How To Create A Singleton in Dart

第一种写法（会用一种就好）

借助dart提供的工厂构造函数，可以很方便的创建一个单例对象

class Singleton {
  // 这样写就行了，别问为什么，我也不知道，慢慢理解
  static final Singleton _singleton = Singleton._internal();

  // Use the factory keyword when implementing a constructor that doesn’t always create a new instance of its class.
  // For example, a factory constructor might return an instance from a cache, or it might return an instance of a subtype.
  factory Singleton() => _singleton;

  Singleton._internal(); // private constructor
}

main() {
  var s1 = Singleton();
  var s2 = Singleton();

  print(identical(s1, s2));  // true
  print(s1 == s2);           // true
}

第二种写法（如果别人这样写，能读懂代码就OK）

Another way is to use a static field that use a private constructor. This way it is not possible to create a new instance using Singleton(), the reference to the only instance is available only through the instance static field.

class Singleton {
  Singleton._privateConstructor();

  static final Singleton instance = Singleton._privateConstructor();
}

main() {
  var s1 = Singleton.instance;
  var s2 = Singleton.instance;

  print(identical(s1, s2));  // true
  print(s1 == s2);           // true
}