02-platform

Master Android's component model and lifecycle management. Understand Activities, Fragments, Services, and how they interact through Intents. Learn to handle configuration changes, manage background operations, and implement proper permission handling. Prerequisite: Fundamentals agent (Kotlin, OOP, SOLID) Duration: 78 hours | Level: Beginner to Intermediate Topics: 8 major areas | Code Examples: 35+ real-world patterns ---

Activities are the visible UI component lifecycle. Understanding the complete lifecycle is critical for proper state management.

` onCreate → onStart → onResume → [USER INTERACTION] ↓ onPause → onStop → onDestroy ↑_________| (User returns) ` Complete Lifecycle with Callbacks: `kotlin class MainActivity : AppCompatActivity() { // 1. onCreate(): Called when activity is first created // When: Process created OR activity created (not killed) // State: Activity NOT visible, can initialize UI and restore state override fun onCreate(savedInstanceState: Bundle?) { super.onCreate(savedInstanceState) setContentView(R.layout.activity_main) // Initialize UI components val viewModel: MainViewModel by viewModels() // Restore saved instance state savedInstanceState?.let { val savedValue = it.getString("key") } } // 2. onStart(): Called when activity becomes visible // When: After onCreate OR after onStop // State: Activity visible but NOT in foreground // Note: onStart ↔ onStop protect "visible" resource acquisition override fun onStart() { super.onStart() registerSensorListener() // Acquire resources } // 3. onResume(): Called when activity gains focus // When: After onStart OR after onPause // State: Activity in foreground, receives user input // Note: onResume ↔ onPause protect "focused" resource acquisition override fun onResume() { super.onResume() startCamera() // Camera requires focus startAnimation() // Resume animations } // 4. onPause(): Called when activity loses focus // When: Another activity gains focus (transparent or translucent) // State: Activity partially visible but NOT in foreground // Critical: Keep this short! Next activity won't start until this finishes override fun onPause() { super.onPause() stopCamera() // Release focus-dependent resources pauseAnimation() // Pause animations } // 5. onStop(): Called when activity is no longer visible // When: Another activity fully covers this OR user navigates away // State: Activity NOT visible to user override fun onStop() { super.onStop() unregisterSensorListener() // Release resources saveSessionData() } // 6. onDestroy(): Called before activity is destroyed // When: Activity finishing OR system destroying to free memory // State: Activity being removed from memory override fun onDestroy() { super.onDestroy() viewModel.cleanup() // Final cleanup } } `

By default, configuration change (e.g., rotation) destroys and recreates activity: ` [Current State] → onPause → onStop → onDestroy → onCreate → onStart → onResume [NEW State] ` Problem: User data lost! Solution: Save/Restore state Option 1: Handle in onSaveInstanceState `kotlin class DataActivity : AppCompatActivity() { private var userData: String = "" override fun onSaveInstanceState(outState: Bundle) { super.onSaveInstanceState(outState) // Called BEFORE onStop, guaranteed to be called before destroy outState.putString("user_data", userData) } override fun onCreate(savedInstanceState: Bundle?) { super.onCreate(savedInstanceState) savedInstanceState?.let { userData = it.getString("user_data", "") } } } ` Option 2: Prevent Recreation with configChanges `xml <!-- AndroidManifest.xml --> <activity android:name=".MainActivity" android:configChanges="orientation|screenSize" android:screenOrientation="portrait" /> ` When handled: ` [Current State] → onPause → onStop → onConfigurationChanged → onStart → onResume [State Preserved!] ` Option 3: Use ViewModel (RECOMMENDED) `kotlin @HiltViewModel class DataViewModel @Inject constructor( private val repository: DataRepository ) : ViewModel() { // ViewModel survives configuration changes! private val _userData = MutableLiveData<User>() val userData: LiveData<User> = _userData } class DataActivity : AppCompatActivity() { private val viewModel: DataViewModel by viewModels() override fun onCreate(savedInstanceState: Bundle?) { super.onCreate(savedInstanceState) // ViewModel already has data from previous configuration viewModel.userData.observe(this) { user -> updateUI(user) } } } `