create a BiometricPrompt

createBiometricPrompt.kt

private lateinit var biometricPrompt: BiometricPrompt

override fun onCreate(savedInstanceState: Bundle?) {
   super.onCreate(savedInstanceState)
   setContentView(R.layout.activity_main)
   setSupportActionBar(findViewById(R.id.toolbar))
   // ...   
   biometricPrompt = createBiometricPrompt()
}

private fun createBiometricPrompt(): BiometricPrompt {
   val executor = ContextCompat.getMainExecutor(this)

   val callback = object : BiometricPrompt.AuthenticationCallback() {
       override fun onAuthenticationError(errorCode: Int, errString: CharSequence) {
           super.onAuthenticationError(errorCode, errString)
           Log.d(TAG, "$errorCode :: $errString")
           if (errorCode == BiometricPrompt.ERROR_NEGATIVE_BUTTON) {
               loginWithPassword() // Because in this app, the negative button allows the user to enter an account password. This is completely optional and your app doesn’t have to do it.
           }
       }

       override fun onAuthenticationFailed() {
           super.onAuthenticationFailed()
           Log.d(TAG, "Authentication failed for an unknown reason")
       }

       override fun onAuthenticationSucceeded(result: BiometricPrompt.AuthenticationResult) {
           super.onAuthenticationSucceeded(result)
           Log.d(TAG, "Authentication was successful")
           // Proceed with viewing the private encrypted message.
           showEncryptedMessage(result.cryptoObject)
       }
   }

   val biometricPrompt = BiometricPrompt(this, executor, callback)
   return biometricPrompt
}