## 最近的问题及解决方案

### onPressed

}函数里面一定要定义setState()函数，在这个函数中进行 UI 相关参数修改，不然系统不会即时刷新 UI

This call to setState tells the Flutter framework that something has changed in this State, which causes it to rerun the build method below so that the display can reflect the updated values. If we changed _counter(更新 UI 所需参数) without calling setState(), then the build method would not be called again, and so nothing would appear to happen.