BuildConfig for Kotlin Multiplatform Project.
It currently supports embedding values from gradle file.

• Motivation
• Usage
  • Requirements
  • Gradle Configuration
  • Product Flavor?
  • Overwriting Values
  • HMPP
• Supported Types
• Try out the sample

Passing values from Android/iOS or any other platform code should work, but it's a hassle.
Setting up Android to read values from properties and add those into BuildConfig, and do the equivalent in iOS?
Rather I'd like to do it once.

• Kotlin 1.5.30 or later
• Kotlin Multiplatform Project
• Gradle 7 or later

buildScript {
    repositories {
        mavenCentral()
    }
    dependencies {
        classpath 'org.jetbrains.kotlin:kotlin-gradle-plugin:1.9.0'
        classpath 'com.codingfeline.buildkonfig:buildkonfig-gradle-plugin:latest_version'
    }
}

apply plugin: 'org.jetbrains.kotlin.multiplatform'
apply plugin: 'com.codingfeline.buildkonfig'

kotlin {
    // your target config...
    android()
    iosX64('ios')
}

buildkonfig {
    packageName = 'com.example.app'
    // objectName = 'YourAwesomeConfig'
    // exposeObjectWithName = 'YourAwesomePublicConfig'

    defaultConfigs {
        buildConfigField 'STRING', 'name', 'value'
    }
}

import com.codingfeline.buildkonfig.compiler.FieldSpec.Type.STRING

buildscript {
    repositories {
        mavenCentral()
    }
    dependencies {
        classpath("org.jetbrains.kotlin:kotlin-gradle-plugin:1.9.0")
        classpath("com.codingfeline.buildkonfig:buildkonfig-gradle-plugin:latest_version")
    }
}

plugins {
    kotlin("multiplatform")
    id("com.codingfeline.buildkonfig")
}

kotlin {
    // your target config...
    android()
    iosX64('ios')
}

buildkonfig {
    packageName = "com.example.app"
    // objectName = "YourAwesomeConfig"
    // exposeObjectWithName = "YourAwesomePublicConfig"

    defaultConfigs {
        buildConfigField(STRING, "name", "value")
    }
}

• packageName Set the package name where BuildKonfig is being placed. Required.
• objectName Set the name of the generated object. Defaults to BuildKonfig.
• exposeObjectWithName Set the name of the generated object, and make it public.
• defaultConfigs Set values which you want to have in common. Required.

To generate BuildKonfig files, run generateBuildKonfig task.
This task will be automatically run upon execution of kotlin compile tasks.

Above configuration will generate following simple object.

// commonMain
package com.example.app

internal object BuildKonfig {
    val name: String = "value"
}

If you want to change value depending on your targets, you can use targetConfigs to define target-dependent values.

buildScript {
    repositories {
        mavenCentral()
    }
    dependencies {
        classpath 'org.jetbrains.kotlin:kotlin-gradle-plugin:1.9.0'
        classpath 'com.codingfeline.buildkonfig:buildkonfig-gradle-plugin:latest_version'
    }
}

apply plugin: 'org.jetbrains.kotlin.multiplatform'
apply plugin: 'com.codingfeline.buildkonfig'

kotlin {
    // your target config...
    android()
    iosX64('ios')
}

buildkonfig {
    packageName = 'com.example.app'
    
    // default config is required
    defaultConfigs {
        buildConfigField 'STRING', 'name', 'value'
        buildConfigField 'STRING', 'nullableField', null, nullable: true
    }
    
    targetConfigs {
        // this name should be the same as target names you specified
        android {
            buildConfigField 'STRING', 'name2', 'value2'
            buildConfigField 'STRING', 'nullableField', 'NonNull-value', nullable: true
        }
        
        ios {
            buildConfigField 'STRING', 'name', 'valueForNative'
        }
    }
}

import com.codingfeline.buildkonfig.compiler.FieldSpec.Type.STRING

buildscript {
    repositories {
        mavenCentral()
    }
    dependencies {
        classpath("org.jetbrains.kotlin:kotlin-gradle-plugin:1.9.0")
        classpath("com.codingfeline.buildkonfig:buildkonfig-gradle-plugin:latest_version")
    }
}

plugins {
    kotlin("multiplatform")
    id("com.codingfeline.buildkonfig")
}

kotlin {
    // your target config...
    android()
    iosX64('ios')
}

buildkonfig {
    packageName = "com.example.app"

    // default config is required
    defaultConfigs {
        buildConfigField(STRING, "name", "value")
    }

    targetConfigs {
        // names in create should be the same as target names you specified
        create("android") {
            buildConfigField(STRING, "name2", "value2")
            buildConfigField(STRING, "nullableField", "NonNull-value", nullable = true)
        }

        create("ios") {
            buildConfigField(STRING, "name", "valueForNative")
        }
    }
}

• packageName
  • Sets the package name where BuildKonfig is being placed. Required.
• objectName
  • Sets the name of the generated object. Defaults to BuildKonfig.
• exposeObjectWithName
  • Sets the name of the generated object, and make it public.
• defaultConfigs
  • Sets values which you want to have in common. Required.
• targetConfigs
  • Sets target specific values as closure. You can overwrite values specified in defaultConfigs.
• buildConfigField(type: String, name: String, value: String)
  • Adds new value or overwrite existing one.
• buildConfigField(type: String, name: String, value: String, nullable: Boolean = false, const: Boolean = false)
  • In addition to above method, this can configure nullable and const declarations.

Above configuration will generate following codes.

// commonMain
package com.example.app

internal expect object BuildKonfig {
    val name: String
    val nullableField: String?
}

// androidMain
package com.example.app

internal actual object BuildKonfig {
    actual val name: String = "value"
    actual val nullableField: String? = "NonNull-value"
    val name2: String = "value2"
}

// iosMain
package com.example.app

internal actual object BuildKonfig {
    actual val name: String = "valueForNative"
    actual val nullableField: String? = null
}

Yes(sort of).
Kotlin Multiplatform Project does not support product flavor. Kotlin/Native part of the project has release/debug distinction, but it's not global.
So to mimick product flavor capability of Android, we need to provide additional property in order to determine flavors.

Specify default flavor in your gradle.properties

# ROOT_DIR/gradle.properties
buildkonfig.flavor=dev

// ./mpp_project/build.gradle

buildkonfig {
    packageName = 'com.example.app'
    
    // default config is required
    defaultConfigs {
        buildConfigField 'STRING', 'name', 'value'
    }
    // flavor is passed as a first argument of defaultConfigs 
    defaultConfigs("dev") {
        buildConfigField 'STRING', 'name', 'devValue'
    }
    
    targetConfigs {
        android {
            buildConfigField 'STRING', 'name2', 'value2'
        }
        
        ios {
            buildConfigField 'STRING', 'name', 'valueIos'
        }
    }
    // flavor is passed as a first argument of targetConfigs
    targetConfigs("dev") {
        ios {
            buildConfigField 'STRING', 'name', 'devValueIos'
        }
    }
}

import com.codingfeline.buildkonfig.compiler.FieldSpec.Type.STRING
import com.codingfeline.buildkonfig.gradle.TargetConfigDsl

buildkonfig {
    packageName = "com.example.app"

    // default config is required
    defaultConfigs {
        buildConfigField(STRING, "name", "value")
    }
    // flavor is passed as a first argument of defaultConfigs 
    defaultConfigs("dev") {
        buildConfigField(STRING, "name", "devValue")
    }

    targetConfigs {
        create("android") {
            buildConfigField(STRING, "name2", "value2")
        }

        create("ios") {
            buildConfigField(STRING, "name", "valueIos")
        }
    }
    // flavor is passed as a first argument of targetConfigs
    targetConfigs("dev") {
        create("ios") {
            buildConfigField(STRING, "name", "devValueIos")
        }
    }
}

In a development phase you can change value in gradle.properties as you like.
In CI environment, you can pass value via CLI $ ./gradlew build -Pbuildkonfig.flavor=release

If you configure same field across multiple defaultConfigs and targetConfigs, flavored targetConfigs is the strongest.

Lefter the stronger.

Flavored TargetConfig > TargetConfig > Flavored DefaultConfig > DefaultConfig

a.k.a Intermediate SourceSets. (see Share code on platforms for detail.)
BuildKonfig supports HMPP. However there's some limitations.

When you add a targetConfigs for a intermediate source set, you can't define another targetConfigs for its children source sets.

For example, say your have a source set structure like below.

- commonMain
  - appMain
    - androidMain
    - desktopMain
      - macosX64Main
      - linuxX64Main
      - mingwX64Main
  - jsCommonMain
    - browserMain
    - nodeMain
  - iosMain
    - iosArm64Main
    - iosX64Main

If you add a targetConfigs for appMain, you can't add configs for androidMain, desktopMain, or children of desktopMain. This is because BuildKonfig uses expect/actual to provide different values for each BuildKonfig object. When you provide a configuration for appMain, actual declaration of BuildKonfig object is created in appMain. So any additional actual declarations in children SourceSets leads to compile-time error.

• String
• Int
• Long
• Float
• Boolean

There are two samples; sample and sample-kts. As its name implies, sample-kts a Kotlin DSL sample, and the other is a traditional Groovy DSL.

Have a look at ./sample directory.

# Publish the latest version of the plugin to test maven repository(./build/localMaven)
$ ./gradlew publishAllPublicationsToTestMavenRepository -PRELEASE_SIGNING_ENABLED=false

# Try out the samples.
# BuildKonfig will be generated in ./sample/build/buildkonfig
$ ./gradlew -p sample generateBuildKonfig