Issue
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!
Solution
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.
Answered By - Aorlinn
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