Async Await

This article is referenced from: https://www.cnblogs.com/YMaster/p/6920441.html

async/await rules:

1. async means that this is an async function, and await can only be used in this function.

2. await means to wait here for the operation behind await to complete, and then execute the next code.

3. The await is preferably followed by a time-consuming operation or an asynchronous operation.