封装好的observer.js，用于非父子组件传值，直接调用$on和$emit方法

const eventList = {}

const $on = (eventName,callback)=>{

    if(!eventList[eventName]){

        eventList[eventName] = [];

    }

    eventList[eventName].push(callback)

}

const $emit = (eventName,params)=>{

    if(eventList[eventName]){

        let arr = eventList[eventName];

        arr.map((cb)=>{

            cb(params);

        })

    }

}

const $off = (eventName,callback)=>{

    if(eventList[eventName]){

        if(callback){

            let index = eventList[eventName].indexOf(callback);

            eventList[eventName].splice(index,1);

        }else{

            eventList[eventName].length = 0;

        }

    }

}

export default {

    $on,

    $emit,

    $off

}