Develop Reference

RecyclerView is not updated when notifyDataSetChanged is used with MVVM

I am working with MVVM. Main screen shows movie's posters only during debugging (and not during regular run).
The problem is in observation of RecyclerView population. There is Observer in MainActivity. I expect that notifyDataSetChanged method will cause
posters to appear after receiving data from the API, but it doesn't happen.
My cleaned code related to this issue only is available in https://github.com/RayaLevinson/Test
I am missing some important point related to Observer. Please help me! Thank you.
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
mRecyclerView = findViewById(R.id.recycler_view_movie);
mMainActivityViewModal = ViewModelProviders.of(this).get(MainActivityViewModel.class);
mMainActivityViewModal.init();
mMainActivityViewModal.getMovies().observe(this, new Observer<List<Movie>>() {
#Override
public void onChanged(#Nullable List<Movie> movies) {
mAdapter.notifyDataSetChanged();
}
});
initRecyclerView();
}
private void initRecyclerView() {
mAdapter = new RecyclerViewAdapter(this, mMainActivityViewModal.getMovies().getValue());
mRecyclerView.setLayoutManager(new GridLayoutManager(this, 2));
mRecyclerView.setAdapter(mAdapter);
}
MovieRepository.java
public class MovieRepository {
private static final String TAG = "MovieRepository";
private static String mSortBy = "popular";
private static MovieRepository instance;
private List<Movie> movies = new ArrayList<>();
public static MovieRepository getInstance() {
if (instance == null) {
instance = new MovieRepository();
}
return instance;
}
public MutableLiveData<List<Movie>> getMovies() {
setMovies();
MutableLiveData<List<Movie>> data = new MutableLiveData<List<Movie>>();
data.setValue(movies);
return data;
}
private void setMovies() {
Context context = GlobalApplication.getAppContext();
if (NetworkUtils.isNetworkAvailable(context)) {
movies.clear();
new MovieRepository.FetchMoviesTask().execute(mSortBy);
} else {
alertUserAboutNetworkError();
}
}
private void alertUserAboutNetworkError() {
Context context = GlobalApplication.getAppContext();
// Toast.makeText(context, R.string.networkErr, Toast.LENGTH_LONG).show();
}
private class FetchMoviesTask extends AsyncTask<String, Void, List<Movie>> {
#Override
protected List<Movie> doInBackground(String... params) {
if (params.length == 0) {
return null;
}
String sortBy = params[0];
Log.d(TAG, "In doInBackground " + sortBy);
URL moviesRequestUrl = NetworkUtils.buildUrl(sortBy);
try {
String jsonWeatherResponse = NetworkUtils.getResponseFromHttpUrl(moviesRequestUrl);
return MovieJsonUtils.getMoviesDataFromJson(jsonWeatherResponse);
} catch (Exception e) {
e.printStackTrace();
return null;
}
}
#Override
protected void onPostExecute(List<Movie> parsedMoviesData) {
if (parsedMoviesData != null) {
for (Movie movie : parsedMoviesData) {
movies.add(movie);
Log.d(TAG, "In onPostExecute " + " movie was added");
}
}
}
}
}
MainActivityViewModel.java
public class MainActivityViewModel extends ViewModel {
private MutableLiveData<List<Movie>> mMovies;
private MovieRepository mMoviewRepository;
public void init() {
if (mMovies != null) {
return;
}
mMoviewRepository = MovieRepository.getInstance();
mMovies = mMoviewRepository.getMovies();
}
public LiveData<List<Movie>> getMovies() {
return mMovies;
}
}
RecyclerViewAdapter.java
public class RecyclerViewAdapter extends RecyclerView.Adapter<RecyclerViewAdapter.ViewHolder> {
private static final String TAG = "RecyclerViewAdapter";
private final Context mContext;
private List<Movie> mMovies;
public RecyclerViewAdapter(Context mContext, List<Movie> movies) {
this.mMovies = movies;
this.mContext = mContext;
}
#NonNull
#Override
public ViewHolder onCreateViewHolder(#NonNull ViewGroup parent, int viewType) {
View view = LayoutInflater.from(parent.getContext()).inflate(R.layout.layout_list_item, parent, false);
return new ViewHolder(view);
}
#Override
public void onBindViewHolder(#NonNull final ViewHolder holder, int position) {
Log.d(TAG, "onBindViewHolder called");
Picasso.get()
.load(mMovies.get(holder.getAdapterPosition()).getPosterPath())
.placeholder(R.mipmap.ic_launcher)
.into(holder.image);
}
#Override
public int getItemCount() {
return mMovies.size();
}
public class ViewHolder extends RecyclerView.ViewHolder {
final ImageView image;
final LinearLayout parentLayout;
private ViewHolder(#NonNull View itemView) {
super(itemView);
image = itemView.findViewById(R.id.image);
parentLayout = itemView.findViewById(R.id.parent_layout);
}
}
public void update(List<Movie> movies) {
mMovies.clear();
mMovies.addAll(movies);
notifyDataSetChanged();
}
}

Your MovieRepository#getMovies() executes the Livedata.setValue() before the AsyncTask finishes. You can see that in your debug output.
What you have to do is to call postValue() (cause your on not on the mainthread) in your onPostExecute() method. Then you have to call mAdapter.update() from the onChanged() method.
Also I would recommend to refactor your ViewModel a little bit. Remove the call to the repository from your init() method and create a new method that only calls the load function from the repo. So if you later on would like to support things like endless scrolling, this will help you a lot.
Just a matter of opinion, but i like to create my observables inside my ViewModel and not in the Repository and pass it along as parameter. Thats how it could look like:
Activity
#Override
protected void onCreate(Bundle savedInstanceState) {
...
viewModel = ViewModelProviders.of(this).get(YOUR_VIEW_MODEL.class);
viewModel.init();
viewModel.getItemsObservable().observe(this, new Observer<List<Item>>() {
#Override
public void onChanged(#Nullable List<Item> items) {
// Add/replace your existing adapter
adapter.add/replaceItems(items);
// For better performance when adding/updating elements you should call notifyItemRangeInserted()/notifyItemRangeChanged(). For replacing the whole dataset notifyDataSetChanged() is fine
adapter.notifyDataSetChanged();
// Normally i would put those calls inside the adapter and make appropriate methods but for demonstration.
}
});
initRecyclerView();
viewModel.loadItems()
}
ViewModel
public void init(){
repository = Repository.getInstance();
}
public void loadItems(){
repository.loadItems(getItemsObservable());
}
public LiveData<List<Item>> getItemsObservable() {
if (items == null) {
items = new MutableLiveData<>();
}
return items;
}
Repository
public void loadItems(LiveData<List<Item>> liveData){
List<Item> data = remote.getDataAsync(); // get your data asynchronously
liveData.postValue(data); // call this after you got your data, in your case inside the onPostExecute() method
}

How to update an item on a recyclerview after excuting AsyncTask doInBackground?

I'm creating a chat feature for an application and it works super fine. But I would like to show the user that message has been sent or it still wating for the server's response.
Fields:
List<ChatMessage> chatMessages;
ChatAdapter chatAdapter;
RecyclerView chatRecyclerView;
ImageButton submitMessageBtn;
this how I send a message on my ChatActivity class:
public void submitMessage(final String messageType, final byte[] message){
final ChatMessageResponse messageObject = new ChatMessageResponse();
new AsyncTask<Void, Void, Void>() {
#Override
protected void onPreExecute() {
super.onPreExecute();
messageObject.setMessage( message);
messageObject.setYours(true);
messageObject.setUserNickname(getNickname());
messageObject.setCreationDate(DateTime.now().withZone(DateTimeZone.UTC));
messageObject.setType(messageType);
AddMessage(messageObject);
}
#Override
protected Void doInBackground(Void... voids) {
try {
chatClient.chat().sendMessage(eventId, messageType, message);
runOnUiThread(new Runnable() {
#Override
public void run() {
// Update message on the list after has been sent to server
}
});
} catch (IOException e) {
e.printStackTrace();
}
return null;
}
}.execute();
}
public void AddMessage(ChatMessage message)
{
chatMessages.add(message);
chatAdapter.notifyDataSetChanged();
chatRecyclerView.scrollToPosition(chatMessages.size() -1);
}
When message is immediatly added to the adapter it should look like this:
my ChatAdapter class is setup like this:
public class ChatAdapter extends RecyclerView.Adapter<ChatAdapter.ChatViewHolder> {
private static final int VIEW_TYPE_MESSAGE_THIS_USER = 0;
private static final int VIEW_TYPE_MESSAGE_OTHER_USER = 1;
private final Activity activity;
public List<ChatMessage> chats=new ArrayList<>();
ArrayList<String> usercolor=new ArrayList<>();
Context mContext;
View view;
public ChatAdapter(List<ChatMessage> chats, Context mContext, Activity activity) {
this.chats = chats;
this.mContext = mContext;
this.activity = activity;
}
#Override
public ChatViewHolder onCreateViewHolder(ViewGroup parent, int viewType){
mContext = parent.getContext();
if (viewType == VIEW_TYPE_MESSAGE_OTHER_USER) {
view = View.inflate(mContext, R.layout.message_item_left, null);
} else if (viewType == VIEW_TYPE_MESSAGE_THIS_USER){
view = View.inflate(mContext, R.layout.message_item, null);
}
return new ChatViewHolder(view,(View.OnLongClickListener)activity);
}
#Override
public void onBindViewHolder(final ChatViewHolder holder, int position){
final ChatMessageResponse m = (ChatMessageResponse) chats.get(position);
if (getItemViewType(position) == VIEW_TYPE_MESSAGE_OTHER_USER){
holder.bindToView1(m);
} else if (getItemViewType(position) == VIEW_TYPE_MESSAGE_THIS_USER)
{
holder.bindToView(m);
}
}
#Override
public int getItemCount() {
return chats.size();
}
#Override
public int getItemViewType(int position) {
return chats.get(position).isYours() ? VIEW_TYPE_MESSAGE_THIS_USER : VIEW_TYPE_MESSAGE_OTHER_USER;
}
}
When the server's response is positive the views in the ChatViewHolder (that I don't show the code because is too long) should change visibility state
Someone told me to get a referece for the view and change it on the activity's asynctask or create a Callback listener for my adapter.
But I have no Idea how to do either one of then any help is appreciated.

Are you familiar with the use of "Callbacks" or "Interfaces"? You can create an interface and implement it in your activity. Pass the callback by parameters in the "AsyncTask" and use it there.
//Interface class
/**
* Created by gmora
*/
public interface IProcess {
void updateAdapter(String result);
}
On Activity:
public class YourActivity extends AppCompatActivity {
private IProcess mProcess;
private Adapter mRecyclerAdapter;
private RecyclerView mRecyclerView;
private List<ChatMessage> chats; //update chats on activity and refresh your adapter
#Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_layout);
Toolbar toolbar = (Toolbar) findViewById(R.id.toolbar);
setSupportActionBar(toolbar);
mProcess = new IProceso() {
#Override
public void updateAdapter(String pException) {
//update chats ... and update mAdater.notifyDataChange()...
// or mRecyclerView.setAdapter(new Adpater.... with new list chats)..
}
};
mRecyclerView = find....
// etc....
mRecyclerAdapter = new RecyclerAdapter( chats, ...);
mRecyclerView.setAdapter(mRecyclerAdapter);
}
}
Finally on AsyncTask... create a external class from AsyncTask please!
/**
* Created by gmora.
*/
public class YourAsyncTaskClass extends AsyncTask<String, Void, String > {
private IProcess iProcess;
public StarSearchPrinterTask(IProcess pIProcess) {
this.iProcess= pIProcess;
}
#Override
protected void onPreExecute() {
//loading... its optional
}
#Override
protected String doInBackground(String... interfaceType) {
// execute webservice or api and get results..
return results;
}
#Override
protected void onPostExecute(String results) {
mIProceso.updateAdapter(results);
}
}

Passing ArrayList from AsyncTask to PagerAdapter

hope you fine and well,
i have the following main class :
public class MainActivity extends AppCompatActivity {
Activity activity;
ViewPager viewPager;
CustomAdapter adapter;
#Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
viewPager=(ViewPager)findViewById(R.id.view_pager);
adapter=new CustomAdapter(this);
viewPager.setAdapter(adapter);
ConnectionAsyncTask connectionAsyncTask = new
ConnectionAsyncTask(MainActivity.this);
connectionAsyncTask.execute("http://www.mocky.io/v2/570d3677270000f600dc29b6");
}
public void showUploader()
{
// findViewById(R.id.progressBar1).setVisibility(View.VISIBLE);
}
public void hideUploader()
{
//findViewById(R.id.progressBar1).setVisibility(View.GONE);
}
public void DisplyOnTextView(List< Student > students) {
List <Student> my = students ;
}
}
when i run the app, this main activity will use another class to read json data from link as follows:
public class StudentJasonParser {
public static List<Student> getObjectFromJason(String jason)
{
List<Student> students;
try {
JSONArray jsonArray = new JSONArray(jason);
students = new ArrayList<>();
for(int i=0;i<jsonArray.length();i++)
{
JSONObject jsonObject = new JSONObject();
jsonObject= (JSONObject) jsonArray.get(i);
Student student = new Student();
student.setID(jsonObject.getInt("id"));
student.setName(jsonObject.getString("name"));
student.setUrl(jsonObject.getString("url"));
student.setDes(jsonObject.getString("des\n"));
student.setRate(jsonObject.getDouble("rate"));
student.setLon(jsonObject.getDouble("lon"));
student.setLat(jsonObject.getDouble("lat"));
students.add(student);
}
} catch (JSONException e) {
e.printStackTrace();
return null;
}
return students;
}
}
now this class will return the data to the following class :
public class ConnectionAsyncTask extends AsyncTask<String,String,String> {
Activity activity;
public ConnectionAsyncTask(Activity activity) {
this.activity=activity;
}
#Override
protected void onPreExecute() {
//((MainActivity)activity).DisplyOnTextView();
((MainActivity)activity).showUploader();
}
#Override
protected String doInBackground(String... params) {
String content =HttpManager.getData(params[0]);
return content;
}
#Override
protected void onProgressUpdate(String... values) {
super.onProgressUpdate(values);
}
#Override
protected void onPostExecute(String s) {
((MainActivity)activity).hideUploader();
List<Student> students= StudentJasonParser.getObjectFromJason(s);
if (students != null) {
((MainActivity) activity).DisplyOnTextView(students);
}
}
}
this line : ((MainActivity)activity).DisplyOnTextView(students);
will return the fetched data to the main class in the following function (mentioned in the main class ! )
public void DisplyOnTextView(List< Student > students) {
List <Student> my = students ;
}
now what i want is to pass this list to the following class in order to use it in the imageView and textView in the viewPager instead of the pre-defined data in the class :
public class CustomAdapter extends PagerAdapter {
private int[] images = {R.drawable.sample_0,R.drawable.sample_1};
private Context ctx;
private LayoutInflater LayoutInflater;
public CustomAdapter(Context ctx)
{
this.ctx=ctx;
}
#Override
public int getCount() {
return images.length;
}
#Override
public boolean isViewFromObject(View view, Object object) {
return (view==(LinearLayout)object);
}
#Override
public Object instantiateItem(ViewGroup container, int position) {
LayoutInflater = (LayoutInflater)ctx.getSystemService(Context.LAYOUT_INFLATER_SERVICE);
View view = LayoutInflater.inflate(R.layout.slide_layout,container,false);
ImageView imageView =(ImageView) view.findViewById(R.id.image_view);
TextView textView = (TextView)view.findViewById(R.id.image_count);
imageView.setImageResource(images[position]);
textView.setText(""+position);
container.addView(view);
return view;
}
#Override
public void destroyItem(ViewGroup container, int position, Object object) {
container.removeView((LinearLayout) object);
}
}
any idea ?!
thanks, regards.

what i want is to pass this list to the following class in order to use it in the imageView and textView in the viewPager
Then simply pass in the list as a parameter to the adapter and add a member variable for it. The usage of this adapter is at the bottom of this post, because I want to mention some other stuff.
class CustomAdapter extends PagerAdapter {
private Context ctx;
private List<Student> data;
public CustomAdapter(Context ctx, List<Student> students) {
this.ctx = ctx;
this.data = students;
}
If you want to use that data variable in the instantiateItem method, then you can do Student s = this.data.get(position); and use the various methods on the Student object to load the TextView or ImageView.
Please note that you will need an image loading library (Picasso, Glide, Fresco, etc.) to load a URL into an ImageView. While on the topic of libraries, though, you will save yourself much development time by looking into Gson for JSON parsing and Retrofit or Volley for HTTP network calls with JSON data.
As for your usage of the AsyncTask, passing around the Activity variable is bad practice. Try to use an asynchronous callback to the Activity instead.
public interface AsyncResponse<T> {
void onResponse(T response);
}
public class ConnectionAsyncTask extends AsyncTask<String, Void, List<Student>> {
private AsyncResponse<List<Student>> callback;
public ConnectionAsyncTask(AsyncResponse<List<Student>> callback) {
this.callback = callback;
}
#Override
protected List<User> doInBackground(String... params) {
String url = params[0];
final List<Student> students = new ArrayList<Student>();
// TODO: JSON stuff
return students;
}
#Override
protected void onPostExecute(List<Student> result) {
if (this.callback != null) {
this.callback.onResponse(result);
} else {
Log.w("ConnectionAsyncTask", "Ignoring result");
}
}
}
public class SampleViewPagerActivity extends Activity {
private ViewPager pager;
private PagerAdapter adapter;
private ArrayList<Student> students;
private ProgressDialog progress;
#Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
// 1. Inflate a layout
setContentView(R.layout.viewpager_activity);
// 2. Initialize the views
this.pager = (ViewPager) findViewById(R.id.pager);
this.progress = new ProgressDialog(this);
this.progress.setTitle("Loading");
this.progress.setMessage("Please wait");
// 3. Populate the views with data
this.students = new ArrayList<Student>();
this.adapter = new CustomAdapter(this, students);
this.pager.setAdapter(adapter);
// This code runs later, after 'execute' is called and the response is returned
ConnectionAsyncTask task = new ConnectionAsyncTask(new AsyncResponse<List<Student>>() {
#Override
public void onResponse(List<Student> response) {
students.clear();
students.addAll(response);
adapter.notifyDataSetChanged();
progress.hide();
}
});
// Optionally show some progress while waiting
this.progress.show();
// TODO: Use real URL
task.execute("http://www.somesite.com/data");
}
}

Running an AsynTask in a GridFragment is causing a null pointer on rotation

I have a FragmentActivity that has 5 Fragments
On my 2nd fragment is a gridview that displays many images.
That GridFragment is starting an AsyncTask with callback to get the arraylist of images.
It then sets an adapter using the following as arguments (listener, context, arraylist) context is getActivity()
when adapter starts it tries to do LayoutInflater.from(Context);
That is where im getting my null pointer. If the async task is complete it will not crash. but it i rotate while async task is working it crashes.
Is there any way around this?
Fragment
public class IconsFrag extends GridFragmentIcons implements AdapterIcons.AdapterListener {
AsyncTaskIconsAll aTask;
Button button;
final String TAG = "IconsFrag";
private ArrayList<Integer> mThumbs;
private final String KEY_LIST_DATA = "icons_cache";
private final String KEY_LIST_POSITION = "icons_position";
private int mPosition = -1;
private AdapterIcons mAdapter;
#Override
public void onActivityCreated(Bundle savedInstanceState) {
Log.i(TAG, "onActivityCreated");
super.onCreate(savedInstanceState);
if (savedInstanceState == null){
Log.i(TAG, "savedInstanceState null");
aTask = new AsyncTaskIconsAll();
aTask.updateActivity(this, getActivity(), new AsyncTaskIconsAll.Callback() {
#Override
public void onData(ArrayList<Integer> data) {
mThumbs = data;
mAdapter = new AdapterIcons(IconsFrag.this, getActivity(), mThumbs);
getGridView().setNumColumns(getResources().getInteger(R.integer.column_count_icon));
setGridAdapter(mAdapter);
getGridView().setOnItemClickListener(null);
}
});
aTask.execute();
}
AsyncTask
public class AsyncTaskIconsAll extends AsyncTask<Void, Integer, ArrayList<Integer>> {
private Activity mContext;
private Fragment mFragment;
private ArrayList<Integer> mThumbs;
final String TAG = "AsyncTaskIconsAll";
Callback mCallback;
public static interface Callback{
public void onData(ArrayList<Integer> data);
}
public void updateActivity(Fragment f, Activity a, final Callback c) {
Log.i(TAG, "updateActivity");
mContext = a;
mFragment = f;
mCallback = c;
if(mThumbs != null)
Log.i(TAG, "Callback not null");
mCallback.onData(mThumbs);
}
#Override
protected void onPreExecute() {
}
#Override
protected ArrayList<Integer> doInBackground(Void... unused){
Log.i(TAG, "doInBackground");
mThumbs = new ArrayList<Integer>();
final String[] extras = mContext.getResources().getStringArray(R.array.icon_pack);
for (String extra : extras) {
String uri = "drawable/" + extra;
int res = mContext.getResources().getIdentifier(uri, null, mContext.getPackageName());
if (res != 0) {
mThumbs.add(res);
}
}
return mThumbs;
}
protected void onProgressUpdate(Integer... progress) {
}
#Override
protected void onPostExecute(ArrayList<Integer> icons) {
Log.i(TAG, "onPostExecute");
mThumbs = icons;
mCallback.onData(mThumbs);
ProgressBar mProgess = (ProgressBar) mFragment.getView().findViewById(R.id.pending);
mProgess.setVisibility(mFragment.getView().GONE);
}
}
Adapter
public class AdapterIcons extends BaseAdapter implements SpinnerAdapter {
private final String TAG = "AdapterIcons";
private AdapterListener mListener;
private ArrayList<?> mData;
private final LayoutInflater mInflater;
public AdapterIcons(AdapterListener listener, Activity activity) {
this.mData = new ArrayList<Object>();
this.mInflater = LayoutInflater.from(activity);
this.mListener = listener;
}
public AdapterIcons(AdapterListener listener, Context Context, ArrayList<?> data) {
this.mData = (data == null) ? new ArrayList<Object>() : data;
this.mInflater = LayoutInflater.from(Context);
this.mListener = listener;
}
public ArrayList<?> getData () {
return this.mData;
}
public void setData (ArrayList<?> data) {
this.mData = data;
}
public void clearData () {
this.mData.clear();
}
public static abstract interface AdapterListener
{
public abstract View getView(int paramInt, View paramView, ViewGroup paramViewGroup);
}
public Intent.ShortcutIconResource getResource(int position){
Icons icons= new Icons();
ArrayList<Integer> list = (ArrayList<Integer>) mData;
return Intent.ShortcutIconResource.fromContext(icons.getBaseContext(), list.get(position));
}
#Override
public int getCount () {
if (mData == null)
Log.d(TAG, "getCount() Data Set Is Null");
return (mData != null) ? mData.size() : 0;
}
#Override
public Object getItem (int position) {
if (mData == null)
Log.d(TAG, "getItem(int position) Data Set Is Null");
return (mData != null) ? mData.get(position) : null;
}
#Override
public long getItemId (int position) {
if (mData == null)
Log.d(TAG, "getItemId(int position) Data Set Is Null");
return (mData != null) ? position : 0;
}
#Override
public View getView (int position, View convertView, ViewGroup parent) {
return (mListener == null) ? new LinearLayout(mInflater.getContext()) : this.mListener.getView(position, convertView, parent);
}
#Override
public View getDropDownView (int position, View convertView, ViewGroup parent) {
return (mListener == null) ? new LinearLayout(mInflater.getContext()) : this.mListener.getView(position, convertView, parent);
}
}

Take a look on this answer. It's pretty much the same problem. You need to handle orientation changes which changes the activity state in default(if you don't override).
https://stackoverflow.com/a/7618739/1080954

so in your onPostExecute() you try to add items to an activity which is (temporarily) destroyed. Check if your getActivity() == null before doing stuff with the context. Something like:
public void onPostExecute(){
if(getActivity() == null){
// activity is destroyed... skip
return;
}
// proceed like normal
}
This is the best I can do without anymore code. Good luck

Did you notice that you should use brackets {} in here, otherwise mCallback.onData(mThumbs) will always be called:
if(mThumbs != null){
Log.i(TAG, "Callback not null");
mCallback.onData(mThumbs);
}
Also you're passing the fragment and the activity in asyncTask updateActivity() method when you create the asyncTask but they will be null when you rotate your device, the activity is going to be recreated, so when you use them in asyncTask doInBackground() and onPostExecute() you have to check first if they are not null, otherwise you could end up with a NullPointerException.

Populating a ListView using AsyncTaskLoader

I'm having problems using AsyncTaskLoader. This is my first attempt populating a ListView from a SQLite database using a loader.
Everything seems ok, when I rotate the screen the data is cached and no query is done again. But when I press the home button and launch my app again, the data is loaded again.
Note: Usuario means User, so I'm populating the ListView with a list of users.
public class Main extends SherlockFragmentActivity
implements LoaderManager.LoaderCallbacks<ArrayList<Usuario>> {
UsuarioAdapter adapter;
ListView listView;
Database db;
#Override
public void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.main);
listView = (ListView) findViewById(R.id.lista);
db = new Database(this);
adapter = new UsuarioAdapter(this, new ArrayList<Usuario>());
listView.setAdapter(adapter);
getSupportLoaderManager().initLoader(0, null, this);
}
#Override
public Loader<ArrayList<Usuario>> onCreateLoader(int id, Bundle args) {
return new UsuariosLoader(this, db);
}
#Override
public void onLoadFinished(Loader<ArrayList<Usuario>> loader,
ArrayList<Usuario> usuarios) {
//adapter.notifyDataSetChanged();
listView.setAdapter(new UsuarioAdapter(this, usuarios));
// ((BaseAdapter) listView.getAdapter()).notifyDataSetChanged();
}
#Override
public void onLoaderReset(Loader<ArrayList<Usuario>> loader) {
listView.setAdapter(null);
}
}
// THE LOADER
class UsuariosLoader extends AsyncTaskLoader<ArrayList<Usuario>> {
private ArrayList<Usuario> usuarios;
private Database db;
public UsuariosLoader(Context context, Database db) {
super(context);
this.db = db;
}
#Override
protected void onStartLoading() {
if (usuarios != null) {
deliverResult(usuarios); // Use the cache
}
forceLoad();
}
#Override
protected void onStopLoading() {
// The Loader is in a stopped state, so we should attempt to cancel the
// current load (if there is one).
cancelLoad();
}
#Override
public ArrayList<Usuario> loadInBackground() {
db.open(); // Query the database
ArrayList<Usuario> usuarios = db.getUsuarios();
db.close();
return usuarios;
}
#Override
public void deliverResult(ArrayList<Usuario> data) {
usuarios = data; // Caching
super.deliverResult(data);
}
#Override
protected void onReset() {
super.onReset();
// Stop the loader if it is currently running
onStopLoading();
// Get rid of our cache if it exists
usuarios = null;
}
#Override
public void onCanceled(ArrayList<Usuario> data) {
// Attempt to cancel the current async load
super.onCanceled(data);
usuarios = null;
}
}
And I think this snippet is not well done. I'm creating a new Adapter instead of updating the data.
#Override
public void onLoadFinished(Loader<ArrayList<Usuario>> loader,
ArrayList<Usuario> usuarios) {
//adapter.notifyDataSetChanged();
listView.setAdapter(new UsuarioAdapter(this, usuarios));
//((BaseAdapter) listView.getAdapter()).notifyDataSetChanged();
}
Why adapter.notifyDataSetChanged() does not work?
So, basically, my app does not crash but all my data is reloaded again every time I restart the app.
Edit: This is my Adapter code:
class UsuarioAdapter extends BaseAdapter {
private ArrayList<Usuario> usuarios;
private LayoutInflater inflater;
public UsuarioAdapter(Context context, ArrayList<Usuario> usuarios) {
this.usuarios = usuarios;
this.inflater = LayoutInflater.from(context);
}
#Override
public int getCount() { return usuarios.size(); }
#Override
public Object getItem(int pos) { return usuarios.get(pos); }
#Override
public long getItemId(int pos) { return pos; }
#Override
public View getView(int pos, View convertView, ViewGroup arg) {
LinearLayout itemView;
if (convertView == null) {
itemView = (LinearLayout) inflater.inflate(R.layout.list_item, null);
} else {
itemView = (LinearLayout) convertView;
}
ImageView avatar = (ImageView) itemView.findViewById(R.id.avatar);
TextView nombre = (TextView) itemView.findViewById(R.id.nombre);
TextView edad = (TextView)itemView.findViewById(R.id.edad);
// Set the image ... TODO
nombre.setText(usuarios.get(pos).getNombre());
edad.setText(String.valueOf(usuarios.get(pos).getEdad()));
return itemView;
}
}

The call to notifyDataSetChanged() won't change the data your adapter is using. You need to update the data the adapter has, then call that method.
NotifyDataSetChanged() will only tell the adapter it needs to create it's views, but it does not change the data. You need to handle that yourself.
In your adapter add:
public void setUsuario(List<Usuario> usuarios) {
this.usuarios = usuarios;
}
Then in onLoadFinished() call the new method, then notifyDataSetChanged().
listView.getAdapter().setUsuario(usuarios);
listView.getAdapter().notifiyDataSetChanged();

I've found the solution. The onStartLoading was the guilty:
#Override
protected void onStartLoading() {
if (usuarios != null) {
deliverResult(usuarios); // Use cache
} else {
forceLoad();
}
}
In my original post forceLoad was always called. It must be in the else branch.