Module 14 — Kotlin & Android Basics

🎯 Mục tiêu học tập

• Nắm Kotlin fundamentals: null safety, data class, sealed class, scope functions
• Hiểu Android components: Activity lifecycle, Intents, XML layouts
• Biết ViewModel, Coroutines, Flow — so sánh với React patterns
• Đọc/debug được phần Android native side trong dự án React Native

📖 Hướng dẫn học

⚡ Ôn nhanh cho React Native dev

🧭 Vòng lặp học module này

📚 Lý thuyết chi tiết

1. Kotlin Fundamentals

val / var / const

val name = "Linh"       // immutable (như JS const cho primitives)
var age = 25            // mutable (như JS let)
const val API_URL = "https://api.example.com"  // compile-time constant

Null Safety

var name: String = "Linh"    // non-null — KHÔNG thể assign null
var email: String? = null    // nullable — CÓ thể null

// Safe call operator
val length = email?.length        // null nếu email null

// Elvis operator
val len = email?.length ?: 0      // default value nếu null

// Non-null assertion (TRÁNH dùng)
val len2 = email!!.length         // crash nếu null

// Smart cast
if (email != null) {
  println(email.length)           // auto smart-cast thành non-null
}

// let scope function
email?.let { validEmail ->
  sendEmail(validEmail)           // chỉ chạy nếu non-null
}

Data Class

data class User(
  val id: String,
  val name: String,
  val email: String,
  val age: Int = 0
)

val user = User("1", "Linh", "linh@test.com")
val copy = user.copy(age = 25)
val (id, name) = user  // destructuring

Auto-generates: equals(), hashCode(), toString(), copy(), componentN() functions.

Sealed Class vs Enum

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

fun handleResult(result: Result<User>) {
  when (result) {
    is Result.Success -> showUser(result.data)
    is Result.Error -> showError(result.message)
    is Result.Loading -> showLoading()
  }
}

Scope Functions

Function	Object ref	Return	Use case
let	it	Lambda result	Null check + transform
run	this	Lambda result	Object config + compute result
apply	this	Object itself	Object initialization (builder pattern)
also	it	Object itself	Side effects (logging, validation)
with	this	Lambda result	Grouping calls on object

val user = User("1", "Linh", "linh@test.com").apply {
  // this = user, return user
}

user.also {
  Log.d("TAG", "Created user: $it")  // side effect
}

val displayName = user.let {
  "${it.name} (${it.email})"  // transform, return result
}

Extension Functions & Collections

fun String.isValidEmail(): Boolean =
  this.matches(Regex("[a-zA-Z0-9._%+-]+@[a-zA-Z0-9.-]+\\.[a-zA-Z]{2,}"))

val emails = listOf("a@b.com", "invalid", "c@d.org")
val validEmails = emails.filter { it.isValidEmail() }

val users = listOf(user1, user2, user3)
val namesByAge = users
  .filter { it.age > 18 }
  .sortedBy { it.age }
  .map { it.name }
  .joinToString(", ")

2. Android Components

3. Activity Lifecycle

onCreate()     → Activity created (init UI, bind data)
  ↓
onStart()      → Visible (nhưng chưa interactive)
  ↓
onResume()     → Foreground + interactive (user can interact)
  ↓
onPause()      → Partially visible (dialog, multi-window)
  ↓
onStop()       → Not visible (user navigated away)
  ↓
onDestroy()    → Activity destroyed (cleanup)

Configuration Change (rotate screen):
onPause() → onStop() → onDestroy() → onCreate() → onStart() → onResume()

4. XML Layouts

Layout	Mô tả	Khi nào
ConstraintLayout	Flat hierarchy, constraint-based	Complex layouts (recommended)
LinearLayout	Horizontal/Vertical stack	Simple lists, rows
FrameLayout	Stack children on top	Overlays, fragments container

View Binding — type-safe access to views (replace findViewById):

class MainActivity : AppCompatActivity() {
  private lateinit var binding: ActivityMainBinding

  override fun onCreate(savedInstanceState: Bundle?) {
    super.onCreate(savedInstanceState)
    binding = ActivityMainBinding.inflate(layoutInflater)
    setContentView(binding.root)
    binding.btnSubmit.setOnClickListener { submit() }
  }
}

5. RecyclerView

class UserAdapter(private val users: List<User>) :
  RecyclerView.Adapter<UserAdapter.ViewHolder>() {

  class ViewHolder(val binding: ItemUserBinding) :
    RecyclerView.ViewHolder(binding.root)

  override fun onCreateViewHolder(parent: ViewGroup, viewType: Int): ViewHolder {
    val binding = ItemUserBinding.inflate(
      LayoutInflater.from(parent.context), parent, false
    )
    return ViewHolder(binding)
  }

  override fun onBindViewHolder(holder: ViewHolder, position: Int) {
    val user = users[position]
    holder.binding.tvName.text = user.name
    holder.binding.tvEmail.text = user.email
  }

  override fun getItemCount() = users.size
}

DiffUtil: Efficient list updates — calculate diff, animate changes. Tương tự React reconciliation.

6. ViewModel + StateFlow

class UserViewModel : ViewModel() {
  private val _uiState = MutableStateFlow<Result<List<User>>>(Result.Loading)
  val uiState: StateFlow<Result<List<User>>> = _uiState.asStateFlow()

  fun loadUsers() {
    viewModelScope.launch {
      _uiState.value = Result.Loading
      try {
        val users = repository.getUsers()
        _uiState.value = Result.Success(users)
      } catch (e: Exception) {
        _uiState.value = Result.Error(e.message ?: "Unknown error")
      }
    }
  }
}

Tại sao ViewModel: Survive configuration changes (rotate screen). Activity destroy + recreate nhưng ViewModel vẫn sống. Tương tự concept store trong Zustand/Redux.

7. Coroutines

	launch	async
Return	Job (fire and forget)	Deferred<T> (có result)
Use case	Side effects (update UI, save DB)	Parallel computation (cần return value)
Get result	Không	.await()

viewModelScope.launch {
  val user = withContext(Dispatchers.IO) {
    api.getUser(userId)
  }
  _uiState.value = Result.Success(user)
}

// Parallel
viewModelScope.launch {
  val userDeferred = async(Dispatchers.IO) { api.getUser(id) }
  val postsDeferred = async(Dispatchers.IO) { api.getPosts(id) }
  val user = userDeferred.await()
  val posts = postsDeferred.await()
}

Dispatchers

Dispatcher	Thread	Use case
Main	UI thread	Update UI, LiveData
IO	Background (shared pool)	Network, DB, file I/O
Default	CPU-intensive (shared pool)	Sorting, parsing, heavy computation

8. Flow

9. Hilt DI (Basics)

@HiltAndroidApp
class MyApp : Application()

@Module
@InstallIn(SingletonComponent::class)
object NetworkModule {
  @Provides
  @Singleton
  fun provideApiService(): ApiService =
    Retrofit.Builder()
      .baseUrl(BASE_URL)
      .build()
      .create(ApiService::class.java)
}

@HiltViewModel
class UserViewModel @Inject constructor(
  private val repository: UserRepository
) : ViewModel()

10. Android files hay đụng trong React Native

File	Dùng khi	Lưu ý
android/app/build.gradle	Dependency, signing, buildTypes, versionCode	Đổi native dependency thường cần clean/rebuild
AndroidManifest.xml	Permissions, activity, deep link intent-filter	Sai intent-filter làm deep link không vào app
MainActivity.kt	Activity entry point, splash, new architecture config	Tránh làm heavy work trong onCreate
MainApplication.kt	Package/native module registration	Kiểm tra khi autolinking không hoạt động

<uses-permission android:name="android.permission.CAMERA" />

<activity
  android:name=".MainActivity"
  android:exported="true"
  android:windowSoftInputMode="adjustResize">
  <intent-filter>
    <action android:name="android.intent.action.VIEW" />
    <category android:name="android.intent.category.DEFAULT" />
    <category android:name="android.intent.category.BROWSABLE" />
    <data android:scheme="myapp" android:host="product" />
  </intent-filter>
</activity>

❓ Câu hỏi phỏng vấn + Đáp án

🧩 Mini exercise thực tế React Native Android

⚠️ Lỗi thường gặp

🏢 Case đi làm thật

🧠 Ghi nhớ nhanh

❔ Câu hỏi tự kiểm tra

✅ Checklist tự đánh giá

📎 Tài nguyên