android-patterns

name	android-patterns
description	Core Android development patterns for Kotlin, including coroutines, lifecycle management, and functional programming idioms.

Android Development Patterns

Modern Android patterns with Kotlin, coroutines, and functional programming.

Kotlin Idioms

Immutability

// ✅ Prefer val over var
val name: String = "John"

// ✅ Immutable collections
val items: List<Item> = listOf(item1, item2)

// ✅ Data class with copy
data class User(val id: String, val name: String)
val updatedUser = user.copy(name = "Jane")

Null Safety

// ✅ Safe call
val length = name?.length

// ✅ Elvis operator
val name = nullableName ?: "Unknown"

// ✅ let for null checks
nullableUser?.let { user ->
    processUser(user)
}

// ✅ Early return with null check
fun processUser(user: User?) {
    user ?: return
    // user is smart-cast to non-null
}

Scope Functions

// let - Transform and return
val result = nullable?.let { transform(it) }

// run - Configure and return result
val result = service.run {
    configure()
    execute()
}

// with - Operate on object
with(binding) {
    title.text = "Title"
    subtitle.text = "Subtitle"
}

// apply - Configure and return self
val user = User().apply {
    name = "John"
    email = "[e-mail]"
}

// also - Side effects, return self
val user = User().also {
    logger.log("Created user: ${it.id}")
}

Extensions

// ✅ Extension functions
fun String.isValidEmail(): Boolean {
    return android.util.Patterns.EMAIL_ADDRESS.matcher(this).matches()
}

// ✅ Extension properties
val Context.screenWidth: Int
    get() = resources.displayMetrics.widthPixels

// Usage
if (email.isValidEmail()) { ... }
val width = context.screenWidth

Lifecycle Patterns

ViewModel

class HomeViewModel(
    private val repository: HomeRepository,
    savedStateHandle: SavedStateHandle
) : ViewModel() {

    private val _state = MutableStateFlow(HomeState())
    val state: StateFlow<HomeState> = _state.asStateFlow()
    
    init {
        loadData()
    }
    
    private fun loadData() {
        viewModelScope.launch {
            _state.update { it.copy(isLoading = true) }
            
            repository.getItems()
                .onSuccess { items -> _state.update { it.copy(items = items, isLoading = false) } }
                .onFailure { error -> _state.update { it.copy(error = error.message, isLoading = false) } }
        }
    }
}

Lifecycle-aware Collection

@Composable
fun HomeScreen(viewModel: HomeViewModel = koinViewModel()) {
    val state by viewModel.state.collectAsStateWithLifecycle()
    
    HomeContent(state = state)
}

Functional Patterns

Result Type

// ✅ Use Result for operations that can fail
suspend fun fetchUser(id: String): Result<User> = runCatching {
    api.getUser(id).toDomain()
}

// ✅ Chain operations
repository.fetchUser(id)
    .map { it.profile }
    .mapCatching { decryptProfile(it) }
    .onSuccess { displayProfile(it) }
    .onFailure { showError(it) }

Higher-Order Functions

// ✅ Pass functions as parameters
fun <T> retry(
    times: Int,
    block: suspend () -> T
): T {
    repeat(times - 1) {
        try { return block() }
        catch (e: Exception) { delay(1000) }
    }
    return block() // Last attempt
}

// Usage
val result = retry(3) { api.fetchData() }

Sealed Classes

sealed interface UiState<out T> {
    data object Loading : UiState<Nothing>
    data class Success<T>(val data: T) : UiState<T>
    data class Error(val message: String) : UiState<Nothing>
}

// Exhaustive when
when (state) {
    is UiState.Loading -> LoadingIndicator()
    is UiState.Success -> Content(state.data)
    is UiState.Error -> ErrorMessage(state.message)
}

Resource Management

Context Extensions

fun Context.dp(value: Int): Int = 
    (value * resources.displayMetrics.density).toInt()

fun Context.showToast(message: String) {
    Toast.makeText(this, message, Toast.LENGTH_SHORT).show()
}

String Resources

// strings.xml
<string name="welcome_message">Welcome, %1$s!</string>

// Usage
stringResource(R.string.welcome_message, userName)

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Remember: Kotlin is concise. Embrace its idioms for cleaner, safer code.