PositionalDataSource returns nothing from Parse Server databse

Question

I'm trying to load Products items from a Parse Server database using data source type of PositionalDataSource()to be able to fetch pages of items while scrolling down.

This what i've done so far :

My PositionalDataSource() class

class ParsePositionalDataSource: PositionalDataSource<Product>() {

    private fun getQuery() : ParseQuery<Product> {
        return ParseQuery.getQuery(Product::class.java).orderByDescending("createdAt")
    }

    override fun loadRange(params: LoadRangeParams, callback: LoadRangeCallback<Product>) {
        val query = getQuery()

        query.limit = params.loadSize
        query.skip = params.startPosition

        val products = query.find()

        callback.onResult(products)
    }

    override fun loadInitial(params: LoadInitialParams, callback: LoadInitialCallback<Product>) {
        val query = getQuery()

        query.limit = params.requestedLoadSize
        query.skip = params.requestedStartPosition

        val count = query.count()
        val products = query.find()

        callback.onResult(products, params.requestedStartPosition, count)
    }
}

And then a data source factory :

class ParseDataSourceFactory : DataSource.Factory<Int, Product>() {

    override fun create(): DataSource<Int, Product> {
        return ParsePositionalDataSource()
    }


}

And finally my ViewModal class

class CategoryTabViewModel() : ViewModel() {

    private var pagedListConfig: PagedList.Config = PagedList.Config.Builder().setEnablePlaceholders(true)
        .setPrefetchDistance(5)
        .setInitialLoadSizeHint(5)
        .setPageSize(5).build()

    private val sourceFactory = ParseDataSourceFactory()

    private val _productList =  LivePagedListBuilder(sourceFactory, pagedListConfig).build()

    val x = _productList

    //val productList: LiveData<PagedList<Product>> = _productList

    fun getProductList(): LiveData<PagedList<Product>> {
        return _productList
    }
}

I've checked if loadinitial() returns data by setting a breakpoint after : val products = query.find()and yes, productscontains a list of Product

But in my ViewModal, _productList it's always null or empty list.

Do I miss something ?

score 0 · Answer 1 · answered Feb 12 '21 at 11:05

I've found the solution by my self while digging in debugger console output.

I saw a line that says :

E/RecyclerView: No layout manager attached; skipping layout

And it works just fine after i added the layoutmanager in my RecyclerView

<androidx.recyclerview.widget.RecyclerView
        ...
        app:layoutManager="androidx.recyclerview.widget.LinearLayoutManager"
        ... />

And the documentation says :

A LayoutManager is responsible for measuring and positioning item views within a RecyclerView as well as determining the policy for when to recycle item views that are no longer visible to the user. By changing the LayoutManager a RecyclerView can be used to implement a standard vertically scrolling list, a uniform grid, staggered grids, horizontally scrolling collections and more. Several stock layout managers are provided for general use.

And

When writing a RecyclerView.LayoutManager you almost always want to use layout positions whereas when writing an RecyclerView.Adapter, you probably want to use adapter positions.

It seems that RecyclerView.Adapter needs RecyclerView.LayoutManager to be set.

PositionalDataSource returns nothing from Parse Server databse

1 Answers1