useTask$()
使用 useTask$() 在初始渲染之前以及跟踪值发生变化时执行函数。该函数在渲染之前执行，但不能延迟渲染，因此如果 useTask$() 是异步的，则渲染将在 useTask$() 完全执行之前发生。
跟踪存储更改
The useTask$() hook receives a track() function used for setting up subscriptions that will automatically rerun the useTask$() hook. The set of track() properties resets on each useTask$() execution, therefore it is important to always use track() to set up subscriptions anew. Because of this it is possible for the useTask$() to subscribe to different properties over time.
清理
The useTask$() hook can return a cleanup callback. This is useful for cleaning up any resources before new execution.