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Make your Android application work on iOS – tutorial

Here you can learn how to make your existing Android application cross-platform so that it works both on Android and iOS. You'll be able to write code and test it for both Android and iOS only once, in one place.

This tutorial uses a sample Android application with a single screen for entering a username and password. The credentials are validated and saved to an in-memory database.

If you aren't familiar with KMM, you can learn how to create and configure a KMM application from scratch first.

Prepare an environment for development

  1. Install Android Studio 4.2 or Android Studio 2020.3.1 Canary 8 or higher and other tools for KMM development on macOS.

  2. In Android Studio, create a new project from version control: https://github.com/Kotlin/kmm-integration-sample.

  3. Switch to the Project view.

    Project view

Make your code cross-platform

To have a KMM application that works on iOS, you'll first make your code cross-platform, and then you’ll reuse your cross-platform code in a new iOS application.

To make your code cross-platform:

  1. Decide what code to make cross-platform.

  2. Create a shared module for cross-platform code.

  3. Add a dependency on the shared module to your Android application.

  4. Make the business logic cross-platform.

  5. Run your cross-platform application on Android.

Decide what code to make cross-platform

Decide which code of your Android application is better to share for iOS and which to keep native. A simple rule is: share what you want to reuse as much as possible. The business logic is often the same for both Android and iOS, so it's a great candidate for reuse.

In your sample Android application, the business logic is stored in the package com.jetbrains.simplelogin.androidapp.data. Your future iOS application will use the same logic, so you should make it cross-platform, as well.

Business logic to share

Create a shared module for cross-platform code

The cross-platform code that is used for both iOS and Android is stored in the shared module. KMM provides a special wizard for creating such modules.

In your Android project, create a KMM shared module for your cross-platform code. Later you'll connect it to your existing Android application and your future iOS application.

  1. In Android Studio, click File | New | New Module.

  2. In the list of templates, select KMM Shared Module, enter the module name shared, and select the Regular framework in the list of iOS framework distribution options.
    This is required for connecting the shared module to the iOS application.

    KMM shared module
  3. Click Finish.

The wizard will create the KMM shared module, update the configuration files, and create files with classes that demonstrate the benefits of Kotlin Multiplatform. You can learn more about the KMM project structure.

Add a dependency on the shared module to your Android application

To use cross-platform code in your Android application, connect the shared module to it, move the business logic code there, and make this code cross-platform.

  1. Ensure that compileSdkVersion and minSdkVersion in build.gradle.kts of the shared module are the same as those in the build.gradle of your Android application in the app module.
    If they are different, update them in the build.gradle.kts of the shared module. Otherwise, you'll encounter a compile error.

  2. Add a dependency on the shared module to the build.gradle of your Android application.

    dependencies { implementation project(':shared') }
  3. Synchronize the Gradle files by clicking Sync Now in the warning.

    Synchronize the Gradle files
  4. To make sure that the shared module is successfully connected to your application, dump the greeting() function result to the log by updating the onCreate() method of the LoginActivity class.

    override fun onCreate(savedInstanceState: Bundle?) { super.onCreate(savedInstanceState) Log.i("Login Activity", "Hello from shared module: " + (Greeting().greeting())) }
  5. Search for Hello in the log, and you'll find the greeting from the shared module.

    Greeting from the shared module

Make the business logic cross-platform

You can now extract the business logic code to the KMM shared module and make it platform-independent. This is necessary for reusing the code for both Android and iOS.

  1. Move the business logic code com.jetbrains.simplelogin.androidapp.data from the app directory to the com.jetbrains.simplelogin.shared package in the shared/src/commonMain directory. You can drag and drop the package or refactor it by moving everything from one directory to another.

    Drag and drop the package with the business logic code
  2. When Android Studio asks what you'd like to do, select to move the package, and then approve the refactoring.

    Refactor the business logic package
  3. Ignore all warnings about platform-dependent code and click Continue.

    Warnings about platform-dependent code
  4. Remove Android-specific code by replacing it with cross-platform Kotlin code or connecting to Android-specific APIs using expect and actual declarations. See the following sections for details.

Replace Android-specific code with cross-platform code

To make your code work well on both Android and iOS, replace all JVM dependencies with Kotlin dependencies wherever possible.

  1. In the login() function of the LoginDataSource class, replace IOException, which is not available in Kotlin, with RuntimeException.

    // Before return Result.Error(IOException("Error logging in", e))
    //After return Result.Error(RuntimeException("Error logging in", e))
  2. For email validation, replace the Patterns class from the android.utils package with a Kotlin regular expression matching the pattern in the LoginDataValidator class:

    // Before private fun isEmailValid(email: String) = Patterns.EMAIL_ADDRESS.matcher(email).matches()
    // After private fun isEmailValid(email: String) = emailRegex.matches(email) companion object { private val emailRegex = ("[a-zA-Z0-9\\+\\.\\_\\%\\-\\+]{1,256}" + "\\@" + "[a-zA-Z0-9][a-zA-Z0-9\\-]{0,64}" + "(" + "\\." + "[a-zA-Z0-9][a-zA-Z0-9\\-]{0,25}" + ")+").toRegex() }

Connect to platform-specific APIs from the cross-platform code

A universally unique identifier (UUID) for fakeUser in LoginDataSource is generated using the java.util.UUID class, which is not available for iOS.

val fakeUser = LoggedInUser(java.util.UUID.randomUUID().toString(), "Jane Doe")

Since the Kotlin standard library doesn't provide functionality for generating UUIDs, you still need to use platform-specific functionality for this case.

Provide the expect declaration for the randomUUID() function in the shared code and its actual implementations for each platform – Android and iOS – in the corresponding source sets. You can learn more about connecting to platform-specific APIs.

  1. Remove the java.util.UUID class from the common code:

    val fakeUser = LoggedInUser(randomUUID(), "Jane Doe")
  2. Create a Utils.kt file in the shared/src/commonMain directory and provide the expect declaration:

    package com.jetbrains.simplelogin.shared expect fun randomUUID(): String
  3. Create a Utils.kt file in the shared/src/androidMain directory and provide the actual implementation for randomUUID() in Android:

    package com.jetbrains.simplelogin.shared import java.util.* actual fun randomUUID() = UUID.randomUUID().toString()
  4. Create a Utils.kt file in the shared/src/iosMain directory and provide the actual implementation for randomUUID() in iOS:

    package com.jetbrains.simplelogin.shared import platform.Foundation.NSUUID actual fun randomUUID(): String = NSUUID().UUIDString()

For Android and iOS, Kotlin will use different platform-specific implementations.

Run your cross-platform application on Android

Run your cross-platform application for Android to make sure it works.

Android login application

Make your cross-platform application work on iOS

Once you've made your Android application cross-platform, you can create an iOS application and reuse the shared business logic in it.

  1. Create an iOS project in Xcode.

  2. Connect the framework to your iOS project.

  3. Use the shared module from Swift.

Create an iOS project in Xcode

  1. In Xcode, click File | New | Project.

  2. Select a template for an iOS app and click Next.

    iOS project template
  3. As the product name, specify simpleLoginIOS and click Next.

    iOS project settings
  4. As the location for your project, select the directory that stores your cross-platform application, for example, kmm-integrate-into-existing-app.

In Android Studio, you'll get the following structure:

iOS project in Android Studio

You can rename the simpleLoginIOS directory to iosApp for consistency with other top-level directories of your cross-platform project.

Renamed iOS project directory in Android Studio

Connect the framework to your iOS project

Once you have the framework, you can connect it to your iOS project manually.

Connect your framework to the iOS project manually:

  1. In Xcode, open the iOS project settings by double-clicking the project name.

  2. On the Build Phases tab of the project settings, click the + and add New Run Script Phase.

    Add run script phase
  3. Add the following script:

    cd "$SRCROOT/.." ./gradlew :shared:embedAndSignAppleFrameworkForXcode
    Add the script
  4. Move the Run Script phase before the Compile Sources phase.

    Move the Run Script phase
  5. On the Build Settings tab, specify the Framework Search Path under Search Paths$(SRCROOT)/../shared/build/xcode-frameworks/$(CONFIGURATION)/$(SDK_NAME).

    Framework search path
  6. On the Build Settings tab, specify the Other Linker flags under Linking$(inherited) -framework shared

    Linker flag
  7. Build the project in Xcode. If everything is set up correctly, the project will successfully build.

Use the shared module from Swift

  1. In Xcode, open the ContentView.swift file and import the shared module.

    import shared
  2. To check that it is properly connected, use the greeting() function from the KMM module:

    import SwiftUI import shared struct ContentView: View { var body: some View { Text(Greeting().greeting()) .padding() } }
    Greeting from the KMM module
  3. In ContentView.swift, write code for using data from the KMM module and rendering the application UI.

  4. In simpleLoginIOSApp.swift, import the shared module and specify the arguments for the ContentView() function:

    import SwiftUI import shared @main struct SimpleLoginIOSApp: App { var body: some Scene { WindowGroup { ContentView(viewModel: .init(loginRepository: LoginRepository(dataSource: LoginDataSource()), loginValidator: LoginDataValidator())) } } }
    Simple login application

Enjoy the results – update the logic only once

Now your application is cross-platform. You can update the business logic in one place and see results on both Android and iOS.

  1. In Android Studio, change the validation logic for a user's password in the checkPassword() function of the LoginDataValidator class:

    package com.jetbrains.simplelogin.shared.data class LoginDataValidator { //... fun checkPassword(password: String): Result { return when { password.length < 5 -> Result.Error("Password must be >5 characters") password.toLowerCase() == "password" -> Result.Error("Password shouldn't be \"password\"") else -> Result.Success } } //... }
  2. Update gradle.properties to connect your iOS application to Android Studio for running it on a simulated or real device right there:

  3. Synchronize the Gradle files by clicking Sync Now in the warning.

    Synchronize the Gradle files

You will see the new run configuration simpleLoginIOS for running your iOS application right from Android Studio.

iOS run configuration
iOS application password error
Android application password error

You can review the final code for this tutorial.

What else to share?

You've shared the business logic of your application, but you can also decide to share other layers of your application. For example, the ViewModel class code is almost the same for Android and iOS applications, and you can share it if your mobile applications should have the same presentation layer.

What's next?

Last modified: 10 November 2021