虽然网上有一些现成的类，但是觉得过于庞大（或者是有简单的我没找到），所以自己写了一个很简单的，需要什么功能就慢慢往里加。-some ready-made category, but think it is too large (or is a simple, I could not find). So they wrote a very simple, functional needs and poured slowly on the increase.