因为我不太喜欢使用new关键字或其他构造函数，比如对单例对象的调用，我更喜欢使用一个名为inst的静态getter:

// the singleton class
class Dao {
    // singleton boilerplate
        Dao._internal() {}
        static final Dao _singleton = new Dao._internal();
        static get inst => _singleton;

    // business logic
        void greet() => print("Hello from singleton");
}

使用示例:

Dao.inst.greet();       // call a method

// Dao x = new Dao();   // compiler error: Method not found: 'Dao'

// verify that there only exists one and only one instance
assert(identical(Dao.inst, Dao.inst));

这就是我如何在我的项目中实现单例

灵感来自flutter firebase => FirebaseFirestore.instance.collection('collectionName')

class FooAPI {
  foo() {
    // some async func to api
  }
}

class SingletonService {
  FooAPI _fooAPI;

  static final SingletonService _instance = SingletonService._internal();

  static SingletonService instance = SingletonService();

  factory SingletonService() {
    return _instance;
  }

  SingletonService._internal() {
    // TODO: add init logic if needed
    // FOR EXAMPLE API parameters
  }

  void foo() async {
    await _fooAPI.foo();
  }
}

void main(){
  SingletonService.instance.foo();
}

来自我的项目的例子

class FirebaseLessonRepository implements LessonRepository {
  FirebaseLessonRepository._internal();

  static final _instance = FirebaseLessonRepository._internal();

  static final instance = FirebaseLessonRepository();

  factory FirebaseLessonRepository() => _instance;

  var lessonsCollection = fb.firestore().collection('lessons');
  
  // ... other code for crud etc ...
}

// then in my widgets
FirebaseLessonRepository.instance.someMethod(someParams);

下面是一个组合了其他解决方案的简洁示例。可以通过以下方式访问单例对象:

使用指向实例的单例全局变量。 通用单例。模式实例。 使用默认构造函数，它是一个返回实例的工厂。

注意:您应该只实现三个选项中的一个，以便使用单例的代码保持一致。

Singleton get singleton => Singleton.instance;
ComplexSingleton get complexSingleton => ComplexSingleton._instance;

class Singleton {
  static final Singleton instance = Singleton._private();
  Singleton._private();
  factory Singleton() => instance;
}

class ComplexSingleton {
  static ComplexSingleton _instance;
  static ComplexSingleton get instance => _instance;
  static void init(arg) => _instance ??= ComplexSingleton._init(arg);

  final property;
  ComplexSingleton._init(this.property);
  factory ComplexSingleton() => _instance;
}

如果需要进行复杂的初始化，则必须在稍后在程序中使用实例之前进行。

例子

void main() {
  print(identical(singleton, Singleton.instance));        // true
  print(identical(singleton, Singleton()));               // true
  print(complexSingleton == null);                        // true
  ComplexSingleton.init(0); 
  print(complexSingleton == null);                        // false
  print(identical(complexSingleton, ComplexSingleton())); // true
}

在库中使用全局变量怎么样?

single.dart:

library singleton;

var Singleton = new Impl();

class Impl {
  int i;
}

main.dart:

import 'single.dart';

void main() {
  var a = Singleton;
  var b = Singleton;
  a.i = 2;
  print(b.i);
}

或者这是不受欢迎的?

单例模式在Java中是必要的，因为在Java中不存在全局变量的概念，但在Dart中似乎不需要绕这么长的路。

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。

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。