单例构造函数
单例模式比较简单，它保证一个类只能有一个实例，并且提供一个全局的访问入口。采用单例模式的类应尽量保证无法被实例化，否则就缺失了单例的实际意义，因此开发者在定义单例类时不要去实现默认的无名构造函数：
class Singleon {
Singleon._();
static Singleon _instance;
static Singleon getInstance() {
if (_instance == null) {
_instance = Singleon._();
}
return _instance;
}
}
main(List<String> args) {
// var singleonInstance = new Singleon(); // The class 'Singleon' doesn't have a default constructor.
var singleonInstance = Singleon.getInstance();
var singleonInstance2 = Singleon.getInstance();
print(singleonInstance == singleonInstance2); // true
}
class Singleon {
private constructor() {}
private static _instance
static getInstance(): Singleon {
if (this._instance == null) {
this._instance = new Singleon()
}
return this._instance
}
}
// const singleonInstance = new Singleon() // 类“Singleon”的构造函数是私有的，仅可在类声明中访问。
const singleonInstance = Singleon.getInstance()
const singleonInstance2 = Singleon.getInstance()
console.log(singleonInstance == singleonInstance2) // true