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.