App architecture: activities, fragments, and Firebase

class ApplicationClass : Application() {

    private val ONESIGNAL_APP_ID = BuildConfig.ONESIGNAL_APP_ID
    private var startedActivities = 0
    private var isChangingConfig = false

    override fun onCreate() {
        super.onCreate()

        OneSignal.Debug.logLevel = LogLevel.VERBOSE
        OneSignal.initWithContext(this, ONESIGNAL_APP_ID)

        CoroutineScope(Dispatchers.IO).launch {
            OneSignal.Notifications.requestPermission(false)
        }

        registerActivityLifecycleCallbacks(object : ActivityLifecycleCallbacks {
            override fun onActivityStarted(activity: Activity) {
                if (startedActivities == 0 && !isChangingConfig) {
                    actualizarEstadoOnline(true)
                }
                startedActivities++
            }

            override fun onActivityStopped(activity: Activity) {
                isChangingConfig = activity.isChangingConfigurations
                startedActivities--
                if (startedActivities == 0 && !isChangingConfig) {
                    actualizarEstadoOnline(false)
                }
            }
            // ... other no-op overrides
        })
    }

    companion object {
        fun actualizarEstadoOnline(online: Boolean) {
            val uid = FirebaseAuth.getInstance().uid ?: return
            val ref = FirebaseDatabase.getInstance().getReference("Usuarios").child(uid)
            if (online) {
                ref.child("online").setValue(true)
                ref.child("online").onDisconnect().setValue(false)
            } else {
                ref.child("online").setValue(false)
            }
        }
    }
}

object Constantes {

    const val MENSAJE_TIPO_TEXTO  = "TEXTO"
    const val MENSAJE_TIPO_IMAGEN = "IMAGEN"

    fun obtenerTiempoDis(): Long = System.currentTimeMillis()

    fun obtenerFecha(tiempo: Long): String {
        val calendario = Calendar.getInstance(Locale.ENGLISH)
        calendario.timeInMillis = tiempo
        return DateFormat.format("dd/MM/yyyy", calendario).toString()
    }

    fun obtenerFechaHora(tiempo: Long): String {
        val calendar = Calendar.getInstance(Locale.ENGLISH)
        calendar.timeInMillis = tiempo
        return DateFormat.format("dd/MM/yyyy hh:mm:a", calendar).toString()
    }

    fun rutaChat(receptorUid: String, emisorUid: String): String {
        val arrayUid = arrayOf(receptorUid, emisorUid)
        Arrays.sort(arrayUid)
        return "${arrayUid[0]}_${arrayUid[1]}"
    }
}

com.example.applacasadelbordadito/
├── Adaptadores/          # AdaptadorChat, AdaptadorUsuario
├── Bordado/              # PatronBordado model, BordadoCanvasView, adapters
├── Cafe/                 # Cafe model, AgregarCafeActivity, adapters
├── Carrito/              # CarritoItem model, CarritoActivity, QRActivity
├── Chat/                 # ChatActivity, ListaChatActivity
├── Fragmentos/           # FragmentInicio, FragmentCafe, FragmentBordado, FragmentCuenta
├── Historial/            # Orden model, HistorialActivity, HistorialDetalleActivity
├── Modelos/              # Usuario, Chat (message model)
├── Notificaciones/       # FcmUtil
├── Opciones_Login/       # Login_email
├── Perfil/               # PaymentMethod, EditarPerfil, TarjetaAgregarActivity
├── Taller/               # TallerActivity
├── ApplicationClass.kt
├── Constantes.kt
├── MainActivity.kt
├── OpcionesLogin.kt
├── Registro_email.kt
└── VerificarEmailActivity.kt