Endless Scrolling with AdapterViews and RecyclerView

Overview

A common application feature is to load automatically more items as the user scrolls through the items (aka infinite scroll). This is done by triggering a request for more data once the user crosses a threshold of remaining items before they've hit the end.

The approaches for ListView, GridView and RecyclerView (the successor to ListView) are documented here. Both are similar in code except that the LayoutManager in the RecyclerView needs to be passed in to provide the necessary information to implement infinite scrolling.

In both cases, the information needed to implement the scrolling include determining the last visible item within the list and some type of threshold value to start fetching more data before the last item has been reached. This data can be used to decide when to load more data from an external source:

To provide the appearance of endless scrolling, it's important to fetch data before the user gets to the end of the list. Adding a threshold value therefore helps anticipate the need to append more data:

Implementing with ListView or GridView

Every AdapterView (such as ListView and GridView) has support for binding to the OnScrollListener events which are triggered whenever a user scrolls through the collection. Using this system, we can define a basic EndlessScrollListener which supports most use cases by creating our own class that extends OnScrollListener:

import android.widget.AbsListView; 

public abstract class EndlessScrollListener implements AbsListView.OnScrollListener {
	// The minimum number of items to have below your current scroll position
	// before loading more.
	private int visibleThreshold = 5;
	// The current offset index of data you have loaded
	private int currentPage = 0;
	// The total number of items in the dataset after the last load
	private int previousTotalItemCount = 0;
	// True if we are still waiting for the last set of data to load.
	private boolean loading = true;
	// Sets the starting page index
	private int startingPageIndex = 0;

	public EndlessScrollListener() {
	}

	public EndlessScrollListener(int visibleThreshold) {
		this.visibleThreshold = visibleThreshold;
	}

	public EndlessScrollListener(int visibleThreshold, int startPage) {
		this.visibleThreshold = visibleThreshold;
		this.startingPageIndex = startPage;
		this.currentPage = startPage;
	}

	// This happens many times a second during a scroll, so be wary of the code you place here.
	// We are given a few useful parameters to help us work out if we need to load some more data,
	// but first we check if we are waiting for the previous load to finish.
	@Override
	public void onScroll(AbsListView view, int firstVisibleItem, int visibleItemCount, int totalItemCount) 
        {
		// If the total item count is zero and the previous isn't, assume the
		// list is invalidated and should be reset back to initial state
		if (totalItemCount < previousTotalItemCount) {
			this.currentPage = this.startingPageIndex;
			this.previousTotalItemCount = totalItemCount;
			if (totalItemCount == 0) { this.loading = true; } 
		}
		// If it's still loading, we check to see if the dataset count has
		// changed, if so we conclude it has finished loading and update the current page
		// number and total item count.
		if (loading && (totalItemCount > previousTotalItemCount)) {
			loading = false;
			previousTotalItemCount = totalItemCount;
			currentPage++;
		}
		
		// If it isn't currently loading, we check to see if we have breached
		// the visibleThreshold and need to reload more data.
		// If we do need to reload some more data, we execute onLoadMore to fetch the data.
		if (!loading && (firstVisibleItem + visibleItemCount + visibleThreshold) >= totalItemCount ) {
		    loading = onLoadMore(currentPage + 1, totalItemCount);
		}
	}

	// Defines the process for actually loading more data based on page
	// Returns true if more data is being loaded; returns false if there is no more data to load.
	public abstract boolean onLoadMore(int page, int totalItemsCount);

	@Override
	public void onScrollStateChanged(AbsListView view, int scrollState) {
		// Don't take any action on changed
	}
}

Notice that this is an abstract class, and that in order to use this, you must extend this base class and define the onLoadMore method to actually retrieve the new data. We can define now an anonymous class within any activity that extends EndlessScrollListener and bind that to the AdapterView. For example:

public class MainActivity extends Activity {
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        // ... the usual 
        ListView lvItems = (ListView) findViewById(R.id.lvItems);
        // Attach the listener to the AdapterView onCreate
        lvItems.setOnScrollListener(new EndlessScrollListener() {
          @Override
          public boolean onLoadMore(int page, int totalItemsCount) {
              // Triggered only when new data needs to be appended to the list
              // Add whatever code is needed to append new items to your AdapterView
              customLoadMoreDataFromApi(page); 
              // or customLoadMoreDataFromApi(totalItemsCount); 
              return true; // ONLY if more data is actually being loaded; false otherwise.
          }
        });
    }
    
    // Append more data into the adapter
    public void customLoadMoreDataFromApi(int offset) {
      // This method probably sends out a network request and appends new data items to your adapter. 
      // Use the offset value and add it as a parameter to your API request to retrieve paginated data.
      // Deserialize API response and then construct new objects to append to the adapter
    }
}

Now as you scroll, items will be automatically filling in because the onLoadMore method will be triggered once the user crosses the visibleThreshold. This approach works equally well for a GridView and the listener gives access to both the page as well as the totalItemsCount to support both pagination and offset based fetching.

Implementing with RecyclerView

We can use a similar approach with the RecyclerView by defining an interface EndlessRecyclerViewScrollListener that requires an onLoadMore() method to be implemented. The LayoutManager, which is responsible in the RecyclerView for rendering where items should be positioned and manages scrolling, provides information about the current scroll position relative to the adapter. For this reason, we need to pass an instance of what LayoutManager is being used to collect the necessary information to ascertain when to load more data.

Implementing endless pagination for RecyclerView requires the following steps:

1. Copy over the EndlessRecyclerViewScrollListener.java into your application.
2. Call addOnScrollListener(...) on a RecyclerView to enable endless pagination. Pass in an instance of EndlessRecyclerViewScrollListener and implement the onLoadMore which fires whenever a new page needs to be loaded to fill up the list.
3. Inside the aforementioned onLoadMore method, load additional items into the adapter either by sending out a network request or by loading from another source.

To start handling the scroll events for steps 2 and 3, we need to use the addOnScrollListener() method in our Activity or Fragment and pass in the instance of the EndlessRecyclerViewScrollListener with the layout manager as shown below:

public class MainActivity extends Activity {
    @Override
    protected void onCreate(Bundle savedInstanceState) {
       // Configure the RecyclerView
       RecyclerView rvItems = (RecyclerView) findViewById(R.id.rvContacts);
       LinearLayoutManager linearLayoutManager = new LinearLayoutManager(this);
       rvItems.setLayoutManager(linearLayoutManager);
       // Add the scroll listener
       // keep an instance so that you can call `resetState()` for new searches
       EndlessRecyclerViewScrollListener scrollListener = new EndlessRecyclerViewScrollListener(linearLayoutManager) {
           @Override
           public void onLoadMore(int page, int totalItemsCount, RecyclerView view) {
               // Triggered only when new data needs to be appended to the list
               // Add whatever code is needed to append new items to the bottom of the list
               customLoadMoreDataFromApi(page);
           }
      });
      rvItems.addOnScrollListener(scrollListener);
  }

  // Append more data into the adapter
  // This method probably sends out a network request and appends new data items to your adapter. 
  public void customLoadMoreDataFromApi(int page) {
      // Send an API request to retrieve appropriate data using the offset value as a parameter.
      //  --> Deserialize API response and then construct new objects to append to the adapter
      //  --> Notify the adapter of the changes
  }
}

You can refer to this code sample for usage and this code sample for the full endless scroll source code.

All of the code needed is already incorporated in the EndlessRecyclerViewScrollListener.java code snippet above. However, if you wish to understand how the endless scrolling is calculated, the detailed explanation is available here.

Troubleshooting

If you are running into problems, please carefully consider the following suggestions:

• For the ListView, make sure to setup the setOnScrollListener listener in the onCreate method of the Activity or onCreateView in a Fragment and not much later otherwise you may encounter unexpected issues.

• In order for the pagination system to continue working reliably, you should make sure to clear the adapter of items (or notify adapter after clearing the array) before appending new items to the list. For RecyclerView, it is highly recommended to make more granular updates when notifying the adapter. See this [video talk] (https://youtu.be/imsr8NrIAMs?t=8m27s) for more context.

• In order for this pagination system to trigger, keep in mind that as customLoadMoreDataFromApi is called, new data needs to be appended to the existing data source. In other words, only clear items from the list when on the initial "page". Subsequent "pages" of data should be appended to the existing data.

• For the RecyclerView, if you intend to clear the contents of the list and start endless scrolling again, make sure to clear the contents of the list and notify the adapter the contents have changed as soon as possible. Make sure also to reset the state of the EndlessRecyclerViewScrollListener too:

  // endless scroll listener needs to be reset whenever performing a new search
  scrollListener.resetState();

• If you see Cannot call this method in a scroll callback. Scroll callbacks might be run during a measure & layout pass where you cannot change the RecyclerView data., you mean need to do the following inside your onLoadMore() method:

  // delay a frame before notifying the adapter since the scroll listeners can be called when RecyclerView data
  // cannot be changed
  view.post(new Runnable() {
              @Override
              public void run() {
                // notify adapter
                adapter.notifyItemRangeInserted(curSize, allContacts.size() - 1);
              }
         });

See this Stack Overflow article for more details.

Displaying Progress with Custom Adapter

To display the last row as a ProgressBar indicating that the ListView is loading data, we do the trick in the Adapter. Having defined two types of views in getItemViewType(int position), we can display the last row differently from a normal data row. It can be a ProgressBar or some text to indicate that the ListView has reached the last row by comparing the size of data List to the number of items on the server side. See this gist for sample code.

References

• http://benjii.me/2010/08/endless-scrolling-listview-in-android/
• http://stackoverflow.com/questions/1080811/android-endless-list
• http://stackoverflow.com/questions/12583419/android-listview-automatically-load-more-data-when-scroll-to-the-bottom