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. The Kotlin Multiplatform plugin provides a special wizard for creating such modules.

In your Android project, create a Kotlin Multiplatform 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 Kotlin Multiplatform 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 Kotlin Multiplatform shared module, update the configuration files, and create files with classes that demonstrate the benefits of Kotlin Multiplatform. You can learn more about the 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. In the build.gradle.kts file of the shared module, ensure that compileSdk and minSdk are the same as those in the build.gradle.kts of your Android application in the app module.

   If they're 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.kts of your Android application.

   dependencies {
       implementation (project(":shared"))
   }
   

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

   

4. In the app/src/main/java/ directory, open the LoginActivity class in the com.jetbrains.simplelogin.androidapp.ui.login package.

5. To make sure that the shared module is successfully connected to your application, dump the greet() function result to the log by updating the onCreate() method:

   override fun onCreate(savedInstanceState: Bundle?) {
       super.onCreate(savedInstanceState)
   
       Log.i("Login Activity", "Hello from shared module: " + (Greeting().greet()))
   }
   

6. Follow Android Studio suggestions to import missing classes.

7. Debug the app. On the Logcat tab, 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 Kotlin Multiplatform 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-integration-sample.

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, switch to All build settings and 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 greet() function from the shared module of your cross-platform app:

   import SwiftUI
   import shared
   
   struct ContentView: View {
       var body: some View {
           Text(Greeting().greet())
           .padding()
       }
   }
   

   

3. In ContentView.swift, write code for using data from the shared module and rendering the application UI:

   import SwiftUI import shared struct ContentView: View { @State private var username: String = "" @State private var password: String = "" @ObservedObject var viewModel: ContentView.ViewModel var body: some View { VStack(spacing: 15.0) { ValidatedTextField(titleKey: "Username", secured: false, text: $username, errorMessage: viewModel.formState.usernameError, onChange: { viewModel.loginDataChanged(username: username, password: password) }) ValidatedTextField(titleKey: "Password", secured: true, text: $password, errorMessage: viewModel.formState.passwordError, onChange: { viewModel.loginDataChanged(username: username, password: password) }) Button("Login") { viewModel.login(username: username, password: password) }.disabled(!viewModel.formState.isDataValid || (username.isEmpty && password.isEmpty)) } .padding(.all) } } struct ValidatedTextField: View { let titleKey: String let secured: Bool @Binding var text: String let errorMessage: String? let onChange: () -> () @ViewBuilder var textField: some View { if secured { SecureField(titleKey, text: $text) } else { TextField(titleKey, text: $text) } } var body: some View { ZStack { textField .textFieldStyle(RoundedBorderTextFieldStyle()) .autocapitalization(.none) .onChange(of: text) { _ in onChange() } if let errorMessage = errorMessage { HStack { Spacer() FieldTextErrorHint(error: errorMessage) }.padding(.horizontal, 5) } } } } struct FieldTextErrorHint: View { let error: String @State private var showingAlert = false var body: some View { Button(action: { self.showingAlert = true }) { Image(systemName: "exclamationmark.triangle.fill") .foregroundColor(.red) } .alert(isPresented: $showingAlert) { Alert(title: Text("Error"), message: Text(error), dismissButton: .default(Text("Got it!"))) } } } extension ContentView { struct LoginFormState { let usernameError: String? let passwordError: String? var isDataValid: Bool { get { return usernameError == nil && passwordError == nil } } } class ViewModel: ObservableObject { @Published var formState = LoginFormState(usernameError: nil, passwordError: nil) let loginValidator: LoginDataValidator let loginRepository: LoginRepository init(loginRepository: LoginRepository, loginValidator: LoginDataValidator) { self.loginRepository = loginRepository self.loginValidator = loginValidator } func login(username: String, password: String) { if let result = loginRepository.login(username: username, password: password) as? ResultSuccess { print("Successful login. Welcome, \(result.data.displayName)") } else { print("Error while logging in") } } func loginDataChanged(username: String, password: String) { formState = LoginFormState( usernameError: (loginValidator.checkUsername(username: username) as? LoginDataValidator.ResultError)?.message, passwordError: (loginValidator.checkPassword(password: password) as? LoginDataValidator.ResultError)?.message) } } }

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()))
           }
       }
   }