Hello guys I need advice I have a `PagedListAdapter` I popul

Question

Hello guys I need advice I have a `PagedListAdapter` I populate it using `LivePagedListBuilder` Each list item has `Favorite button` How it would be better to handle `Favorite button` state I need to know if an item is already added to favorites and when the button is pressed I need to update `Favorites database` to delete or add this item I m using architecture components in my app Thank you My solution Inside `onBindViewHolder` I call the callback to my activity and pass an item and its position ViewModel checks if this item is favorited or not and returns a result back to the activity the activity calls `adapter notifyItemChanged position ` Same goes for `Favorite button` click listener I need to pass an adapter position to ViewModel I don t like it I think this solution is really bad but I can t come up with something better

Ben Abramovitch · Answer

Do you need to pass the position around Could you not just pass an ID and find it on both ends that way List indexOf it id == clickedId

Serhii K. · Answer

I need the position because I m using `PagedListAdapter`

Ben Abramovitch · Answer

to do the update right You can find it by the ID again

Ben Abramovitch · Answer

```val index = List indexOf it id == clickedId if index = 1 notifyItemUpdated index ```

Ben Abramovitch · Answer

add the id to the view holder when binding it and use a custom click listener that returns that id

Serhii K. · Answer

Don t need to pass the position to the view holder ViewHolder class has adapterPosition property

Ben Abramovitch · Answer

What s your concern you don t want to pass a position into the view model

Ben Abramovitch · Answer

I need to pass an adapter position to ViewModel I don t like it I think this solution is really bad but I can t come up with something better

Serhii K. · Answer

I just want to know if there re any better solutions to achieve this

Serhii K. · Answer

Not only about passing the position

Serhii K. · Answer

The approach in general

Ben Abramovitch · Answer

I don t think it s bad overall The click action needs to make it to the view model so it can do it s logic It then needs to indicate the change so the list updates

Serhii K. · Answer

and what about initial state

Serhii K. · Answer

before the button is pressed

Ben Abramovitch · Answer

set it on the row when you bind it

Ben Abramovitch · Answer

if you need to get data from 2 sources before binding that s okay

Ben Abramovitch · Answer

then in the onBind you d just check against the favorite list and set it enabled disabled

Serhii K. · Answer

I don t have the favorite list To know if an item is in that list I need to get this information from Room Do you think I need to have a copy of the favorite list in memory

Ben Abramovitch · Answer

I would probably do that ya

Serhii K. · Answer

what if it has a lot of items

Ben Abramovitch · Answer

Favourites or rows from the server Whats a realistic expectation here for favorited items Dozens Hundreds Thousands Millions

Serhii K. · Answer

Favorites Let s assume that thousands

Ben Abramovitch · Answer

Is the view model fetching the data for the paged adapter

Serhii K. · Answer

Yes

Serhii K. · Answer

You re thinking about combining two requests

Serhii K. · Answer

So I will get a list with `isFavorited` info

Ben Abramovitch · Answer

Ya get the rows and then before you tell the adapter you have results query room for each row

Ben Abramovitch · Answer

add a field to the viewholder with isFavorited

Ben Abramovitch · Answer

and if it doesn t have the ID of the record add the ID as well

Ben Abramovitch · Answer

then you can just pass the ID around and avoid passing positions

Ben Abramovitch · Answer

I m just wondering if precaching the favourites is a good or bad idea at that quantity vs how many hits to room

Ben Abramovitch · Answer

can see how much it slows down the update querying for each ID seperately

Serhii K. · Answer

I like the approach with getting `isFavorited` before the list is passed to the adapter In that case I don t need to have in memory list of favorites

Ben Abramovitch · Answer

no you wouldn t but I m just wondering about performance every time the list gets updated you d have to query each individual row for it s status If it was precached you d only do it once and never again while on that page but then you d have to update it if they toggled it

Ben Abramovitch · Answer

either way would work though

Ben Abramovitch · Answer

Ya thinking more it s probably better not in memory and just get it each time

Serhii K. · Answer

I think It can be achieved with only one query to Room `SELECT FROM MyTable WHERE id IN 1 2 3 4 ` where 1 2 3 4 are ids

Serhii K. · Answer

but after that I will need to map the result

Serhii K. · Answer

< Ben Abramovitch> thank you for your help

Ben Abramovitch · Answer

Ya that would be a good way too No problem

Chris Craik · Answer

I agree that tracking adapter position isn t great it s leaking UI impl details and they re not guaranteed to be stable if something is inserted at the top of the list all the positions change I d suggest to first embed the favorite flag in the items coming out of your repository so that each item will know if it s favorites by default You can keep it only in the favorite table and reference both tables in your query or store it additionally in the item table you re querying Then when an item is favorited tell the viewmodel the id and favorite status to update the DB asynchronously and update the local item in the adapter at the same time Your async message to the viewmodel will take a while to write and readback but that s ok because you re immediately overriding the state in the adapter You dont want to override the data in the PagedList though because paging expects data to be immutable so it can diff on a background thread So what you do instead is have a map lt gt of local overrides item ID gt favorite state which you check in onbind If the id is present in the map the value overrides the favorite value you get out of your adapter s getItem The adapter map based on unique id is a common way to do selection and this is mostly just that

Serhii K. · Answer

< Chris Craik> thank you for your answer

zhuinden · Answer

Why do you need to call `adapter notifyItemChange ` at all You should be able to handle this just by writing to DB and then `LivePagedListBuilder` would emit a new PagedList and PagedListAdapter would handle the diff

Serhii K. · Answer

I am getting that PagedList not from DB