在Dart中创建Singleton没有什么棘手的。您可以在顶级(全局)位置声明任何变量，默认情况下这是一个单例。也可以将变量声明为类的静态成员。这是单元素a。

class A {}

final a = A();

However, the above does not allow you to replace the instance for testing. The other issue is that as the app grows in complexity, you may want to convert global or static variables to transient dependencies inside your classes. If you use dependency injection, you can change a dependency inside your composition at any time. This is an example of using ioc_container to configure a singleton instance of A in the root of an app. You can change this to a transient dependency any time by using add instead of addSingletonService

import 'package:ioc_container/ioc_container.dart';

class A {}

void main(List<String> arguments) {
  final builder = IocContainerBuilder()..addSingletonService(A());
  final container = builder.toContainer();
  final a1 = container<A>();
  final a2 = container<A>();
  print(identical(a1, a2));
}

上面打印为真，因为应用程序只会生成一个A实例。

如何在飞镖扑动中创建类的单例实例

  class ContactBook {
      ContactBook._sharedInstance();
      static final ContactBook _shared = ContactBook._sharedInstance();
      factory ContactBook() => _shared;
    }

这也是一种创建Singleton类的方法

class Singleton{
  Singleton._();
  static final Singleton db = Singleton._();
}

你可以只使用Constant构造函数。

class Singleton {
  const Singleton(); //Constant constructor
  
  void hello() { print('Hello world'); }
}

例子:

Singleton s = const Singleton();
s.hello(); //Hello world

根据文件:

恒定的构造函数 如果类生成永不更改的对象，则可以使这些对象成为编译时常量。为此，定义一个const构造函数，并确保所有实例变量都是final变量。

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;
  }
}

创建单例

class PermissionSettingService {
  static PermissionSettingService _singleton = PermissionSettingService._internal();

  factory PermissionSettingService() {
    return _singleton;
  }

  PermissionSettingService._internal();
}

重置单例

// add this function inside the function
  void reset() {
   _singleton = PermissionSettingService._internal();
  }