Câu hỏi phỏng vấn Architecture & Design Patterns

Phần này tổng hợp các câu hỏi về Architecture patterns và Design principles trong Android development.

1. MVVM (Model-View-ViewModel)

Q: MVVM pattern là gì? Giải thích từng layer?

Trả lời:

MVVM là architecture pattern tách biệt UI logic khỏi business logic:

Layer	Responsibility	Android Components
Model	Data + business logic	Repository, Room, Retrofit
View	Display UI, handle input	Activity, Fragment, Composable
ViewModel	Hold UI state, transform data	ViewModel class


// Model (Repository)
class UserRepository(private val api: UserApi) {
    suspend fun getUser(id: String): User = api.fetchUser(id)
}
 
// ViewModel
class UserViewModel(private val repository: UserRepository) : ViewModel() {
    
    private val _uiState = MutableStateFlow<UiState>(UiState.Loading)
    val uiState: StateFlow<UiState> = _uiState.asStateFlow()
    
    fun loadUser(id: String) {
        viewModelScope.launch {
            _uiState.value = UiState.Loading
            try {
                val user = repository.getUser(id)
                _uiState.value = UiState.Success(user)
            } catch (e: Exception) {
                _uiState.value = UiState.Error(e.message)
            }
        }
    }
}
 
// View (Compose)
@Composable
fun UserScreen(viewModel: UserViewModel) {
    val uiState by viewModel.uiState.collectAsStateWithLifecycle()
    
    when (uiState) {
        is UiState.Loading -> CircularProgressIndicator()
        is UiState.Success -> UserContent(uiState.user)
        is UiState.Error -> ErrorText(uiState.message)
    }
}

Lợi ích:

✅ Separation of concerns
✅ Testable business logic
✅ Survives configuration changes

📚 Tìm hiểu thêm: MVVM trong Android

Q: ViewModel survive configuration change như thế nào?

Trả lời:

ViewModelStore được retain bởi ViewModelStoreOwner (Activity/Fragment) qua configuration changes.

Internal mechanism:

ViewModelStore lưu trữ ViewModel instances theo key
Khi configuration change, NonConfigurationInstances giữ reference đến ViewModelStore
Activity mới nhận lại cùng ViewModelStore


// ViewModel tồn tại qua rotation
class MainViewModel : ViewModel() {
    private val _data = MutableStateFlow<List<Item>>(emptyList())
    val data = _data.asStateFlow()
    
    init {
        loadData()  // Called only once, not on rotation
    }
    
    override fun onCleared() {
        // Called when Activity is truly finishing
    }
}

📚 Tìm hiểu thêm: ViewModel

2. MVI (Model-View-Intent)

Q: MVI khác gì MVVM?

Trả lời:

MVI thêm unidirectional data flow với Intent (user actions):

Aspect	MVVM	MVI
Data Flow	Bidirectional	Unidirectional
State	Multiple LiveData/Flows	Single immutable state
Events	Direct function calls	Intent objects
Complexity	Simpler	More structured


// Intent (User Actions)
sealed class UserIntent {
    data class LoadUser(val id: String) : UserIntent()
    object Refresh : UserIntent()
    data class UpdateName(val name: String) : UserIntent()
}
 
// State (Single source of truth)
data class UserState(
    val isLoading: Boolean = false,
    val user: User? = null,
    val error: String? = null
)
 
// ViewModel processes intents
class UserViewModel : ViewModel() {
    
    private val _state = MutableStateFlow(UserState())
    val state = _state.asStateFlow()
    
    fun processIntent(intent: UserIntent) {
        when (intent) {
            is UserIntent.LoadUser -> loadUser(intent.id)
            is UserIntent.Refresh -> refresh()
            is UserIntent.UpdateName -> updateName(intent.name)
        }
    }
    
    private fun loadUser(id: String) {
        viewModelScope.launch {
            _state.update { it.copy(isLoading = true) }
            try {
                val user = repository.getUser(id)
                _state.update { it.copy(isLoading = false, user = user) }
            } catch (e: Exception) {
                _state.update { it.copy(isLoading = false, error = e.message) }
            }
        }
    }
}
 
// View sends intents
@Composable
fun UserScreen(viewModel: UserViewModel) {
    val state by viewModel.state.collectAsStateWithLifecycle()
    
    LaunchedEffect(Unit) {
        viewModel.processIntent(UserIntent.LoadUser("123"))
    }
    
    Button(onClick = { viewModel.processIntent(UserIntent.Refresh) }) {
        Text("Refresh")
    }
}

Khi nào dùng MVI:

✅ Complex state với nhiều interactions
✅ Need time-travel debugging
✅ Team prefers strict patterns

3. Clean Architecture

Q: Clean Architecture trong Android?

Trả lời:

Clean Architecture chia app thành layers với dependency rule: outer layers depend on inner layers.

Layer	Contents	Dependencies
Domain	Use Cases, Entities, Repository interfaces	None (pure Kotlin)
Data	Repository impl, Data Sources, Mappers	Domain
Presentation	UI, ViewModels	Domain


// Domain Layer - Pure Kotlin, no Android dependencies
data class User(val id: String, val name: String)
 
interface UserRepository {
    suspend fun getUser(id: String): User
    suspend fun saveUser(user: User)
}
 
class GetUserUseCase(private val repository: UserRepository) {
    suspend operator fun invoke(id: String): Result<User> {
        return runCatching { repository.getUser(id) }
    }
}
 
// Data Layer
class UserRepositoryImpl(
    private val api: UserApi,
    private val dao: UserDao
) : UserRepository {
    
    override suspend fun getUser(id: String): User {
        return try {
            api.fetchUser(id).also { dao.insert(it.toEntity()) }
        } catch (e: Exception) {
            dao.getById(id)?.toUser() ?: throw e
        }
    }
}
 
// Presentation Layer
class UserViewModel(
    private val getUserUseCase: GetUserUseCase
) : ViewModel() {
    
    fun loadUser(id: String) {
        viewModelScope.launch {
            getUserUseCase(id)
                .onSuccess { _user.value = it }
                .onFailure { _error.value = it.message }
        }
    }
}

📚 Tìm hiểu thêm: Clean Architecture

4. Dependency Injection

Q: Dependency Injection là gì? Hilt vs Koin?

Trả lời:

DI là pattern cung cấp dependencies từ bên ngoài thay vì tạo trong class.


// ❌ Without DI - tight coupling
class UserViewModel {
    private val repository = UserRepository(
        UserApi(RetrofitClient.create()),
        UserDatabase.getInstance(context).userDao()
    )
}
 
// ✅ With DI - loose coupling
class UserViewModel(
    private val repository: UserRepository  // Injected
) : ViewModel()

Hilt vs Koin:

Aspect	Hilt	Koin
Type	Compile-time	Runtime
Error Detection	Compile time	Runtime
Performance	Faster (no reflection)	Slower (uses reflection)
Setup	More boilerplate	Simpler
Google Support	✅ Official	Community

Hilt Example:


// Module
@Module
@InstallIn(SingletonComponent::class)
object AppModule {
    
    @Provides
    @Singleton
    fun provideUserRepository(
        api: UserApi,
        dao: UserDao
    ): UserRepository = UserRepositoryImpl(api, dao)
}
 
// ViewModel
@HiltViewModel
class UserViewModel @Inject constructor(
    private val repository: UserRepository
) : ViewModel()
 
// Activity
@AndroidEntryPoint
class MainActivity : ComponentActivity()

Koin Example:


// Module
val appModule = module {
    single<UserRepository> { UserRepositoryImpl(get(), get()) }
    viewModel { UserViewModel(get()) }
}
 
// Application
class MyApp : Application() {
    override fun onCreate() {
        super.onCreate()
        startKoin {
            modules(appModule)
        }
    }
}
 
// Usage
class UserViewModel(
    private val repository: UserRepository
) : ViewModel()

📚 Tìm hiểu thêm: Dependency Injection, Koin

5. Repository Pattern

Q: Repository pattern là gì?

Trả lời:

Repository là abstraction layer giữa data sources và business logic:

Responsibilities:

Abstract data sources
Implement caching strategy
Handle offline-first logic
Map DTOs to domain entities


class UserRepository(
    private val api: UserApi,
    private val dao: UserDao,
    private val cache: UserCache
) {
    // Single source of truth pattern
    fun getUsers(): Flow<List<User>> = flow {
        // 1. Emit cached data first
        cache.getUsers()?.let { emit(it) }
        
        // 2. Fetch from network
        try {
            val users = api.fetchUsers()
            dao.insertAll(users.map { it.toEntity() })
            cache.setUsers(users)
            emit(users)
        } catch (e: Exception) {
            // 3. Fallback to local DB
            emit(dao.getAll().map { it.toUser() })
        }
    }
    
    // Offline-first with NetworkBoundResource
    fun getUser(id: String): Flow<Resource<User>> = networkBoundResource(
        query = { dao.getById(id).map { it?.toUser() } },
        fetch = { api.fetchUser(id) },
        saveFetchResult = { dao.insert(it.toEntity()) },
        shouldFetch = { it == null || cache.isExpired(id) }
    )
}

6. Design Principles

Q: SOLID principles trong Android?

Trả lời:

Principle	Description	Android Example
Single Responsibility	One class, one job	Separate ViewModel, Repository
Open/Closed	Open for extension, closed for modification	Use interfaces
Liskov Substitution	Subtypes replaceable	Repository implementations
Interface Segregation	Small, focused interfaces	Separate Dao interfaces
Dependency Inversion	Depend on abstractions	Inject interfaces, not implementations


// Single Responsibility
class UserRepository { /* only data operations */ }
class UserValidator { /* only validation */ }
class UserMapper { /* only mapping */ }
 
// Open/Closed - extend via interfaces
interface DataSource<T> {
    suspend fun get(id: String): T?
    suspend fun save(item: T)
}
 
class ApiDataSource : DataSource<UserDto> { /* ... */ }
class LocalDataSource : DataSource<UserEntity> { /* ... */ }
 
// Dependency Inversion
class UserViewModel(
    private val repository: UserRepository  // Interface, not UserRepositoryImpl
) : ViewModel()

Q: Separation of Concerns trong Android?

Trả lời:

Mỗi component chỉ làm một việc:

Component	Concern	Should NOT
Activity/Fragment	Host UI, handle navigation	Business logic, data fetching
Composable	Render UI	Direct API calls
ViewModel	UI state, orchestrate	Database access directly
Repository	Data operations	UI updates
Use Case	Business rules	Data persistence


// ❌ Bad - Activity does everything
class BadActivity : AppCompatActivity() {
    override fun onCreate(savedInstanceState: Bundle?) {
        val users = Retrofit.create(UserApi::class.java)
            .fetchUsers()  // Network in Activity!
        
        Room.databaseBuilder(this, AppDb::class.java, "db")
            .build().userDao().insertAll(users)  // DB in Activity!
    }
}
 
// ✅ Good - Separation of concerns
@Composable
fun UserScreen(viewModel: UserViewModel = hiltViewModel()) {
    val users by viewModel.users.collectAsStateWithLifecycle()
    
    LazyColumn {
        items(users) { UserItem(it) }
    }
}
 
class UserViewModel(private val repository: UserRepository) : ViewModel() {
    val users = repository.getUsers().stateIn(
        viewModelScope, SharingStarted.WhileSubscribed(), emptyList()
    )
}

📝 Architecture Decision Guide

Scenario	Recommended
Small app, simple state	MVVM
Complex state, many interactions	MVI
Large team, maintainability	Clean Architecture
Quick prototype	MVVM + Koin
Production app, Google stack	MVVM + Hilt
Multiple data sources	Repository pattern

📚 Tìm hiểu thêm: MVVM, Clean Architecture