Flutter 中，两种单例实现

来自，Flutter SharedPreference 包，两种方式，归根结底，属于同一种方式，后一种，借助了Flutter Completer 。

方式一：

class SharedPreferences {
  SharedPreferences._(this._preferenceCache);

  static const String _prefix = 'flutter.';
  static SharedPreferences _instance;
  static Future<SharedPreferences> getInstance() async {
    if (_instance == null) {
      final Map<Object, Object> fromSystem =
          await _kChannel.invokeMethod('getAll');
      assert(fromSystem != null);
      // Strip the flutter. prefix from the returned preferences.
      final Map<String, Object> preferencesMap = <String, Object>{};
      for (String key in fromSystem.keys) {
        assert(key.startsWith(_prefix));
        preferencesMap[key.substring(_prefix.length)] = fromSystem[key];
      }
      _instance = SharedPreferences._(preferencesMap);
    }
    return _instance;
  }
}

方式二：

class SharedPreferences {
  SharedPreferences._(this._preferenceCache);

  static const String _prefix = 'flutter.';
  static Completer<SharedPreferences> _completer;

  static SharedPreferencesStorePlatform get _store =>
      SharedPreferencesStorePlatform.instance;

  /// Loads and parses the [SharedPreferences] for this app from disk.
  ///
  /// Because this is reading from disk, it shouldn't be awaited in
  /// performance-sensitive blocks.
  static Future<SharedPreferences> getInstance() async {
    if (_completer == null) {
      _completer = Completer<SharedPreferences>();
      try {
        final Map<String, Object> preferencesMap =
            await _getSharedPreferencesMap();
        _completer.complete(SharedPreferences._(preferencesMap));
      } on Exception catch (e) {
        // If there's an error, explicitly return the future with an error.
        // then set the completer to null so we can retry.
        _completer.completeError(e);
        final Future<SharedPreferences> sharedPrefsFuture = _completer.future;
        _completer = null;
        return sharedPrefsFuture;
      }
    }
    return _completer.future;
  }
}