded=multithreaded, enable_impeller=enable_impeller)
+              FlutterTesterOptions(multithreaded=multithreaded, impeller_backend=impeller)
           )