如何在WPF应用程序中定义NHibernate会话和事务的范围

I am relatively new to both WPF and NHibernate and attempting to build an application that combines the two, using the MVVM pattern. However, I am struggling to understand when and where my application should open and close NHibernate sessions and transactions.I am relatively new to both WPF and NHibernate