A flutter singleton async-safe example

location_cache.dart

import 'dart:async';
import 'dart:convert';
import 'dart:typed_data';

import 'package:google_maps_flutter/google_maps_flutter.dart';
import 'package:hive/hive.dart';

class LocationCacheSingleton {
  static LocationCacheSingleton _instance;
  static Future<Null> _mutex;

  // Não é thread-safe, apenas async-safe. Entretanto, como o
  //  flutter é single-thread, essa solução não tem problema.
  static Future<LocationCacheSingleton> getInstance() async {
    if (_mutex != null) {
      await _mutex;
    }
    var completer = new Completer<Null>();
    _mutex = completer.future;

    if (_instance == null) {
      _instance = LocationCacheSingleton();
      await _instance.init();
    }

    completer.complete();
    _mutex = null;

    return _instance;
  }

  static String SerializeLatLng(LatLng latlng) {
    ByteData bdata = ByteData.view(Uint8List(16).buffer);
    bdata.setFloat64(0, latlng.latitude);
    bdata.setFloat64(8, latlng.longitude);
    return Base64Codec().encode(bdata.buffer.asUint8List());
  }

  static LatLng DeserializeLatLng(String latlng) {
    ByteData bdata = ByteData.view(Base64Codec().decode(latlng).buffer);
    return LatLng(bdata.getFloat64(0), bdata.getFloat64(8));
  }

  Box box;

  LocationCacheSingleton();

  Future<LocationCacheSingleton> init() async {
    this.box = await Hive.openBox('location');
    return this;
  }

  dynamic get(String key) {
    return this.box.get(key);
  }

  Future<void> put(String key, dynamic value) async {
    return await this.box.put(key, value);
  }
}