iOS: Refactor ShellTestPlatformViewMetal

Migrates DarwinContextMetal to a plain C struct, eliminating the need
for constructor, getters, etc. since it's only used in this translation
unit. The fields themselves cannot be inlined as fields on
ShellTestPlatformViewMetal because the header in which that class is
defined is included in plain C++ (non-Obj-C++) translation units and
therefore cannot contain Obj-C types.

This simplifies the code and simultaneously fixes complicated ARC
behaviour in which the const "DarwinContextMetal::context()" getter
caused refcount to be incremented on the underlying context_ pointer,
but not decremented even if never unassigned. In particular the line
```objc
FML_CHECK([metal_context->context() mainContext]));
```
appeared to cause refcount to be incremented but never decremented.

Regardless of the ARC issue, this significantly simplifies the code.

This also eliminates the last remaining use of fml::scoped_nsobject in
Flutter's codebase. That class will be removed in a followup PR.

Issue: flutter/flutter#137801

shell/common/BUILD.gn

@@ -301,6 +301,9 @@ if (enable_unittests) {
         "shell_test_platform_view_metal.mm",
       ]
 
+      cflags_objc = flutter_cflags_objc_arc
+      cflags_objcc = flutter_cflags_objcc_arc
+
       public_deps += [ "//flutter/shell/platform/darwin/graphics" ]
     }
   }

shell/common/shell_test_platform_view_metal.h

@@ -12,7 +12,7 @@
 namespace flutter {
 namespace testing {
 
-class DarwinContextMetal;
+struct DarwinContextMetal;
 
 class ShellTestPlatformViewMetal final : public ShellTestPlatformView,
                                          public GPUSurfaceMetalDelegate {

shell/common/shell_test_platform_view_metal.mm

@@ -8,64 +8,50 @@
 
 #include <utility>
 
-#include "flutter/fml/platform/darwin/scoped_nsobject.h"
 #include "flutter/shell/gpu/gpu_surface_metal_impeller.h"
 #include "flutter/shell/gpu/gpu_surface_metal_skia.h"
 #include "flutter/shell/platform/darwin/graphics/FlutterDarwinContextMetalImpeller.h"
 #include "flutter/shell/platform/darwin/graphics/FlutterDarwinContextMetalSkia.h"
 
+FLUTTER_ASSERT_ARC
+
 namespace flutter {
 namespace testing {
 
-static fml::scoped_nsprotocol<id<MTLTexture>> CreateOffscreenTexture(id<MTLDevice> device) {
+// This is out of the header so that shell_test_platform_view_metal.h can be included in
+// non-Objective-C TUs.
+struct DarwinContextMetal {
+  FlutterDarwinContextMetalSkia* context;
+  FlutterDarwinContextMetalImpeller* impeller_context;
+  id<MTLTexture> offscreen_texture;
+};
+
+static std::unique_ptr<DarwinContextMetal> CreateDarwinContext(
+    bool impeller,
+    const std::shared_ptr<const fml::SyncSwitch>& is_gpu_disabled_sync_switch) {
+  FlutterDarwinContextMetalSkia* skia_context = nil;
+  FlutterDarwinContextMetalImpeller* impeller_context = nil;
+  id<MTLDevice> device = nil;
+  if (impeller) {
+    impeller_context = [[FlutterDarwinContextMetalImpeller alloc] init:is_gpu_disabled_sync_switch];
+    FML_CHECK(impeller_context.context);
+    device = impeller_context.context->GetMTLDevice();
+  } else {
+    skia_context = [[FlutterDarwinContextMetalSkia alloc] initWithDefaultMTLDevice];
+    FML_CHECK(skia_context.mainContext);
+    device = skia_context.device;
+  }
   auto descriptor =
       [MTLTextureDescriptor texture2DDescriptorWithPixelFormat:MTLPixelFormatBGRA8Unorm
                                                          width:800
                                                         height:600
                                                      mipmapped:NO];
   descriptor.usage = MTLTextureUsageRenderTarget | MTLTextureUsageShaderRead;
-  return fml::scoped_nsprotocol<id<MTLTexture>>{[device newTextureWithDescriptor:descriptor]};
+  id<MTLTexture> offscreen_texture = [device newTextureWithDescriptor:descriptor];
+  return std::make_unique<DarwinContextMetal>(
+      DarwinContextMetal{skia_context, impeller_context, offscreen_texture});
 }
 
-// This is out of the header so that shell_test_platform_view_metal.h can be included in
-// non-Objective-C TUs.
-class DarwinContextMetal {
- public:
-  explicit DarwinContextMetal(
-      bool impeller,
-      const std::shared_ptr<const fml::SyncSwitch>& is_gpu_disabled_sync_switch)
-      : context_(impeller ? nil : [[FlutterDarwinContextMetalSkia alloc] initWithDefaultMTLDevice]),
-        impeller_context_(
-            impeller ? [[FlutterDarwinContextMetalImpeller alloc] init:is_gpu_disabled_sync_switch]
-                     : nil),
-        offscreen_texture_(CreateOffscreenTexture(
-            impeller ? [impeller_context_ context]->GetMTLDevice() : [context_ device])) {}
-
-  ~DarwinContextMetal() = default;
-
-  fml::scoped_nsobject<FlutterDarwinContextMetalImpeller> impeller_context() const {
-    return impeller_context_;
-  }
-
-  fml::scoped_nsobject<FlutterDarwinContextMetalSkia> context() const { return context_; }
-
-  fml::scoped_nsprotocol<id<MTLTexture>> offscreen_texture() const { return offscreen_texture_; }
-
-  GPUMTLTextureInfo offscreen_texture_info() const {
-    GPUMTLTextureInfo info = {};
-    info.texture_id = 0;
-    info.texture = reinterpret_cast<GPUMTLTextureHandle>(offscreen_texture_.get());
-    return info;
-  }
-
- private:
-  const fml::scoped_nsobject<FlutterDarwinContextMetalSkia> context_;
-  const fml::scoped_nsobject<FlutterDarwinContextMetalImpeller> impeller_context_;
-  const fml::scoped_nsprotocol<id<MTLTexture>> offscreen_texture_;
-
-  FML_DISALLOW_COPY_AND_ASSIGN(DarwinContextMetal);
-};
-
 ShellTestPlatformViewMetal::ShellTestPlatformViewMetal(
     PlatformView::Delegate& delegate,
     const TaskRunners& task_runners,
@@ -75,17 +61,11 @@ GPUMTLTextureInfo offscreen_texture_info() const {
     const std::shared_ptr<const fml::SyncSwitch>& is_gpu_disabled_sync_switch)
     : ShellTestPlatformView(delegate, task_runners),
       GPUSurfaceMetalDelegate(MTLRenderTargetType::kMTLTexture),
-      metal_context_(std::make_unique<DarwinContextMetal>(GetSettings().enable_impeller,
-                                                          is_gpu_disabled_sync_switch)),
+      metal_context_(
+          CreateDarwinContext(GetSettings().enable_impeller, is_gpu_disabled_sync_switch)),
       create_vsync_waiter_(std::move(create_vsync_waiter)),
       vsync_clock_(std::move(vsync_clock)),
-      shell_test_external_view_embedder_(std::move(shell_test_external_view_embedder)) {
-  if (GetSettings().enable_impeller) {
-    FML_CHECK([metal_context_->impeller_context() context] != nil);
-  } else {
-    FML_CHECK([metal_context_->context() mainContext] != nil);
-  }
-}
+      shell_test_external_view_embedder_(std::move(shell_test_external_view_embedder)) {}
 
 ShellTestPlatformViewMetal::~ShellTestPlatformViewMetal() = default;
 
@@ -113,16 +93,16 @@ GPUMTLTextureInfo offscreen_texture_info() const {
 // |PlatformView|
 std::unique_ptr<Surface> ShellTestPlatformViewMetal::CreateRenderingSurface() {
   if (GetSettings().enable_impeller) {
-    auto context = [metal_context_->impeller_context() context];
+    auto context = metal_context_->impeller_context.context;
     return std::make_unique<GPUSurfaceMetalImpeller>(
         this, std::make_shared<impeller::AiksContext>(context, nullptr));
   }
-  return std::make_unique<GPUSurfaceMetalSkia>(this, [metal_context_->context() mainContext]);
+  return std::make_unique<GPUSurfaceMetalSkia>(this, metal_context_->context.mainContext);
 }
 
 // |PlatformView|
 std::shared_ptr<impeller::Context> ShellTestPlatformViewMetal::GetImpellerContext() const {
-  return [metal_context_->impeller_context() context];
+  return metal_context_->impeller_context.context;
 }
 
 // |GPUSurfaceMetalDelegate|
@@ -141,7 +121,10 @@ GPUMTLTextureInfo offscreen_texture_info() const {
 
 // |GPUSurfaceMetalDelegate|
 GPUMTLTextureInfo ShellTestPlatformViewMetal::GetMTLTexture(const SkISize& frame_info) const {
-  return metal_context_->offscreen_texture_info();
+  GPUMTLTextureInfo info = {};
+  info.texture_id = 0;
+  info.texture = (__bridge GPUMTLTextureHandle)metal_context_->offscreen_texture;
+  return info;
 }
 
 // |GPUSurfaceMetalDelegate|