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 (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 && (totalItemCount - visibleItemCount)<=(firstVisibleItem + visibleThreshold)) {
		    onLoadMore(currentPage + 1, totalItemCount);
		    loading = true;
		}
	}

	// Defines the process for actually loading more data based on page
	public abstract void 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 void 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); 
	    }
        });
    }
    
    // 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.

Adapter

To display the last row as a Progressbar indicating that the ListView is loading data, we do the trick in 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.

/**
 *  A child class shall subclass this Adapter and 
 *  implement method getDataRow(int position, View convertView, ViewGroup parent),
 *  which supplies a View present data in a ListRow.
 *  
 *  This parent Adapter takes care of displaying Progressbar in a row or 
 *  indicating that it has reached the last row.
 * 
 */
public abstract class GenericAdapter<T> extends BaseAdapter {
	
	// the main data list to save loaded data
	protected List<T> dataList;
	
	protected Activity mActivity;
	
	// the serverListSize is the total number of items on the server side,
	// which should be returned from the web request results
	protected int serverListSize = -1;
	
	// Two view types which will be used to determine whether a row should be displaying 
	// data or a Progressbar
	public static final int VIEW_TYPE_LOADING = 0;
	public static final int VIEW_TYPE_ACTIVITY = 1;
	
	
	public GenericAdapter(Activity activity, List<T> list) {
		mActivity = activity;
		dataList = list;
	}
	
	
	public void setServerListSize(int serverListSize){
		this.serverListSize = serverListSize;
	}


/**
	 * disable click events on indicating rows
	 */
	@Override
	public boolean isEnabled(int position) {
		 
		return getItemViewType(position) == VIEW_TYPE_ACTIVITY;
	}

	/**
	 * One type is normal data row, the other type is Progressbar
	 */
	@Override
	public int getViewTypeCount() {
		return 2;
	}
	
	
	/**
	 * the size of the List plus one, the one is the last row, which displays a Progressbar
	 */
	@Override
	public int getCount() {
		return dataList.size() + 1;
	}

	
	/**
	 * return the type of the row, 
	 * the last row indicates the user that the ListView is loading more data
	 */
	@Override
	public int getItemViewType(int position) {
		// TODO Auto-generated method stub
		return (position >= dataList.size()) ? VIEW_TYPE_LOADING
				: VIEW_TYPE_ACTIVITY;
	}

	@Override
	public T getItem(int position) {
		// TODO Auto-generated method stub
		return (getItemViewType(position) == VIEW_TYPE_ACTIVITY) ? dataList
				.get(position) : null;
	}

	@Override
	public long getItemId(int position) {
		// TODO Auto-generated method stub
		return (getItemViewType(position) == VIEW_TYPE_ACTIVITY) ? position
				: -1;
	}

	/**
	 *  returns the correct view 
	 */
	@Override
	public  View getView(int position, View convertView, ViewGroup parent){
		if (getItemViewType(position) == VIEW_TYPE_LOADING) {
			// display the last row
			return getFooterView(position, convertView, parent);
		}
		View dataRow = convertView;
		dataRow = getDataRow(position, convertView, parent);
		
		return dataRow;
	};
	
	/**
	 * A subclass should override this method to supply the data row.
	 * @param position
	 * @param convertView
	 * @param parent
	 * @return
	 */
	public abstract View getDataRow(int position, View convertView, ViewGroup parent);

	
	/**
	 * returns a View to be displayed in the last row.
	 * @param position
	 * @param convertView
	 * @param parent
	 * @return
	 */
	public View getFooterView(int position, View convertView,
			ViewGroup parent) {
		if (position >= serverListSize && serverListSize > 0) {
			// the ListView has reached the last row
			TextView tvLastRow = new TextView(mActivity);
			tvLastRow.setHint("Reached the last row.");
			tvLastRow.setGravity(Gravity.CENTER);
			return tvLastRow;
		}
		
		View row = convertView;
		if (row == null) {
			row = mActivity.getLayoutInflater().inflate(
					R.layout.progress, parent, false);
		}

		return row;
	}

}

Troubleshooting

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

• Make sure to setup the setOnScrollListener listener in the onCreate method of the Activity and not later on 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.

• 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.

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