How to setup protobuf in Android/IOS Kotlin Multiplatform project

Question

First time using protobu and could not find an example of how to connect it to Kotlin Multiplatform. I created Multiplatform project Android/IOS with "shared" module where I need to use protobuf. Project structure:

Code in build.gradle shared.module lvl build.gradle shared.module lvl

plugins {
    kotlin("multiplatform")
    id("com.android.library")
    id("com.google.protobuf")
}

kotlin {
    android()
    listOf(
        iosX64(),
        iosArm64(),
        iosSimulatorArm64()
    ).forEach {
        it.binaries.framework {
            baseName = "shared"
        }
    }

    sourceSets {
        val commonMain by getting {
            dependencies {
                implementation("org.jetbrains.kotlinx:kotlinx-coroutines-core:1.6.4")
            }

        }
        val commonTest by getting {
            dependencies {
                implementation(kotlin("test"))
            }
        }
        val androidMain by getting
        val androidTest by getting
        val iosX64Main by getting
        val iosArm64Main by getting
        val iosSimulatorArm64Main by getting
        val iosMain by creating {
            dependsOn(commonMain)
            iosX64Main.dependsOn(this)
            iosArm64Main.dependsOn(this)
            iosSimulatorArm64Main.dependsOn(this)
        }
        val iosX64Test by getting
        val iosArm64Test by getting
        val iosSimulatorArm64Test by getting
        val iosTest by creating {
            dependsOn(commonTest)
            iosX64Test.dependsOn(this)
            iosArm64Test.dependsOn(this)
            iosSimulatorArm64Test.dependsOn(this)
        }
    }
}


android {
    namespace = "com.example.someproject"
    compileSdk = 33
    defaultConfig {
        minSdk = 21
        targetSdk = 33
    }

    protobuf {
        protoc {
            artifact = ("com.google.protobuf:protoc:3.21.9")
        }
        generateProtoTasks {
            all().forEach { task ->
                task.builtins {
                    java {
 //                   option 'lite' //Error - Unresolved reference: option
                    }
                    kotlin {
//                    option 'lite' //Error - Unresolved reference: option
                    }
                }
            }
        }
    }
    dependencies {
        implementation("com.google.protobuf:protobuf-javalite:3.21.9")
    }
}

My build.gradle project lvl: build gradle project lvl

buildscript {
    dependencies {
        classpath 'com.google.protobuf:protobuf-gradle-plugin:0.9.1'
    }
}
plugins {
    id 'com.android.application' version '7.3.1' apply false
    id 'com.android.library' version '7.3.1' apply false
    id 'org.jetbrains.kotlin.android' version '1.7.20' apply false
    id 'com.google.protobuf' version '0.9.1' apply false
}

I tried to add path to my .proto files with sourceSets, but I got errors:

android {
    namespace = "com.example.someproject"
    compileSdk = 33
    defaultConfig {
        minSdk = 21
        targetSdk = 33
    }
    sourceSets {
        main { //Error - Unresolved reference: main
            proto { //Error - Unresolved reference: proto
                srcDir 'src/main/protobuf' //Error - Unresolved reference: srcDir
            }
            java {
                srcDirs 'build/generated/source/proto/main/java' //Error - Unresolved reference: srcDirs
                srcDirs 'build/generated/source/proto/main/kotlin' //Error - Unresolved reference: srcDirs
            }
        }
    }

    protobuf {
        protoc {
            artifact = ("com.google.protobuf:protoc:3.21.9")
        }
        generateProtoTasks {
            all().forEach { task ->
                task.builtins {
                    java {
 //                   option 'lite' //Error - Unresolved reference: option
                    }
                    kotlin {
//                    option 'lite' //Error - Unresolved reference: option
                    }
                }
            }
        }
    }
    dependencies {
        implementation("com.google.protobuf:protobuf-javalite:3.21.9")
    }
}

When I try to build project without sourceSets, I didn't get generated java/kotlin files in build/generated/source - path

And my question is: "How setup protobuf in Android/IOS project Kotlin multiplatform?"

Answer 1

If you want to use ProtoBuf library in shared module in common code, then you have to use library that supports Kotlin Multiplatform. You can take a look at official kotlinx.serialization library, it supports ProtoBuf. And you can use it in commonMain module.

Also take a look at community libraries: 1, 2

Or learn more about platform-specific implementations and use expect/actual mechanism

Another option will be use your ProtoBuf library and plugin in androidApp only and for iOS part use another iOS specific ProtoBuf library.