模型类充当数据的占位符。

viewmodel填充这些模型。

每当模型发生变化时，viewmodel会通知UI由于数据的变化需要刷新它。

Model classes act as the placeholder for your data.
The viewmodel populates these models.
Whenever, there are changes to the model, the viewmodel notifies the UI that it needs to be refreshed due to change in data.