Endless Scrolling with AdapterViews

Overview

A common application feature is to have an AdapterView (such as a ListView or GridView) that automatically loads 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.

Every AdapterView 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:

public abstract class EndlessScrollListener implements OnScrollListener {
	// The minimum amount 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 (!loading && (totalItemCount < previousTotalItemCount)) {
			this.currentPage = this.startingPageIndex;
			this.previousTotalItemCount = totalItemCount;
			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) {
			if (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 loadMore to fetch the data.
		if (!loading && (totalItemCount - visibleItemCount) <= (firstVisibleItem + visibleThreshold)) {
			loadMore(currentPage + 1, totalItemCount);
			loading = true;
		}
	}

	// Defines the process for actually loading more data based on page
	public abstract void loadMore(int page, int totalItemsCount);

	@Override
	public void onScrollStateChanged(AbsListView view, int scrollState) {
	}
}

Notice that this is an abstract class, and that in order to use this, you must extend this base class and define the loadMore 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);
        lvItems.setOnScrollListener(new EndlessScrollListener() {
	    @Override
	    public void loadMore(int page, int totalItemsCount) {
                // whatever code is needed to append new items to your AdapterView
	        customLoadMoreFromActivity(page);
	    }
        });
    }
}

Now as you scroll, items will be automatically filling in because the loadMore 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.

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