afternoon :)

hello, Eric!

how can I help you today?

I am posting a List<Item> to my fragment from my viewmodel livedata in order to manually perform paging using notifyItemRangeInserted(offset, numItems). The problem I am facing is I don't want to use a variable offset in my fragment. is there a method I can use that provides an offset?

well... that depends on what you're looking to get an offset from

the way I currently implemented it, offset=0, 10, 20, basically adds pagesize every time

actually List<Item>.size

is the List<Item> being published only representing one page? or is it all loaded items, including a new page's worth?

single page

so publish 1=page 1, publish 2=page 2

what are you doing when the user scrolls up?

nothing, it works as is because I am adding to my list

"I am adding to my list" does not seem to mesh with "single page" from my earlier question, though

so, when you get a List<Item> from this LiveData, are you appending it to some master List<Item> that represents the entire data set (or the portion loaded to date)?

I am appending it to a mster List<item> that represents the entire set of data, but I post per page

OK, then if "actually List<Item>.size" refers to the size of the master List<Item>, off the cuff, that seems reasonable

IOW, it should be whatever your Adapter is returning for getItemCount()

which I assume is the size() of your master List<Item>

yes, thanks

is there a way for the viewmodel to know if my screen was rotated? I want to post my stored list in my viewmodel and return instead of calling into my model layer

I could do this fine if I didn't paginate by seeing if my list was null but since I call it multiple times for a paginated list I can't

well, IMHO, this feels like you're going about things incorrectly -- the viewmodel should be holding the master List<Item>, so it survives a configuration change

blending in with the rest of your approach, the LiveData would publish the updated master List<Item> along with the page offset and size to pass to notifyItemRangeInserted()

my assumption is that right now, your master List<Item> is just being held in your Adapter, which is why you're losing it on the configuration change

FWIW, if you are doing this in Kotlin, the Jetpack Paging 3 library is rather nice and is much easier to use for custom data sources than was its predecessor

actually, now that I think about it my live data is expecting a page of data, not a master list so I don't know if I can even differentiate between them in my observer

I tried using the paging library with rxjava before and it was pretty difficult, not sure if it was improved

Paging 3 + Kotlin + RxJava works well -- I'm using that for a customer's project right now

again, item 1 means that IMHO you are thinking about the problem incorrectly

if you wish to hold onto certain data across a configuration change, the viewmodel should be owning that data

if your master List<Item> is being owned by the Adapter, that's a problem

so, for example, another way to handle your case is to use ListAdapter (the RecyclerView edition) and just call submitList() with each larger edition of your master List<Item>

let it figure out which notify...() calls to make

onCreateView will call my viewmodel to get data from my model. My viewmodel does contain my master list, so I'm not sure I understand y9ou

"My viewmodel does contain my master list" -- not according to other entriese that you posted in the chat, such as "I want to post my stored list in my viewmodel", though it is possible that I am getting lost in your various lists

I meant my viewmodel posts pages but also keeps track of a master list. sorry it was confusing

then item #1 should already be solved ("my viewmodel knowing my screen was rotated to avoid fetching from my model")

if your problem is that your fragment is doing something in onViewCreated() (or wherever) that is telling your viewmodel to fetch from your model, that is your problem

but in onCreateView I call viewModel.getData()

then getData() needs to be smart enough to look at the master List<Item> and say "hey, I already have data; I do not need to fetch it again"

or something like that

alternatively, get rid of getData() and have your viewmodel load the data from the model in the constructor/init block

yes, that was my reaso for asking if my viewmodel knew if a rotation just occured

anyway, I am out of time for today's chat -- the transcript will be posted to https://commonsware.com/office-hours/ shortly

the next chat is Tuesday in the 8:30am US Eastern time slot

ok I appreiate it

have a pleasant day!