观察者模式

class StateTracker{    constructor(){        this.observers = [];        this.internalState= 10;    }    // 改变内部状态，触发状态的观察者列表    change(val){        this.internalState= val;        this.observers.forEach(observer=>observer(val));    }// 注册观察者    registerObserver(ObserverFn){        this.obserers.push(ObserverFn)    }}

class PubSubHandler{
       constructor(){
           this.eventPool = {};
       }
       //移除
       off(topicName){
           delete this.observers[topicName]
       }
       //发布
       trigger(topicName,...args){
           this.eventPool[topicName] && 
           this.eventPool[topicName].forEach(callback=>callback(...args));
       }
       //订阅
       on(topicName,callback){
           let topic = this.eventPool[topicName] ;
           if(!topic){
               this.eventPool[topicName] =[]
           }
           this.eventPool[topicName].push(callback)
       }
   }

var single = () => {
    var instance;
    var createInstance = () => {
        this.a = 1;
        this.b = 2;
    }
    return {
        getInstance:() => {
            if(!instance) {
                instance = createInstance();
            }
            return instance;
        }
    }
}