Android Using GridView with OnScrollListener

Question

I\'m building a gridview that contains alot of images likely around 1000++ pictures. To reduce the loading time for the gridview, I want to implement an onScrollListener usi

Answer 1

I suppose you want to load images from Urls. For that purpose, you could use this solution: Lazy load of images in ListView

In order to control your paging using onScrollListener, I think you want to see the Endless scrolling example. This is a simplistic example, but the only thing you have to add is a check if you have reached the last item of your xml.

Inside your AsyncTask (called at each new loading), you would only parse a bit of data from your xml (corresponding to 20 items in your case). This is an example of use based on your code:

public class MainGridView extends SherlockActivity implements OnScrollListener {

    public static final String KEY_SONG = "song"; 
    public static final String KEY_ID = "id";
    public static final String KEY_TITLE = "title";
    public static final String KEY_ARTIST = "artist";
    public static final String KEY_CAT_ARTIST = "artistcat";
    public static final String KEY_DURATION = "duration";
    public static final String KEY_THUMB_URL = "thumb_url";
    public static final String KEY_BIG_URL = "big_url";
    public static final String KEY_CAT_URL = "cat_url";

    private final static int ITEMS_PPAGE = 20;

    private ProgressDialog mDialog;
    private ArrayList<HashMap<String, String>> mSongsList;

    private static String IMAGE_POSITION;
    private GridView mGridView;
    //MainGridViewLazyAdapter mAdapter;
    private ArrayAdapter<String> mAdapter;
    private String cat_url;
    private String artist_url;

    private int mVisibleThreshold = 5;
    private int mCurrentPage = 0;
    private int mPreviousTotal = 0;
    private boolean mLoading = true;
    private boolean mLastPage = false;

    private String mXml;

    @Override
    public void onCreate(Bundle savedInstanceState) {
        setTheme(SherlockBarUtils.getCurrentTheme(this));
        super.onCreate(savedInstanceState);
        setContentView(R.layout.gridview_main);

        getSupportActionBar().setDisplayHomeAsUpEnabled(true);
        //checkInternetConnection();

        mGridView = (GridView) findViewById(R.id.grid_view);
        mGridView.setOnScrollListener(this);

        mAdapter = new ArrayAdapter<String>(this, android.R.layout.simple_list_item_1);
        //mAdapter=new MainGridViewLazyAdapter(MainGridView.this);  
        mGridView.setAdapter(mAdapter);

        new DownloadDataAsyncTask().execute();
    }

    @Override
    public void onScroll(AbsListView view, int firstVisibleItem, int visibleItemCount, int totalItemCount) {

        if (mLoading) {
            if (totalItemCount > mPreviousTotal) {

                mLoading = false;
                mPreviousTotal = totalItemCount;
                mCurrentPage++;

                // Find your own condition in order to know when you 
                // have finished displaying all items
                if (mCurrentPage + 1 > 50) {
                    mLastPage = true;
                }
            }
        }
        if (!mLastPage && !mLoading && 
                (totalItemCount - visibleItemCount) <= (firstVisibleItem + mVisibleThreshold)) {
            new AddItemsAsyncTask().execute();
            mLoading = true;
        }
    }

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

    }

    public class DownloadDataAsyncTask extends AsyncTask<String, String, String> {

        @Override
        protected void onPreExecute() {
            super.onPreExecute();
            mDialog = new ProgressDialog(MainGridView.this);
            mDialog.setTitle("Connect to Server");
            mDialog.setMessage("This process can take a few seconds to a few minutes, depending on your Internet Connection Speed.");
            mDialog.setIndeterminate(false);
            mDialog.setCancelable(false);
            mDialog.show();
        }
        @Override
        protected String doInBackground(String... args) {

            // Downloading XML

            /*
            Intent in = getIntent();
            mSongsList = new ArrayList<HashMap<String, String>>();
            cat_url = in.getStringExtra(KEY_CAT_URL);
            artist_url = in.getStringExtra(KEY_CAT_ARTIST);

            XMLParser parser = new XMLParser();
            mXml = parser.getXmlFromUrl(cat_url); // getting XML from URL
            */

            return null;
        }   

        @Override
        protected void onPostExecute(String args) {

            /*
            getSupportActionBar().setTitle(artist_url);
            */
            mDialog.dismiss();
            mLoading = false;
        }
    }

    public class AddItemsAsyncTask extends AsyncTask<Integer, String, String> {

        @Override
        protected void onPreExecute() {
            super.onPreExecute();
        }
        @Override
        protected String doInBackground(Integer... args) {

            MainGridView.this.runOnUiThread(new Runnable() {

                @Override
                public void run() {

                    // Parsing 20 more items and adding them to the adapter
                    for (int i = mCurrentPage * ITEMS_PPAGE; i < (mCurrentPage + 1) * ITEMS_PPAGE; i++) {
                        MainGridView.this.mAdapter.add(String.valueOf(Math.random() * 5000));
                    }
                    MainGridView.this.mAdapter.notifyDataSetChanged();
                }
            });
            return null;
        }

        @Override
        protected void onPostExecute(String args) {

        }
    }
}

For the convenience, I used texts instead of images inside the GridView. I focused on the paging system so I didn't parse XML, just added some random values on the fly.

Also, you could consider using the same addItemsAsyncTask to download only a part (20 items) of your xml at each loading instead of downloading the whole 1000+ items before creating the GridView.