event.js

/*
* 实现Event类或者EventEmitter类，实现包括on\once\trigger\off等常见的方法
* */
class EventEmitter{
    constructor(){
        this._events={}
    }
    on(event,callback){
        let callbacks = this._events[event] || []
        callbacks.push(callback)
        this._events[event] = callbacks
        return this
    }
    off(event,callback){
        let callbacks = this._events[event]
        this._events[event] = callbacks && callbacks.filter(fn => fn !== callback)
        return this
    }
    emit(...args){
        const event = args[0]
        const params = [].slice.call(args,1)
        const callbacks = this._events[event]
        callbacks.forEach(fn => fn.apply(this, params))
        return this
    }
    once(event,callback){
        let wrapFunc = (...args) => {
            callback.apply(this,args)
            this.off(event,wrapFunc)
        }
        this.on(event,wrapFunc)
        return this
    }
}