Kotlin multiplatform/native interoperability with Objective-C framework

I ended up with this cinterops section in build.gradle

    fromPreset(presets.iosX64, 'ios') {
        // This preset is for iPhone emulator
        // Switch here to presets.iosArm64 (or iosArm32) to build library for iPhone device
        compilations.main {
            outputKinds('FRAMEWORK')
            cinterops {
                firebase {
                    defFile "$projectDir/src/iosMain/cinterop/firebase.def"
                    includeDirs {
                        allHeaders "$projectDir/../iosApp/Pods/FirebaseCore/Firebase/Core/Public",
                                   "$projectDir/../iosApp/Pods/FirebaseDatabase/Firebase/Database/Public"
                    }

                    compilerOpts "-F$projectDir/../iosApp/Pods/Firebase -F$projectDir/../iosApp/Pods/FirebaseCore -F$projectDir/../iosApp/Pods/FirebaseDatabase"
                    linkerOpts "-F$projectDir/../iosApp/Pods/Firebase -F$projectDir/../iosApp/Pods/FirebaseCore -F$projectDir/../iosApp/Pods/FirebaseDatabase"
                }
            }
        }
    }

end this .def file:

language = Objective-C
headers = FirebaseCore.h FirebaseDatabase.h

What's going on here? Cocopods frameworks are placed in Pods directory in your Xcode project. Navigating this folder a bit, you'll find what you need. I'm not sure if there is some standard but firebase place main header file in Public folder. And it contains references to other header files it needs... So you specify these file names in your .def file in the headers section.

Next, you need to specify where to look for these files and others referenced by them. You can do it in the .def file in includeDirs or in build.gradle file. I prefer to use the build.gradle file as it can use variables. So you specify the path to these Public folders. (This is enough for kotlin to see library API, but in order to be able to run the app you need to compile and link this library...)

Then the compiler and linker need to know where library/framework is. So you specify path to root folder of the framework in compilerOpts and linkerOpts prefixing them with -F if it is a framework or -L if it is a library.