JavaScript- singleton

Singleton

It is the only system using
a class has only one instance of

Traditional UML class diagram

• Singleton mode uses characteristics of java (Private)
• es6 No (except the typescript)
• You can only use Java to write the singleton pattern purely UML class diagram

public class SingleObject {
    // 私有化构造函数，外部不能new，只能内部new！！！
    private SingleObject() {

    }
    // 唯一被new出来的对象
    private SingleObject instance = null
    // 获取对象唯一的接口
    public SingleObject getInstance() {
        if (instance == null) {
            // 只能new一次
            instance = new SingleObject();
        }
        return instance
    }
    // 对象方法
    public void login(username, password) {
        System.out.println("login..,");
    }
}

public class SingletonPatternDemo {
    public static void main(String[] args) {
        //不合法的构造函数
        //编译时报错：构造函数SingleObject()不可见！！！
        //SingleObject object = new SingleObject();

        //获取唯一可用的对象
        SingleObject object = SingleObject.getInstance();
        object.login();
    }
}

Singleton pattern in javascript

class SingleObject {
    login() {
        console.log('login...')
    }
}
SingleObject.getInstance = (function () {
    let instance
    return function () {
        if (!instance) {
            instance = new SingleObject();
        }
        return instance
    }
})()

// 无法控制
let obj1 = SingleObject.getInstance()
obj1.login()
let obj2 = SingleObject.getInstance()
obj2.login()
console.log(obj1 === obj2)

//------------------------------------------

let obj4 = SingleObject.getInstance()
obj4.login()
let obj5 = new SingleObject()
obj5.login()
console.log(obj4 === obj5)// false

Scenarios

jQuery

//jQuery 只有一个 '$'
if (window.jQuery != null) {
    return window.jQuer
}else {
    //初始化......
}

Analog login box

class LoginForm {
    constructor() {
        this.state = 'hide'
    }
    show() {
        if (this.state === 'show') {
            alert('已经显示')
            return
        }
        this.state = 'show'
        console.log('登录框已显示')
    }
    hide() {
        if (this.state === 'hide') {
            alert('已经隐藏')
            return
        }
        this.state = 'hide'
        console.log('登录框已隐藏')
    }
}
LoginForm.getInstance = (function () {
    let instance
    return function () {
        if (!instance) {
            instance = new LoginForm();
        }
        return instance
    }
})()

// 一个页面中调用登录框
let login1 = LoginForm.getInstance()
login1.show()
// login1.hide()

// 另一个页面中调用登录框
let login2 = LoginForm.getInstance()
login2.show()

// 两者是否相等
console.log('login1 === login2', login1 === login2)

Design Principles verification

• In line with the principle of a single, unique object is instantiated only
• Open Closed Principle can not specific, but not in violation of the open closed principle