[Bron Davies (brondavies)]

Summary:

Fixes #3137.

UIImagePNGRepresentation and UIImageJPEGRepresentation are defined in RCTUIImage.m (Objective-C), so their symbols are C, but RCTUIImage.h declared them without extern "C". An Objective-C++ caller then asks for C++-mangled names and fails to link on macOS (react-native-svg's RNSVGSvgView.mm, for one). They now sit in an extern "C" block, like UIImageGetScale and UIImageGetCGImageRef above them.

That on its own breaks RNTester's link: RCTImageLoader.mm kept a copy of both functions from when React-RCTImage couldn't depend on RCTUIKit (#1611). As C++ the copies had mangled names and never clashed; declared extern "C" they become duplicate symbols. React-Core depends on React-RCTUIKit now (React-Core.podspec), and RCTImageLoader.mm already sees RCTUIImage.h, so the copy is removed and the image loader uses RCTUIKit's implementation, which is the same code.

Test Plan:

• Two files, before and after. The two RCTUIKit files and a one-line Objective-C++ caller:

  K=packages/react-native/ReactApple/Libraries/RCTUIKit
  xcrun clang -c -fobjc-arc -fmodules -I$K $K/RCTUIImage.m -o RCTUIImage.o
  xcrun clang++ -c -fobjc-arc -fmodules -I$K caller.mm -o caller.o   # calls UIImagePNGRepresentation
  xcrun clang++ caller.o RCTUIImage.o -framework AppKit

  On main the link fails with "UIImagePNGRepresentation(NSImage*)", referenced from: _main in caller.o and ld's note that _UIImagePNGRepresentation is in RCTUIImage.o, possibly missing extern "C". With this change it links.
• RNTester-macOS (Debug, arm64, Xcode 27.1, macOS 26.6.2) builds and links. With only the header change it fails with duplicate symbol '_UIImagePNGRepresentation' (and JPEG) in RCTImageLoader.o and RCTUIImage.o; with the copy removed, nm shows one T _UIImagePNGRepresentation and one T _UIImageJPEGRepresentation in RNTester-macOS.debug.dylib.
• The header change has been in our app (react-native-macos 0.83.0, dynamic frameworks) as a patch-package patch, where it lets react-native-svg 15.16 link.

Not built for iOS or visionOS: both changes are inside TARGET_OS_OSX.

[azure-pipelines]

Azure Pipelines:
There may be pipelines that require an authorized user to comment /azp run to run.