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}

修改了@Seth Ladd关于谁更喜欢Swift风格的单例，比如。shared的答案:

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

  // variables
  String username;
  String password;
}

示例:

Auth.shared.username = 'abc';

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;
}

这也是一种创建Singleton类的方法

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

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}

这里有一个简单的答案:

类应具有其类型的私有和静态属性。 构造函数应该是私有的，以防止外部对象初始化。 检查实例是否为空，如果是，创建一个实例并返回，否则返回现有实例。

实现(延迟加载)

class Singleton {
  static Singleton? _instance;

  Singleton._();

  static Singleton get instance => _instance ??= Singleton._();

  void someMethod(){
    ...
  }

  ...
}

实现(急装)

class Singleton {
  static Singleton _instance = Singleton._();

  Singleton._();

  static Singleton get instance => _instance;

  void someMethod(){
    ...
  }

  ...
}

使用

Singleton.instance.someMethod();