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4. Android Development in Kotlin – Some Advance Concepts

1 2 App Bundles

  • App Bundle:
    • It is the compiled version of any kotlin/java code, along with any resources and static files that are needed for the app to run.
    • It used to be a bundle for each device architecture, but now it is a single bundle for all architectures that should be submitted to the play store.
    • The play store takes the bundle and generates the a signed APK bundle for each device architecture.
  • The App bundle is not a full APK, thus it can NOT be installed on a device, however it can be used to generate a signed APK, which Google Play does on your behalf.
  • The Play Store generates hundreds of signed APKs from the app bundle, each signed APK is optimized for a different device configuration.
  • bundletool is a command line tool that can be used to:
    • Generate APK set from an app bundle, targeting a specific device configuration(s).
    • Deploy an app bundle to a connected device.
    • Perform E2E testing on a device using an app bundle and test APKs.
    • Estimate the download size of an app bundle.
  • Basically, Google Play uses the same bundletool, so it is a good way to experiment with changes before submitting to the Play Store.

3 SQLite Database in Android

  • You can use SQLite to create databases on the user’s device and access them from your app at any time.
  • It is an Offline database on the user’s device, thus, no internet connection is required to access it.
  • The data is stored in a text file that’s convenient for SQLite engine to read and write.
  • The permissions required to use SQLite are:
    • android.permission.READ_EXTERNAL_STORAGE
    • android.permission.WRITE_EXTERNAL_STORAGE
    • add them to the AndroidManifest.xml file in a <uses-permission> tag.

4 5 Network Connection in Android

  • The Android platform includes the `HttpsURLConnection`` client, which supports TLS, streaming uploads and downloads, configurable timeouts, IPv6, and connection pooling.
  • Third party libraries: Retrofit (on top of OkHttp), and Ktor (kotlin native, uses coroutines, be JetBrains).
  • The DnsResolver interface can be used to resolve host names to IP addresses, and vice versa.
  • Use the Repository pattern to abstract the data layer from the rest of the app.
  • Network requests can NOT be done one the main thread, as a NetworkOnMainThreadException will be thrown.
  • Functions that perform network operations should be called from a coroutine or from a different thread (aka, should be suspend functions).
  • The ViewModel class is designed to store and manage UI-related data in a lifecycle conscious way. That is, it holds component state between renderers (analogy to useState in React).
  • Best practices:
    • Before doing a request, check the network connection using ConnectivityManager or NetworkInfo.
    • Avoid large data transfers if device is not connected to a Wi-Fi network.

6 8 Exceptions in Kotlin

  • Kotlin Try Catch is used to handle the code that could throw an exception at run-time.
try {
   //code that code potentially throw and exception
 } catch(e: Some_Exception) {
   //code to handle if this exception is occurred
 } catch(e: Exception) {
   //you can have multiple catch blocks
   //code to handle if this exception is occurred
 } finally {
   //code that will be executed regardless of whether an exception is thrown or not
 }

 val x = try { someFunction() } catch(e: Exception) { 0} //x will be 0 if an exception is thrown from someFunction()
  • Exception is the parent class of all exceptions in Kotlin.
  • Other exceptions include: ArithmeticException, IOException, ClassNotFoundException.
  • Exceptions that do not have a suitable catch block are called uncaught exceptions, and they will cause the app to crash (still be throw even if you have a catch block for other exceptions).
  • Exception has:
    • message property: a string that describes the exception.
    • printStackTrace() method: prints the stack trace of the exception.
  • Ypu can throw Exception("message") to throw an exception.
  • Error and Exception are both subclasses of Throwable, but Error is used to indicate serious problems that a reasonable application should not try to catch, while Exceptions are caught.
  • Example of errors: OutOfMemoryError, StackOverflowError.
  • To catch all exceptions (including errors), use catch(e: Throwable).
  • checked exceptions are java feature where a function states what errors it throws, and the callers must handle them. Kotlin does not have this feature.

7 9 Generics in Kotlin

class TouchedByAnAngle<T> {
    private T thingy;
    public T getThingy() { return thingy; }
    public void setThingy(T replacement) { thingy = replacement; }
}

val m = TouchedByAnAngle<Int>()
m.setThingy(5)
m.getThingy() //5

data class Person(val name: String, val age: Int)
val p = TouchedByAnAngle<Person>()
p.setThingy(Person("John", 20))
p.getThingy() //Person(name=John, age=20)
  • Any is possible to be used as a generic type, but it is not recommended.
val critters = mutableListOf(Frog(), Axolotl(), "ribbit!") //MutableList<Any> as Any is the parent of all classes in the list
  • Advantage of generics: Type safety, no need for type casting, and compile time checking.
  • As extension function allows to add methods to class without inherit a class or any design pattern.

10 Emulator

  • Advantages of Emulator: Flexibility (run any Android version, any device, any screen size), Simulate any thing (GPS, network, battery, etc), and Test apps without physical device faster.
  • Android Virtual Device (AVD): is a configuration that defines the characteristics of an Android phone, tablet, Wear OS, Android TV, or Automotive OS device that you want to simulate in the Android Emulator.
  • The Emulator saves the state of each AVD between each session; the data is stored under a specific folder in the user’s home directory.

References

  1. About android app bundles. (2022, March 17). Android Developers. https://developer.android.com/guide/app-bundle ↩

  2. Build and test your android app bundle. (2021, July 17). Android Developers. https://developer.android.com/guide/app-bundle/test ↩

  3. Chaitanyamunje. (2021, February 25). How to create and add data to SQLite database in Android? GeeksforGeeks. Retrieved July 3, 2022, from https://www.geeksforgeeks.org/how-to-create-and-add-data-to-sqlite-database-in-android/ ↩

  4. Connect to the network. (2022, June 21). Android Developers. https://developer.android.com/training/basics/network-ops/connecting ↩

  5. Kamal, F. (2020, April 6). Android networking with kotlin tutorial: Getting started. raywenderlich.com. https://www.raywenderlich.com/6994782-android-networking-with-kotlin-tutorial-getting-started ↩

  6. Kotlin exceptions. (2021, January 21). TutorialKart. https://www.tutorialkart.com/kotlin/kotlin-exceptions/ ↩

  7. Kotlin generics. (n.d.). javaTpoint. https://www.javatpoint.com/kotlin-generics ↩

  8. Murphy, M. L. (2021). Elements of kotlin. CommonsWare. https://commonsware.com/Kotlin/Kotlin-FINAL.pdf licensed under Creative commons 4.0. Chapter 18: Exceptions. ↩

  9. Murphy, M. L. (2021). Elements of kotlin. CommonsWare. https://commonsware.com/Kotlin/Kotlin-FINAL.pdf licensed under Creative commons 4.0. Chapter 17: Generics. ↩

  10. Run apps on the android emulator | Android developers. (2021, October 28). Android Developers. https://developer.android.com/studio/run/emulator ↩