Started asserting that metal gpu contexts are shared (flutter/engine#23539)

Started asserting that metal gpu contexts are shared, added ability
for IOSContexts to store their skia contexts so we can make the OpenGL
code work like the Metal code.

engine/src/flutter/shell/platform/darwin/ios/framework/Source/FlutterEngineTest_mrc.mm

@@ -30,8 +30,17 @@ FLUTTER_ASSERT_NOT_ARC
   XCTAssertNotNil(spawn);
   XCTAssertTrue([engine iosPlatformView] != nullptr);
   XCTAssertTrue([spawn iosPlatformView] != nullptr);
-  XCTAssertEqual([engine iosPlatformView]->GetIosContext(),
-                 [spawn iosPlatformView]->GetIosContext());
+  std::shared_ptr<flutter::IOSContext> engine_context = [engine iosPlatformView]->GetIosContext();
+  std::shared_ptr<flutter::IOSContext> spawn_context = [spawn iosPlatformView]->GetIosContext();
+  XCTAssertEqual(engine_context, spawn_context);
+  // If this assert fails it means we may be using the software or OpenGL
+  // renderer when we were expecting Metal.  For software rendering, this is
+  // expected to be nullptr.  For OpenGL, implementing this is an outstanding
+  // change see https://github.com/flutter/flutter/issues/73744.
+  XCTAssertTrue(engine_context->GetMainContext() != nullptr);
+  XCTAssertEqual(engine_context->GetMainContext(), spawn_context->GetMainContext());
+  [engine release];
+  [spawn release];
 }
 
 @end

engine/src/flutter/shell/platform/darwin/ios/ios_context.h

@@ -106,6 +106,20 @@ class IOSContext {
       int64_t texture_id,
       fml::scoped_nsobject<NSObject<FlutterTexture>> texture) = 0;
 
+  //----------------------------------------------------------------------------
+  /// @brief      Accessor for the Skia context associated with IOSSurfaces and
+  ///             the raster thread.
+  /// @details    There can be any number of resource contexts but this is the
+  ///             one context that will be used by surfaces to draw to the
+  ///             screen from the raster thread.
+  /// @returns    `nullptr` on failure.
+  /// @attention  The software context doesn't have a Skia context, so this
+  ///             value will be nullptr.
+  /// @see        For contexts which are used for offscreen work like loading
+  ///             textures see IOSContext::CreateResourceContext.
+  ///
+  virtual sk_sp<GrDirectContext> GetMainContext() const = 0;
+
  protected:
   IOSContext();
 

engine/src/flutter/shell/platform/darwin/ios/ios_context_gl.h

@@ -32,6 +32,9 @@ class IOSContextGL final : public IOSContext {
   // |IOSContext|
   sk_sp<GrDirectContext> CreateResourceContext() override;
 
+  // |IOSContext|
+  sk_sp<GrDirectContext> GetMainContext() const override;
+
   // |IOSContext|
   std::unique_ptr<GLContextResult> MakeCurrent() override;
 

engine/src/flutter/shell/platform/darwin/ios/ios_context_gl.mm

@@ -43,6 +43,16 @@ sk_sp<GrDirectContext> IOSContextGL::CreateResourceContext() {
       GrBackend::kOpenGL_GrBackend, GPUSurfaceGLDelegate::GetDefaultPlatformGLInterface());
 }
 
+// |IOSContext|
+sk_sp<GrDirectContext> IOSContextGL::GetMainContext() const {
+  /// TODO(73744): Currently the GPUSurfaceGL creates the main context for
+  /// OpenGL.  With Metal the IOSContextMetal creates the main context and is
+  /// shared across surfaces.  We should refactor the OpenGL Context/Surfaces to
+  /// behave like the Metal equivalents.  Until then engines in the same group
+  /// will have a heavier memory cost if they are using OpenGL.
+  return nullptr;
+}
+
 // |IOSContext|
 std::unique_ptr<GLContextResult> IOSContextGL::MakeCurrent() {
   return std::make_unique<GLContextSwitch>(

engine/src/flutter/shell/platform/darwin/ios/ios_context_metal.h

@@ -24,7 +24,8 @@ class IOSContextMetal final : public IOSContext {
 
   fml::scoped_nsobject<FlutterDarwinContextMetal> GetDarwinContext() const;
 
-  sk_sp<GrDirectContext> GetMainContext() const;
+  // |IOSContext|
+  sk_sp<GrDirectContext> GetMainContext() const override;
 
   sk_sp<GrDirectContext> GetResourceContext() const;
 

engine/src/flutter/shell/platform/darwin/ios/ios_context_software.h

@@ -20,6 +20,9 @@ class IOSContextSoftware final : public IOSContext {
   // |IOSContext|
   sk_sp<GrDirectContext> CreateResourceContext() override;
 
+  // |IOSContext|
+  sk_sp<GrDirectContext> GetMainContext() const override;
+
   // |IOSContext|
   std::unique_ptr<GLContextResult> MakeCurrent() override;
 

engine/src/flutter/shell/platform/darwin/ios/ios_context_software.mm

@@ -16,6 +16,11 @@ sk_sp<GrDirectContext> IOSContextSoftware::CreateResourceContext() {
   return nullptr;
 }
 
+// |IOSContext|
+sk_sp<GrDirectContext> IOSContextSoftware::GetMainContext() const {
+  return nullptr;
+}
+
 // |IOSContext|
 std::unique_ptr<GLContextResult> IOSContextSoftware::MakeCurrent() {
   // This only makes sense for context that need to be bound to a specific thread.