你如何在Dart中建立一个单身人士?

问题描述 投票:107回答:12

单例模式确保只创建一个类的一个实例。我如何在Dart中构建它?

dart singleton
12个回答
175
投票

感谢Dart的factory constructors,很容易建立一个单身人士:

class Singleton {
  static final Singleton _singleton = new Singleton._internal();

  factory Singleton() {
    return _singleton;
  }

  Singleton._internal();
}

你可以用new构建它

main() {
  var s1 = new Singleton();
  var s2 = new Singleton();
  print(identical(s1, s2));  // true
  print(s1 == s2);           // true
}

0
投票

由于我不是非常喜欢使用new关键字或其他构造函数,例如调用单例,我更喜欢使用名为inst的静态getter,例如:

// 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");
}

示例用法:

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));

0
投票

您好,这样的事情怎么样?非常简单的实现,Injector本身是单例,并且还在其中添加了类。当然可以很容易地扩展。如果您正在寻找更复杂的东西,请查看此包装: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;
  }
}

0
投票

单例实例后无法更改对象的单例

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, idade: 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}

34
投票

以下是在Dart中创建单例的几种不同方法的比较。

1. Factory constructor

class SingletonOne {

  SingletonOne._privateConstructor();

  static final SingletonOne _instance = SingletonOne._privateConstructor();

  factory SingletonOne(){
    return _instance;
  }

}

2. Static field with getter

class SingletonTwo {

  SingletonTwo._privateConstructor();

  static final SingletonTwo _instance = SingletonTwo._privateConstructor();

  static SingletonTwo get instance { return _instance;}

}

3. Static field

class SingletonThree {

  SingletonThree._privateConstructor();

  static final SingletonThree instance = SingletonThree._privateConstructor();

}

How to instanstiate

上面的单例实例化如下:

SingletonOne one = SingletonOne();
SingletonTwo two = SingletonTwo.instance;
SingletonThree three = SingletonThree.instance;

注意:

我最初问这是一个question,但发现上述所有方法都是有效的,选择在很大程度上取决于个人偏好。


31
投票

我发现阅读new Singleton()并不是非常直观。您必须阅读文档才能知道new实际上并没有像通常那样创建新实例。

这是做单身人士的另一种方式(基本上是安德鲁所说的)。

LIB / thing.dart

library thing;

final Thing thing = new Thing._private();

class Thing {
   Thing._private() { print('#2'); }
   foo() {
     print('#3');
   }
}

main.dart

import 'package:thing/thing.dart';

main() {
  print('#1');
  thing.foo();
}

请注意,由于Dart的延迟初始化,直到第一次调用getter时才会创建单例。

如果你愿意,你也可以在单例类上实现单例作为静态getter。即Thing.singleton,而不是顶级的吸气剂。

还阅读了Bob Nystrom对singletons from his Game programming patterns book的看法。


12
投票

如何在库中使用全局变量呢?

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);
}

或者这不赞成?

单例模式在Java中是必要的,其中全局变量的概念不存在,但似乎你不需要在Dart中走很长的路。


9
投票

由const构造函数和工厂执行dart singleton

class Singleton {
  factory Singleton() =>
    const Singleton._internal_();
  const Singleton._internal_();
}


void main() {
  print(new Singleton() == new Singleton());
  print(identical(new Singleton() , new Singleton()));
}

9
投票

这是另一种可能的方式:

void main() {
  var s1 = Singleton.instance;
  s1.somedata = 123;
  var s2 = Singleton.instance;
  print(s2.somedata); // 123
  print(identical(s1, s2));  // true
  print(s1 == s2); // true
  //var s3 = new Singleton(); //produces a warning re missing default constructor and breaks on execution
}

class Singleton {
  static final Singleton _singleton = new Singleton._internal();
  Singleton._internal();
  static Singleton get instance => _singleton;
  var somedata;
}

2
投票

修改@Seth Ladd回答谁更喜欢像.shared这样的Swift风格的单身人士:

class Auth {
  // singleton
  static final Auth _singleton = Auth._internal();
  factory Auth() => _singleton;
  Auth._internal();
  static Auth get shared => _singleton;

  // variables
  String username;
  String password;
}

样品:

Auth.shared.username = 'abc';

1
投票

这是一个结合其他解决方案的简洁示例。访问单例可以通过以下方式完成:

  • 使用指向实例的singleton全局变量。
  • 常见的Singleton.instance模式。
  • 使用默认构造函数,它是返回实例的工厂。

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是一致的。

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,那么在程序中稍后使用该实例之前,您必须这样做。

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
}

0
投票

这应该工作。

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

    _(){

    }
    factory GlobalStore()=> instance;


}
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