Android Paging Library. How to filter uploaded data?

Question

Android

Loading data from the network. The doc says

Note: PagedList is content-immutable. This means that, although new content can be loaded into an instance of PagedList, the loaded items themselves cannot change once loaded

don't you change?

Network only
To display data from a backend server, use the synchronous version of the Retrofit API

why synchronously load data?
Plus, how to find out the number of downloaded data?