How to transform items in a PagedList?(Android Arch Component Paging Library)

Question

Android Architecture Component now introduced Paging Library , which is great.

According to the official demo The DataSource.Factory now supports map and mapByPage methods, which means we may transform items in one DataSource .

But DataSource and DataSource.Factory should be in the model layer, not the Presentor/View layer. However, there are plenty of times when we want to transform data in our Adapter(for RecyclerView or ListView), and obviously, this is the Presentor/View layer logic. By now, an Adapter holds an instance of PagedList , but PageList can't support these operations, which is kinda awkward. Besides, there are still times when we want to add items or remove items to/from a PagedList .

So this is a feature request:

1. Support item transformations on a PagedList

2. Support item adding/removing to/from a PagedList

Any discussion is welcomed.

Answer 1

I think you need use transformations for PagedList instead of LiveData<List<YourModel>> .

For example, I have a list of Pet , each Pet have gender . I need to show paging of Pet and filter gender in runtime.

So, my Dao interface may be:

@Dao
inteface PetDao {
    @Query("SELECT * FROM Pet WHERE Pet.gender = :gender ORDER BY Pet.id ASC")
    fun getPetsByGenderDataFactory(gender: String?): Datasource.Factory<Int, Pet>
}

In ViewModel class, I will use Transformations to filter LiveData<PagedList<Pet>> instead of LiveData<List<Pet>> :

class PetViewModel {

    private val genderMutableData: MutableLiveData<String> = MutableLiveData()
    private val petItemsData: LiveData<PagedList<Pet>> = Transformations.switchMap(this@PetViewModel.genderMutableData) { petGender ->
        LivePagedListBuilder(AppDatabase.getDefault().getPetDao().getPetsByGenderDataFactory(petGender), 20).build()
    }

}

When user change gender of Pet , you just update value of genderMutableData , it will trigger data source for petItemsData and update Pet items:

fun updatePetItemsWithNewGender(gender: String?) {
    this@PetViewModel.genderMutableData.postValue("female")
}

Sorry, I'm using Kotlin for example because you don't flag post in Java language.