ES6 Promise 用法讲解:https://www.cnblogs.com/whybxy/p/7645578.html
廖雪峰 promise讲解:https://www.liaoxuefeng.com/wiki/001434446689867b27157e896e74d51a89c25cc8b43bdb3000/0014345008539155e93fc16046d4bb7854943814c4f9dc2000
