Câu hỏi phỏng vấn Android cơ bản

// Application Context - an toàn cho long-lived objects
val appContext = applicationContext
 
// Activity Context - cho UI operations
val dialog = AlertDialog.Builder(this)  // 'this' là Activity
    .setTitle("Example")
    .show()

// this - cho UI operations
val intent = Intent(this, DetailActivity::class.java)
startActivity(intent)
 
// baseContext - cho custom ContextWrapper
override fun attachBaseContext(newBase: Context) {
    super.attachBaseContext(CustomContextWrapper(newBase))
}

class MyApplication : Application() {
    
    override fun onCreate() {
        super.onCreate()
        // Initialize global dependencies
        initTimber()
        initAnalytics()
        initDependencyInjection()
    }
    
    private fun initTimber() {
        if (BuildConfig.DEBUG) {
            Timber.plant(Timber.DebugTree())
        }
    }
}

<!-- AndroidManifest.xml -->
<application
    android:name=".MyApplication"
    ... >
</application>

<manifest xmlns:android="http://schemas.android.com/apk/res/android"
    package="com.example.app">
    
    <!-- Permissions -->
    <uses-permission android:name="android.permission.INTERNET"/>
    <uses-permission android:name="android.permission.CAMERA"/>
    
    <!-- Hardware features -->
    <uses-feature android:name="android.hardware.camera" 
                  android:required="false"/>
    
    <application
        android:name=".MyApplication"
        android:icon="@mipmap/ic_launcher"
        android:label="@string/app_name"
        android:theme="@style/AppTheme">
        
        <!-- Activities -->
        <activity android:name=".MainActivity"
                  android:exported="true">
            <intent-filter>
                <action android:name="android.intent.action.MAIN"/>
                <category android:name="android.intent.category.LAUNCHER"/>
            </intent-filter>
        </activity>
        
        <!-- Services -->
        <service android:name=".MyService"/>
        
    </application>
</manifest>

// Activity
class MainActivity : AppCompatActivity() {
    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        setContentView(R.layout.activity_main)
    }
}
 
// Service
class MusicService : Service() {
    override fun onBind(intent: Intent?): IBinder? = null
    
    override fun onStartCommand(intent: Intent?, flags: Int, startId: Int): Int {
        // Play music in background
        return START_STICKY
    }
}
 
// BroadcastReceiver
class NetworkReceiver : BroadcastReceiver() {
    override fun onReceive(context: Context, intent: Intent) {
        // Handle network change
    }
}

// Adding Fragment
supportFragmentManager.beginTransaction()
    .replace(R.id.container, HomeFragment())
    .addToBackStack(null)
    .commit()

class MainViewModel : ViewModel() {
    private val _data = MutableLiveData<List<Item>>()
    val data: LiveData<List<Item>> = _data
    
    // Data survives configuration changes
}

override fun onSaveInstanceState(outState: Bundle) {
    super.onSaveInstanceState(outState)
    outState.putString("key", value)
}
 
override fun onCreate(savedInstanceState: Bundle?) {
    super.onCreate(savedInstanceState)
    savedInstanceState?.getString("key")?.let { value = it }
}

@Composable
fun Counter() {
    var count by rememberSaveable { mutableStateOf(0) }
    Button(onClick = { count++ }) {
        Text("Count: $count")
    }
}

<activity android:name=".MainActivity"
          android:configChanges="orientation|screenSize"/>

// ❌ Memory leak - static reference to Activity
companion object {
    var activity: Activity? = null  // DON'T DO THIS
}
 
// ✅ Use WeakReference
class MyHandler(activity: Activity) : Handler(Looper.getMainLooper()) {
    private val activityRef = WeakReference(activity)
    
    override fun handleMessage(msg: Message) {
        activityRef.get()?.let { activity ->
            // Safe to use
        }
    }
}
 
// ✅ Use lifecycle-aware components
viewLifecycleOwner.lifecycleScope.launch {
    repeatOnLifecycle(Lifecycle.State.STARTED) {
        viewModel.data.collect { updateUI(it) }
    }
}

// ❌ ANR - Network trên Main Thread
val data = api.fetchData()  // Blocks UI
 
// ✅ Use Coroutines
viewModelScope.launch {
    val data = withContext(Dispatchers.IO) {
        api.fetchData()
    }
    updateUI(data)
}
 
// ✅ Use WorkManager for persistent tasks
val request = OneTimeWorkRequestBuilder<SyncWorker>().build()
WorkManager.getInstance(context).enqueue(request)

// Explicit Intent - navigate to specific Activity
val intent = Intent(this, DetailActivity::class.java).apply {
    putExtra("item_id", 123)
}
startActivity(intent)
 
// Implicit Intent - let system choose handler
val shareIntent = Intent(Intent.ACTION_SEND).apply {
    type = "text/plain"
    putExtra(Intent.EXTRA_TEXT, "Check this out!")
}
startActivity(Intent.createChooser(shareIntent, "Share via"))
 
// Open URL
val webIntent = Intent(Intent.ACTION_VIEW, Uri.parse("https://google.com"))
startActivity(webIntent)

// DataStore (Recommended)
val dataStore = context.createDataStore("settings")
dataStore.edit { prefs ->
    prefs[THEME_KEY] = "dark"
}
 
// Room Database
@Dao
interface UserDao {
    @Query("SELECT * FROM users")
    fun getAll(): Flow<List<User>>
    
    @Insert
    suspend fun insert(user: User)
}

// build.gradle.kts
android {
    namespace = "com.example.app"
    compileSdk = 35  // API level để compile code
    
    defaultConfig {
        applicationId = "com.example.app"
        minSdk = 24      // Android 7.0 - version thấp nhất hỗ trợ
        targetSdk = 35   // Android 15 - version app được tối ưu cho
        versionCode = 1
        versionName = "1.0"
    }
}

// Nếu targetSdk >= 31 (Android 12)
// - Phải khai báo android:exported cho Activities
// - Approximate location permission xuất hiện
// - PendingIntent yêu cầu mutability flag
 
// Nếu targetSdk >= 33 (Android 13)  
// - POST_NOTIFICATIONS permission bắt buộc request runtime
// - Photo picker thay thế storage permissions
 
// Ví dụ: Request notification permission
if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.TIRAMISU) {
    requestPermissions(arrayOf(Manifest.permission.POST_NOTIFICATIONS), 1)
}

// App với targetSdk = 30 chạy trên Android 14:
// - Không bị ảnh hưởng bởi các restrictions mới của Android 14
// - System "giả lập" behavior của Android 11 cho app

// Yêu cầu hiện tại của Google Play:
// - New apps: targetSdk phải >= 34 (Android 14)
// - App updates: targetSdk phải >= 34
 
// Nếu không cập nhật targetSdk:
// - App bị ẩn khỏi users có device mới
// - App có thể bị xóa khỏi Play Store

// targetSdk = 22: Tất cả permissions cấp lúc install
// targetSdk >= 23: Runtime permissions
 
fun checkCameraPermission() {
    // Với targetSdk >= 23, phải request runtime
    if (ContextCompat.checkSelfPermission(this, Manifest.permission.CAMERA)
        != PackageManager.PERMISSION_GRANTED) {
        ActivityCompat.requestPermissions(
            this, 
            arrayOf(Manifest.permission.CAMERA), 
            CAMERA_REQUEST_CODE
        )
    } else {
        openCamera()
    }
}