Activity/Fragment Lifecycle

Phần này tổng hợp các câu hỏi về Lifecycle - một trong những khái niệm quan trọng nhất của Android development.

1. Activity Lifecycle

Q: Giải thích chi tiết Activity Lifecycle?

Trả lời:

Activity Lifecycle gồm 7 callback methods chính:

flowchart TD
    A["onCreate()"] --> B["onStart()"]
    B --> C["onResume()"]
    C --> D["RUNNING"]
    D --> E["onPause()"]
    E --> F["onStop()"]
    F --> G["onDestroy()"]

    E -.-> C
    F -.-> B
    G -.-> A

    style D fill:#c8e6c9

Callback	Mô tả	Actions
`onCreate()`	Activity được tạo lần đầu	Setup UI, bind data, initialize
`onStart()`	Activity visible cho user	Start animations
`onResume()`	Activity foreground, interactive	Resume video/audio, refresh UI
`onPause()`	Losing focus, still visible	Pause video/audio, save lightweight state
`onStop()`	Not visible	Release resources, stop updates
`onRestart()`	From stopped → started	Refresh data if needed
`onDestroy()`	Being destroyed	Cleanup, unregister listeners

class MainActivity : AppCompatActivity() {

    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        setContentView(R.layout.activity_main)
        Timber.d("onCreate - Initialize UI")
    }

    override fun onStart() {
        super.onStart()
        Timber.d("onStart - Activity visible")
    }

    override fun onResume() {
        super.onResume()
        Timber.d("onResume - Activity interactive")
    }

    override fun onPause() {
        super.onPause()
        Timber.d("onPause - Saving lightweight state")
    }

    override fun onStop() {
        super.onStop()
        Timber.d("onStop - Releasing resources")
    }

    override fun onDestroy() {
        super.onDestroy()
        Timber.d("onDestroy - Cleanup")
    }
}

📚 Tìm hiểu thêm: Activity Lifecycle

Q: onSaveInstanceState() vs onRestoreInstanceState()?

Trả lời:

Dùng để lưu và khôi phục UI state khi Activity bị destroy (configuration change, system kill).

// Lưu state
override fun onSaveInstanceState(outState: Bundle) {
    super.onSaveInstanceState(outState)
    outState.putString("search_query", searchEditText.text.toString())
    outState.putInt("scroll_position", recyclerView.scrollY)
}

// Khôi phục state (cách 1 - trong onCreate)
override fun onCreate(savedInstanceState: Bundle?) {
    super.onCreate(savedInstanceState)
    savedInstanceState?.let { state ->
        val query = state.getString("search_query")
        searchEditText.setText(query)
    }
}

// Khôi phục state (cách 2 - trong onRestoreInstanceState)
override fun onRestoreInstanceState(savedInstanceState: Bundle) {
    super.onRestoreInstanceState(savedInstanceState)
    val position = savedInstanceState.getInt("scroll_position")
    recyclerView.scrollTo(0, position)
}

Timing:

onSaveInstanceState() → called after onStop() (Android 9+) or before onStop() (older)
onRestoreInstanceState() → called after onStart()

💡 Mẹo: Chỉ lưu lightweight data (strings, ids). Dùng ViewModel cho complex data.

Q: Khi nào onDestroy() KHÔNG được gọi?

Trả lời:

onDestroy() không được đảm bảo sẽ được gọi khi:

System kills process do low memory
User force stops app từ Settings
App crashes

// ❌ Don't rely on onDestroy for critical cleanup
override fun onDestroy() {
    super.onDestroy()
    saveImportantData()  // May not be called!
}

// ✅ Use onStop() or lifecycle-aware components
override fun onStop() {
    super.onStop()
    saveImportantData()  // More reliable
}

// ✅ Or use ViewModel's onCleared()
class MyViewModel : ViewModel() {
    override fun onCleared() {
        super.onCleared()
        // Guaranteed to be called when ViewModel is destroyed
        cleanup()
    }
}

2. Fragment Lifecycle

Q: Fragment Lifecycle khác gì Activity Lifecycle?

Trả lời:

Fragment có thêm các callbacks liên quan đến View và host Activity:

flowchart TD
    A["onAttach()"] --> B["onCreate()"]
    B --> C["onCreateView()"]
    C --> D["onViewCreated()"]
    D --> E["onStart()"]
    E --> F["onResume()"]
    F --> G["RUNNING"]
    G --> H["onPause()"]
    H --> I["onStop()"]
    I --> J["onDestroyView()"]
    J --> K["onDestroy()"]
    K --> L["onDetach()"]

    style G fill:#c8e6c9

Callback	Mô tả
`onAttach()`	Fragment attached to Activity
`onCreate()`	Fragment created, no View yet
`onCreateView()`	Create and return View hierarchy
`onViewCreated()`	View created, safe to access views
`onDestroyView()`	View destroyed, Fragment still exists
`onDetach()`	Fragment detached from Activity

class HomeFragment : Fragment(R.layout.fragment_home) {

    private var _binding: FragmentHomeBinding? = null
    private val binding get() = _binding!!

    override fun onViewCreated(view: View, savedInstanceState: Bundle?) {
        super.onViewCreated(view, savedInstanceState)
        _binding = FragmentHomeBinding.bind(view)

        // Safe to access views here
        binding.button.setOnClickListener { /* ... */ }
    }

    override fun onDestroyView() {
        super.onDestroyView()
        _binding = null  // Prevent memory leak
    }
}

⚠️ Quan trọng: Luôn set binding = null trong onDestroyView() để tránh memory leak!

📚 Tìm hiểu thêm: Fragments

Q: Fragment có thể tồn tại khi Activity bị destroy không?

Trả lời:

Có, với setRetainInstance(true) (deprecated) hoặc sử dụng ViewModel:

// ❌ Deprecated approach
class RetainedFragment : Fragment() {
    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        retainInstance = true  // Deprecated in newer versions
    }
}

// ✅ Modern approach - Use ViewModel
class DataFragment : Fragment() {

    private val viewModel: DataViewModel by viewModels()

    override fun onViewCreated(view: View, savedInstanceState: Bundle?) {
        super.onViewCreated(view, savedInstanceState)

        // ViewModel survives configuration changes
        viewModel.data.observe(viewLifecycleOwner) { data ->
            updateUI(data)
        }
    }
}

3. Lifecycle-Aware Components

Q: LifecycleOwner và LifecycleObserver là gì?

Trả lời:

LifecycleOwner là interface cho classes có lifecycle (Activity, Fragment).

LifecycleObserver là interface để observe lifecycle events.

// Custom LifecycleObserver
class LocationObserver(
    private val context: Context
) : DefaultLifecycleObserver {

    private var locationManager: LocationManager? = null

    override fun onStart(owner: LifecycleOwner) {
        // Start listening when Activity/Fragment starts
        locationManager = context.getSystemService(Context.LOCATION_SERVICE) as LocationManager
        // Start location updates
    }

    override fun onStop(owner: LifecycleOwner) {
        // Stop listening when Activity/Fragment stops
        locationManager = null
        // Stop location updates
    }
}

// Usage in Activity/Fragment
class MapFragment : Fragment() {

    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)

        // Automatically manages lifecycle
        lifecycle.addObserver(LocationObserver(requireContext()))
    }
}

Lợi ích:

✅ Automatic cleanup
✅ Avoid memory leaks
✅ Reusable components

Q: viewLifecycleOwner vs this (Fragment)?

Trả lời:

Owner	Lifecycle	Use Case
`this`	Fragment lifecycle	ViewModel observation
`viewLifecycleOwner`	View lifecycle	UI updates, LiveData observation

class HomeFragment : Fragment() {

    private val viewModel: HomeViewModel by viewModels()

    override fun onViewCreated(view: View, savedInstanceState: Bundle?) {
        super.onViewCreated(view, savedInstanceState)

        // ✅ Correct - use viewLifecycleOwner for UI observations
        viewModel.data.observe(viewLifecycleOwner) { data ->
            updateUI(data)  // Safe, auto-removed when view destroyed
        }

        // ❌ Wrong - may cause crashes after view destroyed
        viewModel.data.observe(this) { data ->
            updateUI(data)  // Dangerous!
        }
    }
}

⚠️ Quan trọng: Luôn dùng viewLifecycleOwner khi observe từ onViewCreated() hoặc sau đó.

4. Process Lifecycle

Q: Android xử lý process priority như thế nào?

Trả lời:

Android chia processes thành 5 priority levels:

Priority	Mô tả	Ví dụ
Foreground	User đang interact	Visible Activity
Visible	Visible nhưng không focus	Activity behind dialog
Service	Running service	Music playback
Cached	Background, no active components	Recently closed app
Empty	No active components	Idle process

// Check process importance
val activityManager = getSystemService(Context.ACTIVITY_SERVICE) as ActivityManager
val runningAppProcesses = activityManager.runningAppProcesses

runningAppProcesses.forEach { process ->
    when (process.importance) {
        ActivityManager.RunningAppProcessInfo.IMPORTANCE_FOREGROUND -> "Foreground"
        ActivityManager.RunningAppProcessInfo.IMPORTANCE_VISIBLE -> "Visible"
        ActivityManager.RunningAppProcessInfo.IMPORTANCE_SERVICE -> "Service"
        ActivityManager.RunningAppProcessInfo.IMPORTANCE_CACHED -> "Cached"
        else -> "Other"
    }
}

Khi memory thấp:

System kills lowest priority processes first
Cached → Service → Visible → Foreground

5. Common Scenarios

Q: Lifecycle khi navigate giữa 2 Activities?

Trả lời:

Khi Activity A start Activity B:

A: onPause()
B: onCreate()
B: onStart()
B: onResume()
A: onStop()

Khi back từ B về A:

B: onPause()
A: onRestart()
A: onStart()
A: onResume()
B: onStop()
B: onDestroy()

💡 Mẹo: onPause() của Activity cũ chạy trước onCreate() của Activity mới. Vì vậy không làm heavy work trong onPause().

Q: Lifecycle khi nhận phone call?

Trả lời:

Khi có incoming call (partial cover):

onPause()

Khi call screen covers entirely:

onPause()
onStop()

Khi end call:

onRestart()
onStart()
onResume()

Q: Lifecycle khi Home button pressed?

Trả lời:

onPause()
onStop()
onSaveInstanceState()

Khi return to app:

onRestart()
onStart()
onResume()

💡 Mẹo: onDestroy() không được gọi khi nhấn Home button - Activity vẫn còn trong memory.

📝 Practical Tips

Best Practices theo Lifecycle:

Callback	Nên làm	Không nên
`onCreate()`	Initialize ViewModel, setup UI	Heavy I/O operations
`onStart()`	Register listeners	Start long-running tasks
`onResume()`	Resume animations, refresh UI	Network calls
`onPause()`	Pause animations	Save data to database
`onStop()`	Unregister listeners, save data	Heavy cleanup
`onDestroy()`	Final cleanup	Critical saving

class BestPracticeActivity : AppCompatActivity() {

    private val viewModel: MainViewModel by viewModels()

    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        setContentView(R.layout.activity_main)

        // ✅ Good - observe ViewModel
        viewModel.data.observe(this) { updateUI(it) }
    }

    override fun onStart() {
        super.onStart()
        // ✅ Good - register lifecycle-aware listeners
        registerReceiver(networkReceiver, IntentFilter(ConnectivityManager.CONNECTIVITY_ACTION))
    }

    override fun onStop() {
        super.onStop()
        // ✅ Good - unregister listeners
        unregisterReceiver(networkReceiver)
    }
}

📚 Tìm hiểu thêm: Activity Lifecycle, ViewModel