Make your Android application work on iOS – tutorial

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.

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.

   

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.

   

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.

   

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.

   

2. When Android Studio asks what you'd like to do, select to move the package, and then approve the refactoring.

   

3. Ignore all warnings about platform-dependent code and click Continue.

   

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.

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

Run your cross-platform application on Android

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

Create an iOS project in Xcode

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

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

   

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

   

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:

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

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.

   

3. Add the following script:

   cd "$SRCROOT/.." ./gradlew :shared:embedAndSignAppleFrameworkForXcode 

   

4. Move the Run Script phase before the Compile Sources phase.

   

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

   

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

   

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() } } 

   

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())) } } }