使用空安全操作符和工厂构造函数可以更好地创建单例对象。

class Singleton {
  static Singleton? _instance;

  Singleton._internal();

  factory Singleton() => _instance ??= Singleton._internal();
  
  void someMethod() {
    print("someMethod Called");
  }
}

用法:

void main() {
  Singleton object = Singleton();
  object.someMethod(); /// Output: someMethod Called
}

注意:? ?是一个Null感知操作符，如果左边值为Null，它将返回右边的值，这意味着在我们的例子中_instance ??Singleton._internal();， Singleton._internal()将在第一次调用object时返回，rest _instance将返回。

你好，这样的东西怎么样?非常简单的实现，Injector本身是单例的，并且还添加了类。当然可以很容易地扩展。如果您正在寻找更复杂的东西，请检查这个包:https://pub.dartlang.org/packages/flutter_simple_dependency_injection

void main() {  
  Injector injector = Injector();
  injector.add(() => Person('Filip'));
  injector.add(() => City('New York'));

  Person person =  injector.get<Person>(); 
  City city =  injector.get<City>();

  print(person.name);
  print(city.name);
}

class Person {
  String name;

  Person(this.name);
}

class City {
  String name;

  City(this.name);
}


typedef T CreateInstanceFn<T>();

class Injector {
  static final Injector _singleton =  Injector._internal();
  final _factories = Map<String, dynamic>();

  factory Injector() {
    return _singleton;
  }

  Injector._internal();

  String _generateKey<T>(T type) {
    return '${type.toString()}_instance';
  }

  void add<T>(CreateInstanceFn<T> createInstance) {
    final typeKey = _generateKey(T);
    _factories[typeKey] = createInstance();
  }

  T get<T>() {
    final typeKey = _generateKey(T);
    T instance = _factories[typeKey];
    if (instance == null) {
      print('Cannot find instance for type $typeKey');
    }

    return instance;
  }
}

这是我做单例的方式，接受参数(你可以直接粘贴到https://dartpad.dev/):

void main() {
  
  Logger x = Logger('asd');
  Logger y = Logger('xyz');
  
  x.display('Hello');
  y.display('Hello There');
}


class Logger{
  
  
  Logger._(this.message);
  final String message;
  static Logger _instance = Logger._('??!?*');
  factory Logger(String message){
    if(_instance.message=='??!?*'){
      _instance = Logger._(message);
    }
    return _instance;
  }
  
  void display(String prefix){
    print(prefix+' '+message);
  }
  
}

输入:

Hello asd
Hello There asd

“? ? ! ?*'你看到的只是一个工作区，我做了初始化_instance变量暂时没有使它成为一个Logger?类型(空安全)。

如果碰巧使用Flutter和提供程序包进行状态管理，则创建和使用单例非常简单。

创建实例

无效主体(){ runApp ( MultiProvider ( 供应商:[ ChangeNotifierProvider(create: (context) => SomeModel())， Provider(create: (context) => SomeClassToBeUsedAsSingleton())， ]， 孩子:MyApp (), )， ）; ｝

获取实例

Widget build（BuildContext context） { var instance = Provider.of<SomeClassToBeUsedAsSingleton>（context）; ...

这应该有用。

class GlobalStore {
    static GlobalStore _instance;
    static GlobalStore get instance {
       if(_instance == null)
           _instance = new GlobalStore()._();
       return _instance;
    }

    _(){

    }
    factory GlobalStore()=> instance;


}

实例化后不能更改对象的单例

class User {
  final int age;
  final String name;
  
  User({
    this.name,
    this.age
    });
  
  static User _instance;
  
  static User getInstance({name, age}) {
     if(_instance == null) {
       _instance = User(name: name, age: age);
       return _instance;
     }
    return _instance;
  }
}

  print(User.getInstance(name: "baidu", age: 24).age); //24
  
  print(User.getInstance(name: "baidu 2").name); // is not changed //baidu

  print(User.getInstance()); // {name: "baidu": age 24}