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Passing data from Activity to Fragment using AsyncTask - Android

I'm trying to pass an ArrayList from an AsyncTask in the MainActivity to a fragment, but I'm getting a NullPointerException for invoking
CategoryAdapter.getItemCount() even if I'm passing the array after the BroadCastReceiver Invoke.
What Am I doing wrong?
MainActivity
class GetBooksAsync extends AsyncTask<Void, Void, Void> {
LocalBroadcastManager manager = LocalBroadcastManager.getInstance(getApplicationContext());
#Override
protected Void doInBackground(Void... voids) {
for (ECategories category : ECategories.values()) {
try {
categories.add(new Category(category.toString(), apiClient.getBooks(category)));
} catch (IOException e) {
e.printStackTrace();
}
}
return null;
}
#Override
protected void onPostExecute(Void aVoid) {
super.onPostExecute(aVoid);
Intent intent = new Intent("com.android.mainapp");
intent.putExtra("categories", categories);
manager.sendBroadcast(intent);
replaceFragment(new HomeFragment());
}
}
HomeFragment
#Override
public View onCreateView(LayoutInflater inflater, ViewGroup container,
Bundle savedInstanceState) {
initBroadCastReceiver();
categoryAdapter = new CategoryAdapter(categories,getContext());
View view = inflater.inflate(R.layout.fragment_home, container, false);
recyclerView = view.findViewById(R.id.parent_rv);
recyclerView.setAdapter(categoryAdapter);
recyclerView.setLayoutManager(new LinearLayoutManager(getContext()));
categoryAdapter.notifyDataSetChanged();
return view;
}
private void initBroadCastReceiver() {
manager = LocalBroadcastManager.getInstance(getContext());
MyBroadCastReceiver receiver = new MyBroadCastReceiver();
IntentFilter filter = new IntentFilter();
filter.addAction("com.android.mainapp");
manager.registerReceiver(receiver,filter);
}
class MyBroadCastReceiver extends BroadcastReceiver {
#Override
public void onReceive(Context context, Intent intent) {
//get the categories from the intent
categories = new ArrayList<Category>();
categories = (ArrayList<Category>) intent.getSerializableExtra("categories");
}
}
i've also tried attaching the recyclerView from the OnReceive Method, but it's not getting attached.
Thank you in advance!

I think there are several problems with your code:
Your task is running in a different thread than the UIThread (which schedules the task and processes the result). That means it most probably runs on a different processor/core. Processed values (such as your collection) are cached in a processor and somewhen after execution the data is written back to RAM. But that might happen after the onPostExecute method is called, which takes the collection to another processor cache as well. But when this is done before the collection is returned to the RAM from the task, it's still empty. That's called a race condition.
Now there are several ways to solve that. The simplest one is to use Collections.synchronizedList(categories)
This prevents the processor from caching list values and always return it to the RAM (or using L3 cache which is shared between all processors/cores).
I'm not sure what exactly you pass to the collection. Intents (and it's data) need to be serializable and what you add to your collection is probably not serializable.
Then I would use the AsyncTask parameters:
class GetBooksAsync extends AsyncTask<ECategories, Void, Collection<Category>> {
LocalBroadcastManager manager = LocalBroadcastManager.getInstance(getApplicationContext());
#Override
protected Void doInBackground(ECategories... eCategories) {
Collection<Category> categories = [whatever you want to use];
for (ECategories category : eCategories) {
try {
categories.add(new Category(category.toString(), apiClient.getBooks(category)));
} catch (IOException e) {
e.printStackTrace();
}
}
return categories;
}
#Override
protected void onPostExecute(Collection<Category> categories) {
super.onPostExecute(categories);
Intent intent = new Intent("com.android.mainapp");
intent.putExtra("categories", categories);
manager.sendBroadcast(intent);
replaceFragment(new HomeFragment());
}
}
And note that AsyncTask and LocalBroadcastManager are deprecated.

Is Category serialized?
You can use BroadcastReceiver as an internal class, and then update the data of Adpater when it receives the data, because the code runs very fast, and it is not necessary to register for monitoring, and it will be processed immediately.

I guess the way you pass the data from MainActivity to HomeFragment is incorrect.
WHAT YOU EXPECT
Call MainActivity#GetBooksAsync
Wait till onPostExecute has been called
HomeFragment is ready to receive the broadcast message, then update UI
Broadcast the message from MainActivity to the fragment
WHAT IS HAPPENING HERE
Call MainActivity#GetBooksAsync
Wait till onPostExecute has been called
Broadcast the message from MainActivity. There is no receiver to receive this message!
HomeFragment is ready to receive the broadcast message, then update UI
HOW SHALL YOU PASS THE DATA THEN?
There are several way.
Broadcast data between the UI component like the things you did. But you will need to beaware the life cycle of the components. That is, when you broadcast the data, the receiver must already init and the UI component is in active.
Build a singleton class to store the data. Your activity and fragment treats the singleton class as a common place for the data storage.
Use Intent and the extra property to pass the data IF the data size is small enough.
Use LiveData. I believe it is the most modern way recommended by the community. Though I am not sure how its work.
To verify the fact that it is an life cycle issue,
you can try to add a delay before you sending the broadcast message.
class GetBooksAsync extends AsyncTask<Void, Void, Void> {
...
#Override
protected void onPostExecute(Void aVoid) {
super.onPostExecute(aVoid);
Intent intent = new Intent("com.android.mainapp");
intent.putExtra("categories", categories);
TimerTask task = new TimerTask() {
#Override
public void run() {
manager.sendBroadcast(intent);
}
};
Timer timer = new Timer();
timer.schedule(task, 5 * 1000); // Delay the broadcast after 5 seconds
replaceFragment(new HomeFragment());
}

Your Adapter should be written like this.
class CategoryAdapter extends RecyclerView.Adapter<CategoryAdapter.VHolder>{
private ArrayList<Category> list = new ArrayList<Category>();
public void setList(ArrayList<Category> list) {
this.list = list;
notifyDataSetChanged();
}
public CategoryAdapter(Context context) {
// Do not pass a list in the constructor, because the list may be empty
}
class VHolder extends RecyclerView.ViewHolder {
public VHolder(#NonNull View itemView) {
super(itemView);
}
}
......
}
Your fragment should have a global Adapter for BroadcastReceiver to update data
public class Test extends Fragment {
// Create a global Adapter for BroadcastReceiver to call and update data
private CategoryAdapter adapter;
#Override
public View onCreateView(LayoutInflater inflater, ViewGroup container,
Bundle savedInstanceState) {
adapter = new CategoryAdapter(getContext());
initBroadCastReceiver();
View view = inflater.inflate(R.layout.fragment_home, container, false);
recyclerView = view.findViewById(R.id.parent_rv);
recyclerView.setAdapter(categoryAdapter);
recyclerView.setLayoutManager(new LinearLayoutManager(getContext()));
return view;
}
private void initBroadCastReceiver() {
manager = LocalBroadcastManager.getInstance(getContext());
MyBroadCastReceiver receiver = new MyBroadCastReceiver();
IntentFilter filter = new IntentFilter();
filter.addAction("com.android.mainapp");
manager.registerReceiver(receiver,filter);
}
class MyBroadCastReceiver extends BroadcastReceiver {
#Override
public void onReceive(Context context, Intent intent) {
//get the categories from the intent
ArrayList<Category> categories = (ArrayList<Category>) intent.getSerializableExtra("categories");
adapter.setList(categories);
}
}
}

Attempt to invoke virtual method 'android.view.View android.view.View.findViewById(int)' on a null object reference

I've been looking for a similar problem to mine in order to find a solution, but I seriously couldn't find anything like that.
I was trying to download from parse an array of posts with an asynctask class, and after it gets the posts, it suppose to set the posts array in my page, and perform the setAdapter function in order to set my new posts array.
the problem is, after I've initialized listView and listAdapter in my home fragment,and then I perform the postArray taking from parse function, after it finishes taking the posts array from parse, it cannot update listAdapter because it says the listAdapter and my listView "haven't initialized yet", even though they have.
p.s.
sorry for not posting my code in a convenient way, I don't tend to post my code problems that often.
here's my code:
my home fragment:
public class HomeFragment extends Fragment {
View root;
ArrayList<PostClass> postsArrayList = new ArrayList<>();
static boolean isPostsArrayUpdated = false;
ListAdapter listAdapter;
PullToRefreshListView listView;
public void updatePostsArrayList(ArrayList<PostClass> postsArrayList){
if(!isPostsArrayUpdated){
// First time updating posts array list
listAdapter = new ListAdapter(getActivity(), root);
listView = (PullToRefreshListView) root.findViewById(R.id.list_container);
this.postsArrayList = postsArrayList;
listView.setAdapter(listAdapter);
isPostsArrayUpdated = true;
root.findViewById(R.id.homeFragmentLoadingPanel).setVisibility(View.GONE);
}else{
// Have updated posts before
this.postsArrayList = postsArrayList;
listAdapter.notifyDataSetChanged();
}
}
#Override
public View onCreateView(LayoutInflater inflater, ViewGroup container, Bundle savedInstanceState) {
root = inflater.inflate(R.layout.fragment_home, container, false);
listView = (PullToRefreshListView) root.findViewById(R.id.list_container);
listAdapter = new ListAdapter(getActivity(), root);
Home_Model.getInstance().setPostsArrayList();
return root;
}
public class ListAdapter extends BaseAdapter implements View.OnClickListener{//....}
my home model:
public class Home_Model {
Home_Model(){}
static final Home_Model instance = new Home_Model();
public static Home_Model getInstance() {
return instance;
}
public void setPostsArrayList(){
new setHomePostsArray().execute();
}
public class setHomePostsArray extends AsyncTask<Void, ArrayList<PostClass>, Void>{
ArrayList<String> followersList;
ArrayList<PostClass> postsArrayList;
#Override
protected Void doInBackground(Void... params) {
// Getting posts from parse
String userName = Parse_model.getInstance().getUserClass().get_userName();
followersList = Parse_model.getInstance().getFollowersByUserNameToString(userName);
followersList.add(userName);
postsArrayList = Parse_model.getInstance().getAllUsersPostsByFollowings(followersList);
for (PostClass currPost : postsArrayList) {
for (PostClass currLocalDBPost : LocalDBPostsArray) {
if (currPost.getObjectID().equals(currLocalDBPost.getObjectID())) {
currPost.set_postPicture(currLocalDBPost.get_postPicture());
}
}
}
//Updating home page
onProgressUpdate(postsArrayList);
// Updating local data base in new posts
//checking in local DB if there are any new posts from parse and update them
for (PostClass currPost : postsArrayList) {
boolean isPostExists = false;
for (PostClass currLocalPost : LocalDBPostsArray) {
if (currPost.getObjectID().equals(currLocalPost.getObjectID())) {
isPostExists = true;
}
}
if (!isPostExists) {
ModelSql.getInstance().addPost(currPost);
Log.e("post not exist", "adding local DB");
}
}
//updating followers list in local DB
Parse_model.getInstance().getUserClass().setFollowersArray(followersList);
ModelSql.getInstance().updateFollowersArray(currUser);
return null;
}
#Override
protected void onProgressUpdate(ArrayList<PostClass>... values) {
//pass the updated postsArrayList to home fragment
if(setPostsInHomePageDelegate!= null){
setPostsInHomePageDelegate.setPosts(values[0]);
}
}
}
public interface SetPostsInHomePage {
public void setPosts(ArrayList<PostClass> postsArrayList);
}
SetPostsInHomePage setPostsInHomePageDelegate;
public void setSetPostsInHomePageDelegate(SetPostsInHomePage setPostsInHomePageDelegate) {
this.setPostsInHomePageDelegate = setPostsInHomePageDelegate;
}
main activity:
public class MainActivity extends Activity {
static HomeFragment homeFragment = new HomeFragment();
#Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
// the home fragment has already been opened during the app opening
//...
setPostsImHomePage();
}
//...
public void setPostsImHomePage(){
Home_Model.getInstance().setSetPostsInHomePageDelegate(new Home_Model.SetPostsInHomePage() {
#Override
public void setPosts(ArrayList<PostClass> postsArrayList) {
homeFragment.updatePostsArrayList(postsArrayList);
}
});
}
}

Try to move your method setPostsImHomePage(...) from MainActivity to HomeFragmentand call it in OnCreateView before return root;.

Try initializing homeFragment in onCreate before your method call. It's also helpful to know which line(s) are giving you errors.

Obviously your fragment has no View when the result arrives.
You should properly add the fragment to the Activity using the FragmentManager, then in the Fragment's onActivityCreated() callback (which is called by the system after the Fragment has its view properly set), start your AsyncTask.

Android - Access Servlet AsyncPost task response from fellow classes

I have an app connected to a Java Servlet backend by means of an AsyncPost task. The task returns a String to the client representing a json object serialized with Gson.
It works almost fine, the problem is that I'm unable to access the Servlet response message from the class instantiating the call to the ServletPostAsyncTask: ListViewPrenota.class.
The project is structured as follows:
Both within the Servlet and the Client I created the two classes, Tour.class and Tours.class to store my data:
Tour class:
public class Tour {
// some simple int/string/list fields
}
Tours class:
public class Tours {
private List<Tour> tours;
// ...
}
On Client side, in a ServletPostAsyncTask.class, I receive the aforementioned Gson object within doInBackGround(). Within onPostExecute() I deserialize it, this way:
class ServletPostAsyncTask extends AsyncTask<Pair<Context, String>, Void, String> {
private Context context;
Tours tours;
#Override
protected String doInBackground(Pair<Context, String>... params) {
//connect to Servlet and get the serialized Gson object
}
#Override
protected void onPostExecute(String jsonResponse) {
tours = (new Gson().fromJson(jsonResponse, Tours.class));
}
}
Now, from ListViewPrenota.class in Client I am calling the ServletPostAsyncTask:
ServletPostAsyncTask s = new ServletPostAsyncTask();
s.execute(new Pair<Context, String>(ListViewPrenota.this, "tours"));
Tours ttours = s.tours;
Tour tour = ttours.getTours().get(0);
Problem: I receive a java.lang.NullPointerException pointing to Tour tour = ttours.getTours().get(0);
What is the reasong preventing me to access the newly received Tours object from other classes than ServletPostAsyncTask?
Thank you very much

the problem is you are thinking that code runs serially, if you want to use stuff returned from the AsycTask you need to use it in onPostExecute or have a callback that sends the data after it is done
doInBackground(){
//do heavy work
}
onPostExecute(Data data){
//handle data
//send data via interface to activity or class that needs the data
//or just put everything that needs the data in here
}

Ok, it works. Here's what I was able to come up with:
Callback interface:
interface CallBack {
void callBackMethod(Tours tours);//do job
}
Caller class:
class ServletPostAsyncTask extends AsyncTask<Pair<Context, String>, Tours, String>{
private Context context;
Tours tours;
public ListViewPrenota listViewPrenota;
public ServletPostAsyncTask(ListViewPrenota listView){
this.listViewPrenota = listView;
}
#Override
protected String doInBackground(Pair<Context, String>... params) {
//communicate with Servlet and get a HttpResponse
}
#Override
protected void onPostExecute(String jsonResponse) {
tours = (new Gson().fromJson(jsonResponse, Tours.class));
//the callback starts a thread updating the UI in ListViewPrenota
listViewPrenota.callBackMethod(tours);
Toast.makeText(
context,
"Connected. \nTours size: "+ tours.getTours().size(),
Toast.LENGTH_LONG).show();
}
}
The callback interface's implementation within ListViewPrenota:
public class ListViewPrenota extends FragmentActivity implements CallBack{
private ProgressDialog m_ProgressDialog = null;
private Runnable viewOrders;
private TourAdapter m_adapter;
ListView listView;
private ArrayList<Tour> m_tours =null;
#Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_list_view_prenota);
listView = (ListView) findViewById(R.id.list);
m_tours = new ArrayList<Tour>();
m_adapter = new TourAdapter(this, R.layout.list_row, m_tours);
listView.setAdapter(m_adapter);
getActionBar().setDisplayHomeAsUpEnabled(true); //pulsante drawer
getActionBar().setHomeButtonEnabled(true); //pulsante dietro
ServletPostAsyncTask spat = new ServletPostAsyncTask(ListViewPrenota.this);
String status = spat.getStatus().toString();
spat.execute(new Pair<Context, String>(ListViewPrenota.this,"tours"));
}
public void callBackMethod(final Tours tours){
System.out.println("I've been called back");
viewOrders = new Runnable(){
#Override
public void run() {
getOrders(tours);
}
};
Thread thread = new Thread(null, viewOrders, "MagentoBackground");
thread.start();
m_ProgressDialog = ProgressDialog.show(
ListViewPrenota.this,
"Please wait...",
"Retrieving data ...",
true);
}
public void getOrders(Tours tours){
try{
m_tours = new ArrayList<>();
m_tours.addAll(tours.getTours());
Thread.sleep(2000);
Log.i("ARRAY", "" + m_tours.size());
} catch (Exception e) {
Log.e("BACKGROUND_PROC", e.getMessage());
}
//add tours to the adapter
runOnUiThread(returnRes);
}
private Runnable returnRes = new Runnable() {
#Override
public void run() {
if(m_tours != null && m_tours.size() > 0){
m_adapter.notifyDataSetChanged();
for(int i=0;i<m_tours.size();i++)
m_adapter.add(m_tours.get(i));
}
m_ProgressDialog.dismiss();
m_adapter.notifyDataSetChanged();
}
};
If there's a better way to do it I accept further suggestions.
In the mean time, thank you very much

Show splash screen until app is done loading

My app loads a lot of stuff on startup and after testing it delays too long at the beginning to not have a splash screen. So, I want to display a splash screen until my app is done loading. I do NOT want to display a screen with a timer for X seconds. I found an example here:
Android SplashScreen
I tried implementing the code in the SO topic above but I just don't understand the code. After integrating it in my code I come up with one error that I commented into the code below. But I don't understand a lot of the code and I have commented in the code below the parts I am confused by.
public class MainMenu extends Activity {
private ProgressDialog pd = null;
private Object data = null; //What is this?
#Override
public void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
requestWindowFeature(Window.FEATURE_NO_TITLE);
setContentView(R.layout.mainmenu);
// show the ProgressDialog on this thread
this.pd = ProgressDialog.show(this, "Working...", "Downloading data...", true, false);
// start a new thread that will download all the data
new DownloadTask().execute("Any parameters to download."); //What is DownloadTask()?
}
private class DownloadTask extends AsyncTask<String, Void, Object> {
protected Object doInBackground(String... args) { //Are these parameters correct?
return "replace this with your object"; //What is this?
}
protected void onPostExecute(Object results) {
// pass the resulting data to the main activity
MainMenu.this.data = result; //Error: "result cannot be resolved to a variable"
if(MainMenu.this.pd != null) {
MainMenu.this.pd.dismiss();
}
}
}
}

Let's start with the error:
MainMenu.this.data = result;
Notice the typo? It should be result*s*:
MainMenu.this.data = results;
Addressing the rest of your questions below:
private class DownloadTask extends AsyncTask<String, Void, Object>
The declaration is for an inline class called DownloadTask, and it states that you'll be taking Strings (via String...) as parameters to your doInBackground(String... params).
The second parameter (Void in your case) indicates the datatype used to "publish" the progress via publishProgress(DATATYPE)/onProgressUpdate(DATATYPE... progress). This method is suitable for notifying the user of changes, for example when you've finished downloading a file but still have a few to go.
The last parameter (Object), indicates what type of data you'll be passing on to onPostExecute(DATATYPE), in this example Object. This could either be to update a ListAdapter somewhere, or trigger any other UI change based on the outcome of the actions done in doInBackground.

Show ProgressDialog in onPreexecute and dismiss it in onPostExcute methods
something like this
private class DownloadTask extends AsyncTask<String, Void, Object> {
#Override
protected void onPreExecute() {
mProgressDialog = new ProgressDialog(activity);
mProgressDialog =ProgressDialog.show(activity, "", "Please Wait",true,false);
super.onPreExecute();
}
protected Object doInBackground(String... args) { //Are these parameters correct?
return "replace this with your object"; //What is this?
}
protected void onPostExecute(Object results) {
// pass the resulting data to the main activity
MainMenu.this.data = results; //it should be results
if (mProgressDialog != null || mProgressDialog.isShowing()){
mProgressDialog.dismiss();
}
if(MainMenu.this.pd != null) {
MainMenu.this.pd.dismiss();
}
}

Android Threading Issue

Here's the situation: I've got some lengthy non-UI code that needs to be run in a ListActivity and then have this ListActivity update the UI to contain a the result of this lengthy method (the list). I need a ProgressDialog to be running until it's finished so the user has some feedback.
Here's the code:
public class SolutionListActivity extends ListActivity
{
private String[] solutions;
private String letters;
private ProgressDialog dialog;
private static Solver solver;
/** Called when the activity is first created.
* #param savedInstanceState
*/
#Override
public void onCreate(Bundle savedInstanceState)
{
super.onCreate(savedInstanceState);
//Get the selected letters from LettersActivity
letters = getIntent().getStringExtra("letters");
dialog = ProgressDialog.show(this, "Please Wait...",
"Searching Words...", true);
new Thread()
{
#Override
public void run()
{
if (solver == null)
{
solver = new Solver(SolutionListActivity.this);
solver.readDictionary(0);
solver.readDictionary(1);
}
solutions = solver.solve(letters);
runOnUiThread(new Runnable()
{
#Override
public void run()
{
dialog.dismiss();
//Set up a UI List
setListAdapter(new ArrayAdapter<String>(SolutionListActivity.this, R.layout.list_item, solutions));
ListView lv = getListView();
lv.setTextFilterEnabled(true);
}
});
}
}.start();
The problem is my ProgressDialog won't dismiss and I can't be sure whether the solutions = solver.solve(letters); will be finished before the UI uses it in setListAdapter(new ArrayAdapter<String>(SolutionListActivity.this, R.layout.list_item, solutions));
Any advice you guys have would be helpful.
Thank You,
Calum

Have you tried AsyncTask? It´s built exactly for having threading AND be able to update things in your UI Thread.
Take a look here: http://developer.android.com/resources/articles/painless-threading.html