How to use empty view with pagination using paging library android?

问题

How to determine size of data returned before setting adapter?
How to use emptyview with paging library?
How to set emptyview if pagedlist returns null or no data?

回答1:

Update[24/04/19]: I just found out that the library already provide us a way to listen to empty initial load, using PagedList.BoundaryCallback<YourItem>.

*Note that my old answer is still a valid alternative.

val livedPageList = LivePagedListBuilder(sourceFactory, config)
        .setBoundaryCallback(object: PagedList.BoundaryCallback<YourItem>() {
            override fun onZeroItemsLoaded() {
                super.onZeroItemsLoaded()
                // Handle empty initial load here
            }

            override fun onItemAtEndLoaded(itemAtEnd: YourItem) {
                super.onItemAtEndLoaded(itemAtEnd)
                // Here you can listen to last item on list
            }

            override fun onItemAtFrontLoaded(itemAtFront: YourItem) {
                super.onItemAtFrontLoaded(itemAtFront)
                // Here you can listen to first item on list
            }
        })
        .build()

Original Answer:

Based on this class on google sample Network State. Modify it to handle empty content in initialLoad.

@Suppress("DataClassPrivateConstructor")
data class NetworkState private constructor(
    val status: Status,
    val msg: String? = null
) {

    enum class Status {
        RUNNING,
        SUCCESS_LOADED, // New
        SUCCESS_EMPTY, // New
        FAILED
    }

    companion object {

        val EMPTY = NetworkState(Status.SUCCESS_EMPTY) // New
        val LOADED = NetworkState(Status.SUCCESS_LOADED) // New
        val LOADING = NetworkState(Status.RUNNING)
        fun error(msg: String?) = NetworkState(Status.FAILED, msg)
    }
}

Usage as follow:

class DataSource: PageKeyedDataSource<Long, Item>() {

    val initialLoad: MutableLiveData<NetworkState> = MutableLiveData()

    override fun loadInitial(params: LoadInitialParams<Long>, callback: LoadInitialCallback<Long, Item>) {
        initialLoad.postValue(NetworkState.LOADING)
        apiCallSource.subscribe({ items ->
            if (items.isEmpty()) {
                initialLoad.postValue(NetworkState.EMPTY)
            } else {
                initialLoad.postValue(NetworkState.LOADED)
            }
        }, { error -> 
            // handle error
        })
    }
}

And this is how the activity handle it:

class activity: AppCompatActivity() {

    val viewModel = // init viewmodel

    override fun onCreate(savedInstanceState: Bundle?) {
        viewModel.refreshState.observe(this, Observer { networkState ->
            if (it == NetworkState.LOADING) {
                // Show loading
            } else {
                // Hide loading

                if (it.status == NetworkState.Status.SUCCESS_EMPTY) {
                    // Show empty state for initial load
                }
            }
        }
    }
}

For more details on how to connect DataSource with Activity, see this sample

回答2:

Simply add a listener or callback function to your DataSourceFactory and your DataSource and call it if the list in loadInitial is empty:

class DataSourceFactory(
    private val dataObservable: Observable<List<Data>>,
    private val onEmptyAction: () -> Unit
) : DataSource.Factory<Int, Data >() {

     override fun create(): DataSource {
         return DataSource(observable, onEmptyAction)
     }
}

class DataSource(
    private val observable: Observable<List<Data>>,
    private val onEmptyAction: () -> Unit
) : ItemKeyedDataSource<Int, Data>() {

    private val data = mutableListOf<Data>()

    override fun loadInitial(params: LoadInitialParams<Int>, callback: LoadInitialCallback<Data>) {
        observable
            .subscribe({ data ->
                if (data.isEmpty()) {
                    // Inform someone that this list is empty from the
                    // beginning to be able to show an empty page
                    onEmptyAction()
                }

                // rest of your code & logic
        }, { Timber.e(it) })
    }
}

回答3:

In your fragment/activity you are observing network state:

viewModel.getNetworkState1()?.observe(this, Observer {
                // here you can handle you empty view 
                setEmptyView()

            })

like this:

private fun setNoTransactionsLayout() {
    if(viewModel.listIsEmpty()) {
        yourTextView.visibility = View.VISIBLE
    } else {
        yourTextView.visibility = View.GONE
    }
}

And in view model you have this function:

fun listIsEmpty(): Boolean {
    return yourPagedList?.value?.isEmpty() ?: true
}

来源：https://stackoverflow.com/questions/53313044/how-to-use-empty-view-with-pagination-using-paging-library-android

标签

android

kotlin

paging

mvp