Flutter 单例模板 支持空安全(null safety)

class MyInstance {
  
  
 
  static const tag = "MyInstance";
  
  static MyInstance? _instance;
  
  MyInstance._() {
    // 具体初始化代码
  }
  
  factory MyInstance() {
    _instance ??= MyInstance._();
    return _instance!;
  }
}