发布订阅模式

class EventEmitter {
    constructor() {
        // 缓存列表
        this.list = {}
    }
    // 订阅
    on(event, cb) {
        (this.list[event] || (this.list[event] = [])).push(cb)
    }
    // 发布
    emit(event, ...args) {
        this.list[event] && this.list[event].forEach(cb => {
            cb(...args)
        })
    }
    // 只订阅一次
    once(event, onceCb) {
        const cb = (...args) => {
            onceCb(...args)
            this.off(event, cb)
        }
        this.on(event, cb)
    }
    // 取消订阅
    off(event, offCb) {
        if (this.list[event]) {
            let index = this.list[event].findIndex(cb => cb === offCb)
            this.list[event].splice(index, 1)
            if (this.list[event].length === 0) Reflect.deleteProperty(this.list, event);
        }
    }
}