Android Core
ComponentsThe four pillars every app is built on

class MainActivity : AppCompatActivity() {

    private val viewModel: MainViewModel by viewModels()
    private var _binding: ActivityMainBinding? = null
    private val binding get() = _binding!!

    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        _binding = ActivityMainBinding.inflate(layoutInflater)
        setContentView(binding.root)

        // ViewModel survives rotation — data already loaded
        viewModel.uiState.observe(this) { state -> render(state) }

        // Restore scroll position, selected tab, etc.
        savedInstanceState?.let {
            binding.recycler.scrollToPosition(it.getInt("scroll_pos", 0))
        }
    }

    override fun onResume() {
        super.onResume()
        // Acquire exclusive resources (camera, mic, sensors)
    }

    override fun onPause() {
        super.onPause()
        // FAST — release exclusive resources, save lightweight critical state
    }

    override fun onSaveInstanceState(outState: Bundle) {
        super.onSaveInstanceState(outState)
        outState.putInt("scroll_pos", binding.recycler.computeVerticalScrollOffset())
    }

    override fun onDestroy() {
        _binding = null  // prevent memory leak
        super.onDestroy()
    }
}

class MusicPlayerService : Service() {

    private val binder = LocalBinder()
    private var mediaPlayer: MediaPlayer? = null

    inner class LocalBinder : Binder() {
        fun getService() = this@MusicPlayerService
    }

    override fun onCreate() {
        super.onCreate()
        mediaPlayer = MediaPlayer()
    }

    override fun onStartCommand(intent: Intent?, flags: Int, startId: Int): Int {
        val notification = buildMediaNotification()
        startForeground(NOTIFICATION_ID, notification)  // promote to foreground
        mediaPlayer?.start()
        return START_STICKY  // restart if killed, re-deliver last intent
    }

    override fun onBind(intent: Intent): IBinder = binder

    override fun onDestroy() {
        mediaPlayer?.stop()
        mediaPlayer?.release()
        mediaPlayer = null
        super.onDestroy()
    }
}

// ── Manifest-registered (AndroidManifest.xml) ─────────────────
// <receiver android:name=".BootReceiver" android:exported="false">
//   <intent-filter>
//     <action android:name="android.intent.action.BOOT_COMPLETED"/>
//   </intent-filter>
// </receiver>

class BootReceiver : BroadcastReceiver() {
    override fun onReceive(context: Context, intent: Intent) {
        if (intent.action == Intent.ACTION_BOOT_COMPLETED) {
            // Schedule WorkManager jobs that run on every boot
            WorkManager.getInstance(context).enqueue(OneTimeWorkRequest...)
            // NEVER do long work here — 10 second limit
        }
    }
}

// ── Dynamic registration (in Activity) ────────────────────────
class MainActivity : AppCompatActivity() {
    private val networkReceiver = object : BroadcastReceiver() {
        override fun onReceive(ctx: Context, intent: Intent) {
            viewModel.onNetworkChanged()
        }
    }

    override fun onStart() {
        super.onStart()
        val filter = IntentFilter(ConnectivityManager.CONNECTIVITY_ACTION)
        registerReceiver(networkReceiver, filter)
    }
    override fun onStop() {
        unregisterReceiver(networkReceiver)  // always symmetric
        super.onStop()
    }
}

// ── goAsync() — for work that needs slightly more time ─────────
class DataReceiver : BroadcastReceiver() {
    override fun onReceive(ctx: Context, intent: Intent) {
        val pendingResult = goAsync()  // extends to ~30s, still not for I/O
        CoroutineScope(Dispatchers.Default).launch {
            processData(intent)
            pendingResult.finish()  // must call finish or system kills it
        }
    }
}

class BookProvider : ContentProvider() {

    private val uriMatcher = UriMatcher(UriMatcher.NO_MATCH).apply {
        addURI("com.example.books", "books", 1)     // all books
        addURI("com.example.books", "books/#", 2)  // specific book by ID
    }

    override fun onCreate(): Boolean {
        // Initialize database. Called on main thread — be fast.
        return true
    }

    override fun query(uri: Uri, projection: Array<String>?, selection: String?,
                        selectionArgs: Array<String>?, sortOrder: String?): Cursor? {
        return when (uriMatcher.match(uri)) {
            1 -> db.query("books", projection, selection, selectionArgs, null, null, sortOrder)
            2 -> db.query("books", projection, "_id=?", arrayOf(uri.lastPathSegment!!), null, null, null)
            else -> throw IllegalArgumentException("Unknown URI: $uri")
        }
    }

    override fun insert(uri: Uri, values: ContentValues?): Uri? { ... }
    override fun update(uri: Uri, values: ContentValues?, sel: String?, selArgs: Array<String>?): Int = 0
    override fun delete(uri: Uri, sel: String?, selArgs: Array<String>?): Int = 0
    override fun getType(uri: Uri): String? = when (uriMatcher.match(uri)) {
        1 -> "vnd.android.cursor.dir/vnd.example.books"
        2 -> "vnd.android.cursor.item/vnd.example.books"
        else -> null
    }
}

// ── Accessing via ContentResolver (any app) ────────────────────
val cursor = contentResolver.query(
    Uri.parse("content://com.example.books/books"),
    arrayOf("_id", "title", "author"),
    null, null, "title ASC"
)
cursor?.use { c ->
    while (c.moveToNext()) {
        val title = c.getString(c.getColumnIndexOrThrow("title"))
    }
}

// AndroidManifest.xml
// <provider android:name="androidx.core.content.FileProvider"
//     android:authorities="${applicationId}.fileprovider"
//     android:exported="false" android:grantUriPermissions="true">
//   <meta-data android:name="android.support.FILE_PROVIDER_PATHS"
//       android:resource="@xml/file_paths"/>
// </provider>

// res/xml/file_paths.xml
// <paths> <cache-path name="images" path="images/"/> </paths>

// In code — share a file
val file = File(cacheDir, "images/photo.jpg")
val uri = FileProvider.getUriForFile(this, "${packageName}.fileprovider", file)

val shareIntent = Intent(Intent.ACTION_SEND).apply {
    type = "image/jpeg"
    putExtra(Intent.EXTRA_STREAM, uri)
    addFlags(Intent.FLAG_GRANT_READ_URI_PERMISSION)  // grants read to receiver
}
startActivity(Intent.createChooser(shareIntent, "Share image"))

// ── Explicit Intent ────────────────────────────────────────────
val explicit = Intent(this, DetailActivity::class.java).apply {
    putExtra("product_id", "prod_42")           // extras — arbitrary key-value data
    putExtra("show_reviews", true)
    addFlags(Intent.FLAG_ACTIVITY_SINGLE_TOP)    // behavior flags
}
startActivity(explicit)

// ── Implicit Intent — open URL ─────────────────────────────────
val viewUrl = Intent(Intent.ACTION_VIEW, Uri.parse("https://example.com"))
startActivity(viewUrl)  // browser or WebView app handles this

// ── Implicit Intent — share text ───────────────────────────────
val share = Intent(Intent.ACTION_SEND).apply {
    type = "text/plain"                          // MIME type narrows the match
    putExtra(Intent.EXTRA_TEXT, "Check this out!")
}
startActivity(Intent.createChooser(share, "Share via"))

// ── Always check if an implicit intent can be handled ──────────
if (viewUrl.resolveActivity(packageManager) != null) {
    startActivity(viewUrl)   // safe — at least one app can handle it
} else {
    showMessage("No app available to handle this action")
}

// ── Intent Filter (in manifest — receiving implicit intents) ───
// <activity android:name=".ShareReceiver">
//   <intent-filter>
//     <action android:name="android.intent.action.SEND"/>
//     <category android:name="android.intent.category.DEFAULT"/>
//     <data android:mimeType="image/*"/>
//   </intent-filter>
// </activity>

val intent = Intent(this, MainActivity::class.java).apply {
    putExtra("notification_id", notifId)
    addFlags(Intent.FLAG_ACTIVITY_SINGLE_TOP)
}
val pendingIntent = PendingIntent.getActivity(
    this, requestCode, intent,
    PendingIntent.FLAG_IMMUTABLE or PendingIntent.FLAG_UPDATE_CURRENT
)

val notification = NotificationCompat.Builder(this, CHANNEL_ID)
    .setContentTitle("New message")
    .setContentIntent(pendingIntent)  // fires when notification is tapped
    .setAutoCancel(true)
    .build()

<?xml version="1.0" encoding="utf-8"?>
<manifest xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:tools="http://schemas.android.com/tools">

    <!-- ① Permissions — declared before use ─────────────────── -->
    <uses-permission android:name="android.permission.INTERNET"/>
    <uses-permission android:name="android.permission.CAMERA"/>
    <uses-permission android:name="android.permission.POST_NOTIFICATIONS"/>

    <!-- ② Hardware features ──────────────────────────────────── -->
    <uses-feature android:name="android.hardware.camera" android:required="false"/>

    <application
        android:name=".MyApplication"
        android:label="@string/app_name"
        android:icon="@mipmap/ic_launcher"
        android:theme="@style/Theme.App"
        android:allowBackup="true">

        <!-- ③ Activity ───────────────────────────────────────── -->
        <activity
            android:name=".MainActivity"
            android:exported="true"
            android:launchMode="singleTop"
            android:screenOrientation="portrait">
            <intent-filter>
                <action android:name="android.intent.action.MAIN"/>
                <category android:name="android.intent.category.LAUNCHER"/>
            </intent-filter>
            <!-- Deep link -->
            <intent-filter android:autoVerify="true">
                <action android:name="android.intent.action.VIEW"/>
                <category android:name="android.intent.category.DEFAULT"/>
                <category android:name="android.intent.category.BROWSABLE"/>
                <data android:scheme="https" android:host="myapp.com"/>
            </intent-filter>
        </activity>

        <!-- ④ Service ────────────────────────────────────────── -->
        <service
            android:name=".MusicPlayerService"
            android:exported="false"
            android:foregroundServiceType="mediaPlayback"/>

        <!-- ⑤ BroadcastReceiver ─────────────────────────────── -->
        <receiver
            android:name=".BootReceiver"
            android:exported="false">
            <intent-filter>
                <action android:name="android.intent.action.BOOT_COMPLETED"/>
            </intent-filter>
        </receiver>

        <!-- ⑥ ContentProvider ───────────────────────────────── -->
        <provider
            android:name=".BookProvider"
            android:authorities="${applicationId}.books"
            android:exported="false"/>

    </application>
</manifest>

// Single Activity — container for all navigation destinations
class MainActivity : AppCompatActivity() {

    private val binding by viewBinding(ActivityMainBinding::inflate)

    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        setContentView(binding.root)
        // NavHostFragment manages all screen navigation
        // Each screen is a Fragment or Compose NavHost destination
        // Activity only handles: permissions, back press, window insets
    }

    override fun onNewIntent(intent: Intent) {
        super.onNewIntent(intent)
        // Deep link handling — NavController routes to the right screen
        findNavController(R.id.nav_host).handleDeepLink(intent)
    }
}

// activity_main.xml
// <FragmentContainerView android:name="androidx.navigation.fragment.NavHostFragment"
//     app:navGraph="@navigation/nav_graph"
//     app:defaultNavHost="true"/>

// ── With Compose — NavHost replaces FragmentContainerView ──────
class MainActivity : ComponentActivity() {
    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        setContent {
            val navController = rememberNavController()
            AppNavHost(navController = navController)  // all screens in Compose
        }
    }
}