BiometricsPrompt full implementation

!BiometricsManagerImpl.kt

class BiometricsManagerImpl : BiometricsManager {

    private val executor = MainThreadExecutor()

    override fun authenticate(activity: FragmentActivity): Completable {
        return Completable.create { emitter ->
            val prompt = BiometricPrompt(
                activity,
                executor,
                object : BiometricPrompt.AuthenticationCallback() {
                    @SuppressLint("SwitchIntDef")
                    override fun onAuthenticationError(errorCode: Int, errString: CharSequence) {
                        val exception = when (errorCode) {
                            BiometricConstants.ERROR_HW_NOT_PRESENT,
                            BiometricConstants.ERROR_HW_UNAVAILABLE,
                            BiometricConstants.ERROR_NO_BIOMETRICS ->
                                BiometricsNotAvailableException("Code: $errorCode, Message: $errString")
                            else -> BiometricCancelledException("Code: $errorCode, Message: $errString")
                        }

                        emitter.onError(exception)
                    }

                    override fun onAuthenticationSucceeded(result: BiometricPrompt.AuthenticationResult) {
                        emitter.onComplete()
                    }
                })

            val promptInfo = BiometricPrompt.PromptInfo.Builder()
                .setTitle(activity.getString(R.string.biometric_authentication_title))
                .setDescription(activity.getString(R.string.biometric_authentication_description))
                .setNegativeButtonText(activity.getString(R.string.global_cancel))
                .build()

            prompt.authenticate(promptInfo)
        }
    }

    inner class MainThreadExecutor : Executor {
        private val handler = Handler(Looper.getMainLooper())

        override fun execute(runnable: Runnable) {
            handler.post(runnable)
        }
    }
}

AndroidManifest.xml

<uses-permission android:name="android.permission.USE_BIOMETRIC"/>

BiometricCancelledException.kt

class BiometricCancelledException(message: String? = null) : Exception(message)

BiometricsManager.kt

interface BiometricsManager {
    fun authenticate(activity: FragmentActivity): Completable
}

BiometricsNotAvailableException.kt

class BiometricsNotAvailableException(message: String? = null) : Exception(message)

build.gradle

//Biometrics Prompt
implementation 'androidx.biometric:biometric:1.0.0-alpha03'

//Reactive programming
implementation "io.reactivex.rxjava2:rxkotlin:2.3.0"
implementation 'io.reactivex.rxjava2:rxandroid:2.1.0'

MainActivity.kt

class MainActivity : AppCompatActivity() {

    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        setContentView(R.layout.activity_main)
        
        val biometricsManager: BiometricsManager = BiometricsManagerImpl()
        biometricsManager.authenticate(this)
            .observeOn(AndroidSchedulers.mainThread())
            .subscribe({
                // Authentication Succeeded
            }, { exception ->
                when (exception) {
                    is BiometricsNotAvailableException -> {
                        // Biometrics not available
                    }
                    is BiometricCancelledException -> {
                        // User error
                    }
                }
            })
    } 
}