Loop Cupcakes. Migrating AsyncTask to RxJava.

build.gradle

// build.gradle (:app)

android {
  //..
  compileOptions {
    sourceCompatibility JavaVersion.VERSION_1_8
    targetCompatibility JavaVersion.VERSION_1_8
  }
}

dependencies {
  //.. https://github.com/ReactiveX/RxAndroid
  //.. https://github.com/ReactiveX/RxJava
  implementation 'io.reactivex.rxjava3:rxandroid:3.0.0'
  implementation 'io.reactivex.rxjava3:rxjava:3.0.5'
}

MainActivity.kt

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

    /* Creates a flow of exactly one item, switches to a new thread to perform the operation
     * using the subscribeOn operator, and comes back to the main thread after the chain is
     * fully computed using the observeOn operator, the logging and error handling are run
     * on the main thread. This will print the String "Result is 4" to the Logcat */
    Single.fromCallable { doExpensiveFunction(2) }
      .subscribeOn(Schedulers.newThread())
      .observeOn(AndroidSchedulers.mainThread())
      .subscribe({ Log.d("TAG_", "Result is: $it") }, { Log.e("TAG_ ", "Error: $it") })
  }

  /* Since we're creating an RxJava Single, this method will run on a worker thread created
   * by Schedulers.newThread, if we called this from anywhere else without the fromCallable,
   * this would run on the main thread and would crash or ANR a production app */
  private fun doExpensiveFunction(input: Int): Int {
    /* Thread.sleep simulates a long running operation  */
    Thread.sleep(2000)
    return input * input
  }
}

OldMainActivity.kt

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

    /* Creates and executes the AsyncTask */
    val execute = OldTask().execute(2)
    val result = execute.get()
    /* Prints the string "Result is 4" to the Logcat. This is run on the main thread */
    Log.d("TAG_ ", "Result is $result")
  }
}

/* https://developer.android.com/reference/android/os/AsyncTask */
class OldTask : AsyncTask<Int, Unit, Int>() {

  /* This method runs on a worker thread */
  override fun doInBackground(vararg param: Int?): Int {
    /* Thread.sleep simulates a long running operation  */
    Thread.sleep(2000)
    val input = param.getOrNull(0) ?: 0
    return input * input
  }
}