android-patterns

Android Development Patterns

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

Kotlin Idioms

Immutability

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

// ✅ Read-only collection types (List/Set/Map are read-only, NOT immutable:
// a MutableList held elsewhere can still change behind this reference)
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 displayName = 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 transformed = 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 (receiver must expose vars;
// a data class of vals is configured with copy(), not apply)
val intent = Intent(context, DetailActivity::class.java).apply {
    putExtra(EXTRA_USER_ID, userId)
    flags = Intent.FLAG_ACTIVITY_CLEAR_TOP
}

// also - Side effects, return self
val user = repository.createUser(name).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) } }
        }
    }
}