Singleton Pattern

Singleton Pattern nima?

Singleton Pattern — bu creational design pattern bo‘lib, u class (klass) yoki objectning faqat bir marta instantiate bo’lishini kafolatlaydi va unga global tarzda murojaat qilish imkonini beradi. Ushbu yagona instance butun application bo‘ylab ulashilishi mumkin, shu sababli Singletonlar application ichida global stateni boshqarish uchun juda qulay hisoblanadi.

Frontend ilovalarda bu pattern ko‘pincha:

• global state
• konfiguratsiya
• logger
• API client
• cache yoki service manager

uchun ishlatiladi.

Implementation

ES6 Class

ES2015 class yordamida singleton yaratish quyidagi tarzda amalga oshiriladi:

class ConfigService {
  static instance;

  constructor() {
    if (ConfigService.instance) {
      return ConfigService.instance;
    }

    this.apiUrl = "https://api.example.com";
    ConfigService.instance = this;
  }

  getApiUrl() {
    return this.apiUrl;
  }
}

// Foydalanish
const c1 = new ConfigService();
const c2 = new ConfigService();

console.log(c1 === c2); // true

Bu yerda classni qayta chaqirsak ham, doim bitta instance qaytaradi va xotirani tejashimizga yordam beradi.

Closure

Biz class dan foydalanmasdan ham to‘g‘ridan-to‘g‘ri singleton yaratishimiz mumkin, bu esa ancha sodda va toza code yozishga olib keladi.

Oddiy closure yordamida singleton yaratish uchun biz quyidagilarni bajarishimiz kerak:

const Singleton = (function () {
  let instance;

  function createInstance() {
    return {
      apiUrl: "https://api.example.com",
      logApiUrl: () => console.log("https://api.example.com"),
    };
  }

  return {
    getInstance() {
      if (!instance) {
        instance = createInstance();
      }
      return instance;
    },
  };
})();

// Foydalanish
const a = Singleton.getInstance();
const b = Singleton.getInstance();

console.log(a === b); // true

Pros

Memory: Instantiation ni faqat bitta instance bilan cheklash juda ko‘p memory space ni tejashi mumkin. Har safar yangi instance uchun memory ajratish o‘rniga, biz faqat bitta instance uchun memory ajratamiz va u butun application davomida reference qilinadi.

Cons

Testing: Har safar yangi instance yarata olmaganimiz sababli, barcha testlar oldingi test tomonidan o‘zgartirilgan global instance ga tayanadi. Bu holatda testlarning bajarilish tartibi muhim bo‘ladi va bitta kichik o‘zgarish ham butun test suite ning ishdan chiqishiga olib kelishi mumkin. Testlardan so‘ng, testlar davomida kiritilgan o‘zgarishlarni bekor qilish uchun butun instance’ni reset qilishimiz kerak bo‘ladi.

Note

Yuqoridagi misollarda ko‘rsatilgan class implementation aslida ortiqcha hisoblanadi. JavaScriptda to‘g‘ridan-to‘g‘ri object lar yaratishimiz mumkin bo‘lgani uchun, aynan shu natijaga erishish uchun oddiy object dan foydalanishimiz mumkin. Keling, object orqali singleton yaratishni ko‘rib chiqamiz!

const apiConfig = Object.freeze({
  apiUrl: "https://api.example.com",
  logApiUrl: () => console.log("https://api.example.com"),
});