Paging library overview

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Codelab

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PagingWithNetwork Sample

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Google I/O Session

Watch this video that introduces the Paging Library.

The Paging Library makes it easier for you to load data gradually and gracefully within your app's RecyclerView.

Many apps consume data from a data source that contains a large number of items, but only display a small portion at a time.

The Paging Library helps your app observe and display a reasonable subset of this data. This functionality has several advantages:

• Data requests consume less network bandwidth and fewer system resources. Users who have metered or small data plans appreciate such data-conscious apps.
• Even during data updates and refreshes, the app continues to respond quickly to user input.

If your app already includes logic for paging data and displaying lists, we've provided guidance on how to update your existing app.

Note: To import the Paging library into your Android project, see Adding Components to your Project.

This guide provides an overview of how you can use the Paging Library to request and display data that users want to see while consuming system resources more economically. For guides specific to the layers of your app's architecture, see the following related pages:

• UI components and considerations
• Data components and considerations

Note: The Paging Library helps you display data in your UI's list containers smoothly, regardless of whether you use only a device-internal database or you fetch information from your app's backend. To learn more about how best to use the library based on where your app's data is located, see Support different data architectures.

Library architecture

The Paging Library's key component is the PagedList class, which is a collection that loads chunks of your app's data, or pages, asynchronously. This class serves as a mediator between other pieces of your app's architecture:

Data

Each instance of PagedList loads an up-to-date snapshot of your app's data from its DataSource. Data flows from your app's backend or database into the PagedList object.

The Paging Library supports a variety of app architectures, including a standalone database and a database that communicates with a backend server.

To learn more, see Data components and considerations.

UI

The PagedList class works with a PagedListAdapter to load items into a RecyclerView. These classes work together to fetch and display content as it's loaded, prefetching out-of-view content and animating content changes.

To learn more, see UI components and considerations.

The Paging Library implements the observer pattern that's recommended in the guide to app architecture. In particular, the core components of the library create an instance of LiveData<PagedList> (or the equivalent RxJava2-based classes) that the UI can observe. Your app's UI can then present content from PagedList objects as they're generated, all while respecting your UI controllers' lifecycles.

Support different data architectures

The Paging Library supports app architectures, including ones where your app is fetching data only from a backend server, only from an on-device database, or from a combination of the two sources. This section provides recommendations for each of these cases.

We've provided examples of the recommend patterns to use for different data architectures. To view them, see the PagingWithNetwork sample on GitHub.

Network only

To display data from a backend server, use the synchronous version of the Retrofit API to load information into your own custom DataSource object.

Note: The Paging Library's DataSource objects don't provide any error handling because different apps handle and present error UIs in different ways. If an error occurs, defer to your result callback, and retry the request later. See the PagingWithNetwork sample for an example of this behavior.

Database only

Set up your RecyclerView to observe local storage, preferably using the Room persistence library. That way, whenever data is inserted or modified in your app's database, these changes are automatically reflected in the RecyclerView that's displaying this data.

Network and database

After you've started observing the database, you can listen for when the database is out of data by using PagedList.BoundaryCallback. You can then fetch more items from your network and insert them into the database. If your UI is observing the database, that's all you need to do.

The following code snippet shows an example usage of a boundary callback:

class ConcertViewModel {
    fun search(query: String): ConcertSearchResult {
        val boundaryCallback =
                ConcertBoundaryCallback(query, myService, myCache)
        // Error-handling not shown in this snippet.
        val networkErrors = boundaryCallback.networkErrors
    }
}

class ConcertBoundaryCallback(
        private val query: String,
        private val service: MyService,
        private val cache: MyLocalCache
) : PagedList.BoundaryCallback<Concert>() {
    override fun onZeroItemsLoaded() {
        requestAndSaveData(query)
    }

    override fun onItemAtEndLoaded(itemAtEnd: Concert) {
        requestAndSaveData(query)
    }
}

To see a more extended example of how a Paging Library solution can fetch data from both a network and a database, navigate to the Paging codelab or the PagingWithNetwork sample on GitHub.

Handle network errors

When using a network to fetch or page the data that you're displaying using the Paging Library, it's important to not treat the network as being either "available" or "unavailable" all the time, as many connections are intermittent or flaky:

• A particular server might fail to respond to a network request.
• The device might be connected to a network that's slow or weak.

Instead, your app should check each request for failures and recover as gracefully as possible in cases where the network isn't available. For example, you can provide a "retry" button for users to select if the data refresh step doesn't work. If an error occurs during the data paging step, it's best to retry the paging requests automatically.

Update your existing app

If your app already consumes data from a database or a backend source, it's possible to upgrade directly to functionality that the Paging Library provides. This section shows how to upgrade an app that has a common existing design.

Custom paging solutions

If you use custom functionality to load small subsets of data from your app's data source, you can replace this logic with that from the PagedList class. Instances of PagedList offer built-in connections to common data sources. These instances also provide adapters for RecyclerView objects that you might include in your app's UI.

Data loaded using lists instead of pages

If you use an in-memory list as the backing data structure for your UI's adapter, consider observing data updates using a PagedList class if the number of items in the list can become large. Instances of PagedList can use either LiveData<PagedList> or Observable<List> to pass data updates to your app's UI, minimizing load times and memory usage. Better still, replacing a List object with a PagedList object in your app doesn't require any changes to your app's UI structure or data updating logic.

Associate a data cursor with a list view using CursorAdapter

Your app might use a CursorAdapter to associate data from a Cursor with a ListView. In that case, you usually need to migrate from a ListView to a RecyclerView as your app's list UI container, then replace the Cursor component with either Room or PositionalDataSource, depending on whether instances of Cursor access a SQLite database.

In some situations, such as when working with instances of Spinner, you provide only the adapter itself. A library then takes the data that's loaded into that adapter and displays the data for you. In these situations, change the type of your adapter's data to LiveData<PagedList>, then wrap this list in an ArrayAdapter object before attempting to have a library class inflate these items in a UI.

Load content asynchronously using AsyncListUtil

If you're using AyncListUtil objects to load and display groups of information asynchronously, the Paging Library lets you load data more easily:

• Your data doesn't need to be positional. The Paging Library lets you load data directly from your backend using keys that the network provides.
• Your data can be uncountably large. Using the Paging Library, you can load data into pages until there isn't any data remaining.
• You can observe your data more easily. The Paging library can present your data that your app's ViewModel holds in an observable data structure.

Note: If your existing app accesses a SQLite database, see the section on using the Room persistence library.

Database examples

The following code snippets show several possible ways of having all the pieces work together.

Observing paged data using LiveData

The following code snippet shows all the pieces working together. As concert events are added, removed, or changed in the database, the content in the RecyclerView is automatically and efficiently updated:

@Dao
interface ConcertDao {
    // The Integer type parameter tells Room to use a PositionalDataSource
    // object, with position-based loading under the hood.
    @Query("SELECT * FROM user ORDER BY concert DESC")
    fun concertsByDate(): DataSource.Factory<Int, Concert>
}

class MyViewModel(concertDao: ConcertDao) : ViewModel() {
    val concertList: LiveData<PagedList<Concert>> = LivePagedListBuilder(
            concertDao.concertsByDate(),
            /* page size */ 20
    ).build()
}

class MyActivity : AppCompatActivity() {
    public override fun onCreate(savedState: Bundle?) {
        super.onCreate(savedState)
        val viewModel = ViewModelProviders.of(this)
                .get(MyViewModel::class.java!!)
        val recyclerView = findViewById(R.id.concert_list)
        val adapter = ConcertAdapter()
        viewModel.concertList.observe(this, { pagedList ->
                adapter.submitList(pagedList) })
        recyclerView.setAdapter(adapter)
    }
}

class ConcertAdapter() :
        PagedListAdapter<Concert, ConcertViewHolder>(DIFF_CALLBACK) {
    fun onBindViewHolder(holder: ConcertViewHolder, position: Int) {
        val concert = getItem(position)
        if (concert != null) {
            holder.bindTo(concert)
        } else {
            // Null defines a placeholder item - PagedListAdapter automatically
            // invalidates this row when the actual object is loaded from the
            // database.
            holder.clear()
        }
    }

    companion object {
        private val DIFF_CALLBACK = object : DiffUtil.ItemCallback<Concert>() {
            // Concert details may have changed if reloaded from the database,
            // but ID is fixed.
            override fun areItemsTheSame(oldConcert: Concert,
                    newConcert: Concert): Boolean =
                    oldConcert.id == newConcert.id

            override fun areContentsTheSame(oldConcert: Concert,
                    newConcert: Concert): Boolean =
                    oldConcert == newConcert
        }
    }
}

Observing paged data using RxJava2

If you prefer using RxJava2 instead of LiveData, you can instead create an Observable or Flowable object:

class MyViewModel(concertDao: ConcertDao) : ViewModel() {
    val concertList: Flowable<PagedList<Concert>> = RxPagedListBuilder(
            concertDao.concertsByDate(),
            /* page size */ 50
    ).buildFlowable(BackpressureStrategy.LATEST)
}

You can then start and stop observing the data using the code in the following snippet:

class MyActivity : AppCompatActivity() {
    private lateinit var adapter: ConcertAdapter<Concert>
    private lateinit var viewModel: MyViewModel

    private val disposable = CompositeDisposable()

    public override fun onCreate(savedState: Bundle?) {
        super.onCreate(savedState)
        val recyclerView = findViewById(R.id.concert_list)
        viewModel = ViewModelProviders.of(this).get(MyViewModel::class.java!!)
        adapter = ConcertAdapter()
        recyclerView.setAdapter(adapter)
    }

    override fun onStart() {
        super.onStart()
        disposable.add(viewModel.concertList.subscribe({
                flowableList -> adapter.submitList(flowableList)
        }))
    }

    override fun onStop() {
        super.onStop()
        disposable.clear()
    }
}

The code for the ConcertDao and ConcertAdapter are the same for an RxJava2-based solution as they are for a LiveData-based solution.