Javascript design pattern system explanation and application——study notes 3-singleton pattern

Singleton mode

• Used only in the system
• There is only one instance of a class

Example

• Login box
• shopping cart

note:

• Singleton mode needs to use java features (private)
• Not in ES6 (except for typescript)
• Only java code can be used to demonstrate the content of UML diagrams

Java code demonstration:

public class SingleObject {
    
    
  // 注意，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;
  // 获取对象的唯一接口
  public SingleObject getInstance() {
    
    
    if (instance == null) {
    
    
      // 只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
    // 编译时错误：构造函数SingleObject()是不可见的
    // SingleObject object = new SingleObject();

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

JS uses singleton mode

class SingleObject {
    
    
  login() {
    
    
    console.log('login...')
  }
}

SingleObject.getInstance = (funciton () {
    
    
  let instance
  return function () {
    
    
    if (!instance) {
    
    
      instance = new SingleObject()
    }
    return instance
  }
})()

// 测试：注意这里只能使用静态函数getInstance, 不能使用new SingleObject()。 只能靠文档约束
let obj1 = SingleObject.getInstance()
obj1.login()
let obj2 = SingleObject.getInstance()
obj2.login()
console.log(obj1 === obj2)  // true  两者完全相等
let obj3 = new SingleObject()  // 无法完全控制
obj3.login()
console.log(obj1 === obj3) // false

Scenes

• jQueryonly one$
• Mock login box
• other

jQueryonly one$

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

Mock 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 funciton (0 {
    
    
    if (!instance) {
    
    
      instance = new LoginForm()
    }
    return instance
  })
})()

other

• Shopping cart (similar to login box)
• Store in vuex and redux

Design principle verification

• Comply with the single responsibility principle, only instantiate the only object
• There is no specific open and closed principle, but it absolutely does not violate the open and closed principle