SOURCE

class EventEmitter {
    constructor() {
        this.cache = {}
    }
    on(name, fn) {
        if (this.cache[name]) {
            this.cache[name].push(fn)
        } else {
            this.cache[name] = [fn]
        }
    }
    off(name, fn) {
        let tasks = this.cache[name]
        if (tasks) {
            const index = tasks.findIndex(f => f === fn || f.callback === fn)
            if (index > -1) {
                tasks.splice(index, 1)
            }
        }
    }
    emit(name, once = false, ...args) {
        if (this.cache[name]) {
            // 创建副本，如果回调函数内部注册相同事件，会造成死循环
            let tasks = this.cache[name].slice()
            for (let fn of tasks) {
                fn(...args)
            }
            if (once) {
                delete this.cache[name]
            }
        }
    }
}