Compose Multiplatform Architecture: Patterns in Jetpack Compose

With the rise of Kotlin Multiplatform Mobile (KMM), the ability to share code across multiple platforms like Android and iOS has become a game-changer for developers. Jetpack Compose, Google’s modern UI toolkit, now supports multiplatform development, making it easier than ever to create beautiful and functional cross-platform applications. Selecting the right architecture pattern is crucial for building maintainable, scalable, and testable Compose Multiplatform apps. This post explores the popular architecture patterns you can use to structure your Compose Multiplatform projects.

Understanding Compose Multiplatform

Compose Multiplatform allows developers to write UI code once and deploy it on multiple platforms. This drastically reduces development time and ensures a consistent user experience across different devices. Key advantages include:

• Code Reusability: Share UI and business logic across platforms.
• Native Performance: Compile code to native binaries for optimal performance on each platform.
• Consistent UI: Maintain a unified look and feel across all platforms.

Popular Architecture Patterns for Compose Multiplatform

Choosing the right architecture is essential for managing complexity and ensuring code quality. Here are several popular patterns you can use with Compose Multiplatform:

1. Model-View-Intent (MVI)

MVI is a reactive architecture pattern that enforces a unidirectional data flow. It’s particularly well-suited for complex UIs and state management.

Key Components of MVI:

• Model (State): Represents the current state of the UI. It’s immutable and can only be updated via a new state.
• View (UI): Renders the UI based on the current state. Dispatches intents to the ViewModel based on user interactions.
• Intent: Represents a user’s action or intention. These are dispatched from the View to the ViewModel.
• ViewModel (Presenter): Processes intents, updates the state, and emits new states to the View.

Implementation Example:

First, let’s define the state:

data class CounterState(val count: Int = 0)

Next, the intent:

sealed class CounterIntent {
    object Increment : CounterIntent()
    object Decrement : CounterIntent()
}

And finally, the ViewModel:

import kotlinx.coroutines.flow.MutableStateFlow
import kotlinx.coroutines.flow.StateFlow
import kotlinx.coroutines.flow.update
import androidx.lifecycle.ViewModel
import androidx.lifecycle.viewModelScope
import kotlinx.coroutines.launch

class CounterViewModel : ViewModel() {
    private val _state = MutableStateFlow(CounterState())
    val state: StateFlow = _state

    fun processIntent(intent: CounterIntent) {
        viewModelScope.launch {
            when (intent) {
                CounterIntent.Increment -> incrementCounter()
                CounterIntent.Decrement -> decrementCounter()
            }
        }
    }

    private fun incrementCounter() {
        _state.update { currentState ->
            currentState.copy(count = currentState.count + 1)
        }
    }

    private fun decrementCounter() {
        _state.update { currentState ->
            currentState.copy(count = currentState.count - 1)
        }
    }
}

In the Compose UI:

import androidx.compose.runtime.collectAsState
import androidx.compose.ui.tooling.preview.Preview
import androidx.lifecycle.viewmodel.compose.viewModel
import androidx.compose.material.Button
import androidx.compose.material.Text
import androidx.compose.runtime.Composable

@Composable
fun CounterView(viewModel: CounterViewModel = viewModel()) {
    val state = viewModel.state.collectAsState()

    Column {
        Text(text = "Count: ${state.value.count}")
        Row {
            Button(onClick = { viewModel.processIntent(CounterIntent.Increment) }) {
                Text("Increment")
            }
            Button(onClick = { viewModel.processIntent(CounterIntent.Decrement) }) {
                Text("Decrement")
            }
        }
    }
}

@Preview
@Composable
fun PreviewCounterView() {
    CounterView()
}

Advantages of MVI:

• Unidirectional Data Flow: Simplifies state management and makes debugging easier.
• Immutability: Ensures predictable state updates.
• Testability: Components are highly testable due to clear input-output relationships.

Disadvantages of MVI:

• Complexity: Can be overkill for simple UIs.
• Boilerplate: Requires writing a considerable amount of code.

2. Model-View-ViewModel (MVVM)

MVVM is another popular architecture pattern that separates the UI (View) from the data and business logic (ViewModel).

Key Components of MVVM:

• Model: Represents the data.
• View (UI): Displays the data and notifies the ViewModel of user actions.
• ViewModel: Exposes data streams for the View and processes user commands.

Implementation Example:

Define the ViewModel:

import androidx.lifecycle.LiveData
import androidx.lifecycle.MutableLiveData
import androidx.lifecycle.ViewModel

class MyViewModel : ViewModel() {
    private val _counter = MutableLiveData(0)
    val counter: LiveData = _counter

    fun increment() {
        _counter.value = (_counter.value ?: 0) + 1
    }
}

In the Compose UI:

import androidx.compose.runtime.livedata.observeAsState
import androidx.compose.ui.platform.LocalLifecycleOwner
import androidx.lifecycle.viewmodel.compose.viewModel
import androidx.compose.material.Button
import androidx.compose.material.Text
import androidx.compose.runtime.Composable
import androidx.compose.runtime.getValue
import androidx.compose.ui.tooling.preview.Preview

@Composable
fun MyScreen(viewModel: MyViewModel = viewModel()) {
    val counter by viewModel.counter.observeAsState(0)

    Column {
        Text("Counter: $counter")
        Button(onClick = { viewModel.increment() }) {
            Text("Increment")
        }
    }
}

@Preview
@Composable
fun PreviewMyScreen() {
    MyScreen()
}

Advantages of MVVM:

• Separation of Concerns: Simplifies UI development and makes it easier to maintain.
• Testability: ViewModels are easy to unit test.
• Reusability: ViewModels can be reused across different views.

Disadvantages of MVVM:

• Complexity: Requires a good understanding of data binding and LiveData or StateFlow.
• Boilerplate: Can result in some boilerplate code, especially for simple UIs.

3. Redux

Redux is a predictable state container for JavaScript apps but is increasingly used in other environments, including Kotlin Multiplatform. It’s based on a unidirectional data flow and a single source of truth.

Key Components of Redux:

• State: The single source of truth that represents the application’s state.
• Actions: Plain objects that describe an intention to change the state.
• Reducer: A pure function that takes the previous state and an action, and returns the new state.
• Store: Holds the application’s state, allows access to the state, and dispatches actions.

Implementation Example:

First, define the state:

data class AppState(val counter: Int = 0)

Next, the actions:

sealed class AppAction {
    object Increment : AppAction()
    object Decrement : AppAction()
}

The reducer:

fun appReducer(state: AppState, action: AppAction): AppState {
    return when (action) {
        AppAction.Increment -> state.copy(counter = state.counter + 1)
        AppAction.Decrement -> state.copy(counter = state.counter - 1)
    }
}

And the store (using a simplified implementation):

import kotlinx.coroutines.flow.MutableStateFlow
import kotlinx.coroutines.flow.StateFlow
import kotlinx.coroutines.flow.asStateFlow
import kotlinx.coroutines.flow.update

class Store(
    initialState: AppState,
    private val reducer: (AppState, AppAction) -> AppState
) {
    private val _state = MutableStateFlow(initialState)
    val state: StateFlow = _state.asStateFlow()

    fun dispatch(action: AppAction) {
        _state.update { currentState ->
            reducer(currentState, action)
        }
    }
}

In the Compose UI:

import androidx.compose.runtime.collectAsState
import androidx.compose.runtime.remember
import androidx.compose.ui.tooling.preview.Preview
import androidx.compose.material.Button
import androidx.compose.material.Text
import androidx.compose.runtime.Composable
import androidx.compose.runtime.getValue
import androidx.compose.ui.tooling.preview.Preview

@Composable
fun ReduxCounterView() {
    val store = remember { Store(AppState(), ::appReducer) }
    val state by store.state.collectAsState()

    Column {
        Text("Counter: ${state.counter}")
        Row {
            Button(onClick = { store.dispatch(AppAction.Increment) }) {
                Text("Increment")
            }
            Button(onClick = { store.dispatch(AppAction.Decrement) }) {
                Text("Decrement")
            }
        }
    }
}

@Preview
@Composable
fun PreviewReduxCounterView() {
    ReduxCounterView()
}

Advantages of Redux:

• Predictable State Management: Makes debugging and testing easier.
• Centralized State: Simplifies managing complex state.
• Time-Travel Debugging: Allows you to step through state changes.

Disadvantages of Redux:

• Boilerplate: Requires a lot of boilerplate code.
• Steep Learning Curve: Can be challenging to understand and implement correctly.

Considerations for Choosing an Architecture

When selecting an architecture pattern, consider the following factors:

• Complexity of the UI: Simpler UIs may not require complex patterns like MVI or Redux.
• Team Size: Larger teams may benefit from stricter architectures that enforce consistency.
• Testability: Choose an architecture that promotes testability.
• Performance: Some architectures may introduce overhead, so consider the performance implications.

Conclusion

Choosing the right architecture pattern is a critical step in building successful Compose Multiplatform applications. Patterns like MVI, MVVM, and Redux offer different trade-offs in terms of complexity, testability, and performance. By carefully considering the needs of your project, you can select the pattern that will help you build maintainable, scalable, and testable cross-platform applications with Jetpack Compose.