Android SDK: Activities, Intents & App Lifecycle

The Android SDK provides the APIs to build Android apps. An Activity is the entry point for user interaction — one screen. Apps are built from Activities, Services, BroadcastReceivers, and ContentProviders.

Activity Lifecycle

class MainActivity : AppCompatActivity() {

    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        setContentView(R.layout.activity_main)
        // View created — initialize UI, restore state
    }

    override fun onStart() {
        super.onStart()
        // Activity visible but not interactive
    }

    override fun onResume() {
        super.onResume()
        // Activity in foreground — start animations, sensors
    }

    override fun onPause() {
        super.onPause()
        // Partially obscured — pause animations, save draft state
        // Keep it fast: next activity waits for onPause to complete
    }

    override fun onStop() {
        super.onStop()
        // Activity not visible — release heavy resources
    }

    override fun onDestroy() {
        super.onDestroy()
        // Activity finishing — final cleanup
    }

    override fun onSaveInstanceState(outState: Bundle) {
        super.onSaveInstanceState(outState)
        outState.putString("key", "value")   // save state before rotation/kill
    }
}

Intents

Intents are messages that request an action — launching an Activity, Service, or broadcasting an event. Explicit intents name the target component; implicit intents describe the action and let the system find a handler.

// Explicit intent — launch a specific activity
val intent = Intent(this, DetailActivity::class.java)
intent.putExtra("item_id", 42)
intent.putExtra("title", "Hello")
startActivity(intent)

// Receive extras in target activity
val itemId = intent.getIntExtra("item_id", -1)
val title = intent.getStringExtra("title")

// Implicit intent — let system pick handler
val browserIntent = Intent(Intent.ACTION_VIEW, Uri.parse("https://example.com"))
startActivity(browserIntent)

val shareIntent = Intent.createChooser(
    Intent(Intent.ACTION_SEND).apply {
        type = "text/plain"
        putExtra(Intent.EXTRA_TEXT, "Check this out!")
    },
    "Share via"
)
startActivity(shareIntent)

// Start activity and get result (modern API)
val launcher = registerForActivityResult(
    ActivityResultContracts.StartActivityForResult()
) { result ->
    if (result.resultCode == RESULT_OK) {
        val data = result.data?.getStringExtra("result_key")
    }
}
launcher.launch(intent)

AndroidManifest.xml

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

    <!-- Permissions -->
    <uses-permission android:name="android.permission.INTERNET" />
    <uses-permission android:name="android.permission.ACCESS_FINE_LOCATION" />
    <uses-permission android:name="android.permission.CAMERA" />

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

        <!-- Main activity -->
        <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>

        <!-- Other activities -->
        <activity android:name=".DetailActivity" android:exported="false" />

        <!-- Service -->
        <service android:name=".MyService" android:exported="false" />

        <!-- BroadcastReceiver -->
        <receiver android:name=".MyReceiver" android:exported="false">
            <intent-filter>
                <action android:name="android.intent.action.BOOT_COMPLETED" />
            </intent-filter>
        </receiver>
    </application>
</manifest>

Runtime Permissions (Android 6+)

// Request dangerous permissions at runtime
private val requestPermissionLauncher = registerForActivityResult(
    ActivityResultContracts.RequestPermission()
) { isGranted ->
    if (isGranted) {
        accessCamera()
    } else {
        showPermissionRationale()
    }
}

fun checkAndRequestCamera() {
    when {
        ContextCompat.checkSelfPermission(this, Manifest.permission.CAMERA)
            == PackageManager.PERMISSION_GRANTED -> accessCamera()

        shouldShowRequestPermissionRationale(Manifest.permission.CAMERA) ->
            showRationale()  // user previously denied

        else -> requestPermissionLauncher.launch(Manifest.permission.CAMERA)
    }
}

// Multiple permissions
val launcher = registerForActivityResult(
    ActivityResultContracts.RequestMultiplePermissions()
) { permissions ->
    val cameraGranted = permissions[Manifest.permission.CAMERA] == true
    val locationGranted = permissions[Manifest.permission.ACCESS_FINE_LOCATION] == true
}