AuthenticatorActivity.kt

// Register the callback for the Facebook authentication
LoginManager.getInstance().registerCallback(callbackManager, object : FacebookCallback<LoginResult> {
    override fun onSuccess(result: LoginResult?) {
        Log.d(TAG, "Facebook token = ${result?.accessToken?.token}")
        toast("Access Token received")
   }

    override fun onCancel() {
        Log.d(TAG, "Facebook authentication is cancelled")
        toast("Facebook Authentication is cancelled")
    }

    override fun onError(error: FacebookException?) {
        Log.e(TAG, "Facebook Authentication failed: ${error?.message}")
        toast("Facebook Authentication failed: ${error?.message}")
    }
})