javascript - 使用 React Native 创建单例类

Question

我发现不同的主题解释了如何创建单例，但没有一个对我有用。这是取自这篇文章的例子

export default class Credentials {

    static myInstance = null;

    _userID = "";

    static getInstance() {
        if (myInstance == null) {
            myInstance = new Credentials();
        }

        return myInstance;
    }

    getUserID() {
        return this._userID;
    }

    setUserID(id) {
        this._userID = id;
    }
}

当我调用 Credentials.getInstance() 时，我收到警告

Can't find variable myInstance

Answer 1

JS 没有像静态编译语言那样的隐式字段查找。您需要明确地查找类上的变量:

class Credentials {

    static myInstance = null;

    static getInstance() {
      if (Credentials.myInstance == null) {
        Credentials.myInstance = new Credentials();
      }

      return Credentials.myInstance;
    }
}

谨慎使用这种方法，因为它不是真正的单例，因为 JS 没有适当的类封装。

您可以轻松地直接更改实例:

Credentials.myInstance = 'something else';

正确的封装单例应该通过闭包来实现:

const Credentials = (() => {
  let myInstance = null;
  return class Credentials {
    static getInstance() {
      if (myInstance == null) {
        myInstance = new Credentials();
      }
      return myInstance;
    } 
  }
})()