Kotlin Multiplatform Development stable Help

Update the user interface

To build the user interface, you'll use the Compose Multiplatform toolkit for the Android part of your project and SwiftUI for the iOS one. These are both declarative UI frameworks, and you'll see similarities in the UI implementations. In both cases, you store the data in the phrases variable and later iterate over it to produce a list of Text items.

Update the Android part

The composeApp module contains an Android application, defines its main activity and the UI views, and uses the shared module as a regular Android library. The UI of the application uses the Compose Multiplatform framework.

Make some changes and see how they are reflected in the UI:

  1. Navigate to the App.kt file in composeApp/src/androidMain/kotlin.

  2. Find the Greeting class invocation. Select the greet() function and use the ⌘ B shortcut to go to the function's declaration. You'll see that it's the same class from the shared module you edited in the previous step.

  3. In Greeting.kt, update the greet() function:

    fun greet(): List<String> = buildList { add(if (Random.nextBoolean()) "Hi!" else "Hello!") add("Guess what this is! > ${platform.name.reversed()}!") }

    Now it returns a list of strings.

  4. Go back to App.kt and update its definition:

    @Composable fun App() { MaterialTheme { val greeting = remember { Greeting().greet() } Column( modifier = Modifier.padding(all = 20.dp), verticalArrangement = Arrangement.spacedBy(8.dp), ) { greeting.forEach { greeting -> Text(greeting) Divider() } } } }

    Here the Column composable shows each of the Text items, adds padding around the content, and adds a space between the list items.

  5. Follow Android Studio's suggestions to import the missing dependencies.

  6. Now you can run the Android app to ensure it displays the list:

    Updated UI of Android multiplatform app

Work with the iOS module in Xcode

iosApp is an Xcode project that builds into an iOS application. It depends on and uses the shared module as an iOS framework. The UI of the app is written in Swift.

Implement the same changes as in the Android app:

  1. Navigate to the iosApp folder in the Project window.

  2. Right-click the iosApp.xcodeproj folder and select Open In | Xcode.

  3. In the ContentView.swift file, select the greet() function and use the ⌃ ⌘ J shortcut to jump to the function's definition.

    You'll see the Objective-C declarations for the Kotlin functions defined in the shared module. Kotlin types are represented as Objective-C types when used from Objective-C/Swift. Here the greet() function returns List<String> in Kotlin and is seen from Swift as returning NSArray<NSString>. For more on type mappings, see Interoperability with Swift/Objective-C.

  4. If you try to run the project, the build will fail. The Swift code that uses the greet() function doesn't compile because its declaration is now different. Change the SwiftUI code to display a list of items:

    struct ContentView: View { let phrases = Greeting().greet() var body: some View { List(phrases, id: \.self) { Text($0) } } }
    • The results of the greet() call are stored in the phrases variable (let in Swift is similar to Kotlin's val).

    • The List function produces a list of Text items.

  5. Run the app to see the changes:

    Updated UI of your iOS multiplatform app

Next step

In the next part of the tutorial, you'll learn about dependencies and add a third-party library to expand the functionality of your project.

Proceed to the next part

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Last modified: 23 January 2024