第4节：Flutter设计模式

一、单例

参照SharedPreferences：

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;
  }

创建一个单例的Manager类

class Manager {
  // 工厂模式
  factory Manager() =>_getInstance();
  static Manager get instance => _getInstance();
  
  static Manager _instance;
  Manager._internal() {
    // 初始化
  }
  static Manager _getInstance() {
    if (_instance == null) {
      _instance = new Manager._internal();
    }
    return _instance;
  }
}

调用

// 无论如何初始化，取到的都是同一个对象
Manager manager = new Manager();
Manager manager2 = Manager.instance;