I have an app that downloads data which must be displayed within the fragments of a viewpager. I dont know how to call the ListFragment adaptername.notifyDataSetChanged() in the AsyncTask that does the data download.
For example say i am downloading the temperature forecast for the next seven days:
my app has the following structure:
MainActivity: Starts an AsyncTask to download the data in onCreate() and gives the user choice (button) of which day to look at. Clicking the button launches SecondActivity and passes the day index to the ViewPager (to set the current view).
SecondActivity: Contains a ViewPager that contains 7 of the same ListFragments (The list display the temperature over a period of 5 hours, so the list has 5 entries).
MyListFragment: when this loads it sets the adapter to display each temperature (If the data is downloaded) otherwise it sets the temperature to "loading..."
Now my problem is, if the user waits on the MainActivity until the data downloads they can then proceed to the ViewPager to see the ListFragment temperatures without problem. But if they try click a day and load the ViewPager before the download completes the fragments will forever just say "loading..."
I need a way that I can reload the adapter within the ListFragment from the onPostExecute() of my AsyncTask in MainActivity. To do this though i need to be able to actually access the ListFragment that the ViewPager is displaying. How do update the adapter onPostExecute()?
MainActivity:
protected void onCreate(Bundle savedInstanceState) {
...
new LoadData().execute();
}
protected class LoadData extends AsyncTask<String, Void, String> {
protected String doInBackground(String... params) {
//Download happens here
}
protected void onPostExecute(String result) {
//I need to tell the viewpager in SecondActivity to reload the ListFragment it is currently showing here
}
}
SecondActivity
private ViewPager mPager;
private PagerAdapter mPagerAdapter;
private int[] temperatureArray;
public void onCreate(Bundle savedInstanceState) {
...
Intent intent = getIntent();
int[] defaultTemps = {0, 0, 0, 0, 0};
temperatureArray = getIntArrayExtra("temps", defaultTemps);
mPager = (ViewPager) findViewById(R.id.pagerID);
mPager.setCurrentItem(intent.getIntExtra("page", 0));
mPagerAdapter = new MyPagerAdapter(getSupportFragmentManager());
mPager.setAdapter(mPagerAdapter);
}
private class ScreenSlidePagerAdapter extends FragmentStatePagerAdapter {
...
public Fragment getItem(int position) {
MyListFragment lf = new MyListFragment();
lf.setTemperatures(temperatureArray);
return lf;
}
}
I need a way to be able to refresh the current displayed fragment when i finish downloading in the AsyncTask.
I can suggest doing one of two things, not sure if these are best practices.
You can Have the AsyncTask send a broadcast with a unique action when it has finished loading the information. That would of course have to be done from OnPostExecute:
#Override
protected void onPostExecute(Void result) {
super.onPostExecute(result);
if (result != null) {
//_Do whatever action you normally do, like storing result to database.
//fire up the broadcast
Intent intent = new Intent(Home.ACTION_FEED_LOADING_FINISHED);
mContext.sendBroadcast(intent);
}
}
And then intercept that via a BroadcastReceiver on your Fragment's code.
private BroadcastReceiver mBroadcastReceiver = new BroadcastReceiver() {
#Override
public void onReceive(Context context, Intent intent) {
String action = intent.getAction();
if (action.equals(ACTION_FEED_LOADING_FINISHED)) {
//The AsyncTask Finished loading data
}
}
};
Once the event is received, since the BroadcastReceiver is in the Fragment, where the views are loaded, you can refresh your UI accordingly.
Or,
You can pass the View you want to refresh as a paremeter on your AsyncTask.
Imagine your AsyncTask class like this:
private static class getInternetStuffTask extends AsyncTask<String, Void, Void> {
Then you can create a custom constructor to receive the view as paremeter:
public getInternetStuffTask (final Context context, final ListView list) {
this.mContext = context;
this.mListView= list;
}
Then, during onPostExecute, when data is loaded, you can simply set the adapter to the ListView directly form the AsyncTask.
You're touching on two different problems. First, use a Service for downloading. Services are there for long running operations (like a download) that live outside an Activity's lifecycle.
Second, to communicate between Activities and Fragments you'll want to use an callback interface. The link provided is to the docs which do an excellent job of explaining and providing samples.
Related
I have an activity that grabs data via WebService, from there it creates elements to display the data. Some data is grouped so my solution was to display the grouped data in their own fragments below the main layout, allowing the user to swipe across the groups, probably with a tabs at the top to show the group name.
The problem I came across was that the fragments in the activity are created before that web call takes place, making them empty or using old data. I then created a sharedpreferences listener and placed the fragments layout creation method within it. The main method grabs the data, writes to sharedpreferences the fragment detects the change and creates it's layout, Or so I thought.
Some groups are the same between items, so moving from one to the other won't trigger that onchange event thus not triggering the layout creation method. I then decided to do the following to always trigger the onchange event after the sharedpreferences are written
final Boolean updated = settings.getBoolean("UPDATED_1", false);
SharedPreferences.Editor editor = settings.edit();
editor.putBoolean("UPDATED_" + pageNum, !updated);
I just don't think that's the best solution, it also has it's problems and isn't triggering every time (Which I have yet to troubleshoot)
What's a better solution for all this? I also have a memory leak I haven't diagnosed yet to make things even more of a headache.
I've just thought of moving my data grabbing method to before the ViewPager initialization but I'm not yet sure if this will solve my problem.
I would not recommend waiting until you get the data to show the view as it will affect the User Experience and look sluggish.
Instead, you could implement an AsyncTaskLoader in your fragment so you can inform the Fragment's View with a BroadcastReceiver once you get the data from your server. In the meantime, just show a spinner until the data are retrieved, then you hide it and update your list with a adapter.notifyDataSetChanged();.
Here is an example of a AsyncTaskLoader (In my case it's a database query instead of a server call like you):
public class GenericLoader<T extends Comparable<T>> extends AsyncTaskLoader<ArrayList<T>> {
private Class clazz;
public GenericLoader(Context context, Class<T> clazz) {
super(context);
this.clazz = clazz;
}
#Override
public ArrayList<T> loadInBackground() {
ArrayList<T> data = new ArrayList<>();
data.addAll(GenericDAO.getInstance(clazz).queryForAll());
Collections.sort(data);
return data;
}
}
Then in your Fragment:
public class FragmentMobileData extends Fragment implements ListAdapter.OnItemClickListener, LoaderManager.LoaderCallbacks<ArrayList<EntityCategories.EntityCategory>> {
public static String TAG = "FragmentMobileData";
private ImageListAdapter adapter;
private ArrayList<EntityList> mCategories = new ArrayList<>();
private final BroadcastReceiver mReceiver = new BroadcastReceiver() {
#Override
public void onReceive(Context context, Intent intent) {
Bundle bundle = intent.getExtras();
String result = bundle.getString(DatabaseService.RESULT);
if (DatabaseService.NO_CONNECTION.equals(result)) {
Utils.showToastMessage(getActivity(), "No internet connexion", true);
} else if (DatabaseService.RESULT_TIMEOUT.equals(result)) {
Utils.showToastMessage(getActivity(), "Bad connection. Retry", true);
}
getActivity().getSupportLoaderManager().initLoader(1, null, FragmentMobileData.this).forceLoad();
}
};
#Bind(R.id.progressBarEcard)
ProgressBar spinner;
#Bind(R.id.list)
RecyclerView list;
public FragmentMobileData() {
}
#Override
public void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
}
#Override
public View onCreateView(LayoutInflater inflater, ViewGroup container,
Bundle savedInstanceState) {
View view = inflater.inflate(R.layout.fragment_mobile_plan, container, false);
ButterKnife.bind(this, view);
((AppCompatActivity) getActivity()).getSupportActionBar().setTitle("Mobile");
list.setLayoutManager(new LinearLayoutManager(context));
list.addItemDecoration(new DividerItemDecoration(context, R.drawable.divider));
adapter = new ImageListAdapter(mCategories, this);
list.setAdapter(adapter);
Intent intent = new Intent(context, DatabaseService.class);
intent.setAction(DatabaseService.UPDATE_DATA);
getActivity().startService(intent);
return view;
}
#Override
public void onPause() {
super.onPause();
getActivity().unregisterReceiver(mReceiver);
}
#Override
public void onResume() {
super.onResume();
getActivity().registerReceiver(mReceiver, new IntentFilter(DatabaseService.UPDATE_DATA));
}
#Override
public Loader<ArrayList<EntityCategories.EntityCategory>> onCreateLoader(int id, Bundle args) {
return new GenericLoader(context, EntityCategories.EntityCategory.class);
}
#Override
public void onLoadFinished(Loader<ArrayList<EntityCategories.EntityCategory>> loader, ArrayList<EntityCategories.EntityCategory> data) {
if (mCategories.size() != data.size()) {
mCategories.clear();
mCategories.addAll(data);
adapter.notifyDataSetChanged();
Intent intent = new Intent(context, DownloadFilesService.class);
context.startService(intent);
}
spinner.setVisibility(View.GONE);
}
#Override
public void onLoaderReset(Loader<ArrayList<EntityCategories.EntityCategory>> loader) {
mCategories.clear();
adapter.notifyDataSetChanged();
}
//...
}
Maybe I misunderstood something. But in your case I think there is pretty good alternative to create, for example, your fragment which will display some group of data, then in it's creation stage show progress bar in ui, and meantime do request to the data in background. Then handle result data and show it, and hide progress bar.
This can be achieved with implementing MVP pattern to provide flexibility of code and easy testing. Also you can use rxJava and Retrofit to handle requests in a convenient way. More information about MVP and samples you can find here.
If you don't want to provide this way for some reason. For example, you have undetermined number of groups, which you will receive in future somehow and you want to dynamically build your fragments base on data which you receive, then I suggest you can organize presentation layer in your activity. In this layer your will receive data then pass it to special handler, which will divide it to groups and base on them will ask activity to create fragment. In constructor you will send already received data (so it is need to implement Parcelable interface).
in my app, it launches a fragment, in the onCreate method, I asynchronously make a web service call, which takes about 2 seconds for the data to return. But by that time, the fragment has already completed loading. As a result, there is no data to appear on the screen.
I tried to put a thread sleep for 2 seconds, but that didnt work either.
When the asynchronous call has completed, it should call a method on the fragment. You can achieve this by passing the fragment to the async call and having a callback method.
For example, assuming you are using AsyncTask:
public class MyFragment extends Fragment {
public void onCreate(Bundle b) {
new MyAsyncTask(this).execute(params);
}
public void updateResult(Result r) {
// update the views here
}
}
public MyAsyncTask extends AsyncTask<...> {
private final MyFragment fragment;
public MyAsyncTask(MyFragment fragment) {
this.fragment = fragment;
}
...
#Override
protected void onPostExecute(Result result) {
fragment.updateResult(result);
}
}
In this way, the fragment can load before the data is ready. Then the data can be populated when it is received.
Depending on your design, you may like to have a loading progress animation visible by default in your fragment, then hide it when you make the data visible in the callback.
COMPLETELY EDITED
Ok I will try to be more specific.
I'm developing a small app drawer. Therefore I need a way to let the user choose categories.
This is the main point of the question. If there is another better way than my approach please let me know.
So my try to implement this was to load all apps to an array list and save this to a service to make it available throuout the app. The loading of the apps is done by an asynctask.
MainActivity:
#Override
public void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
packageManager = getPackageManager();
if ((AppDrawerService.getApps()) == null) {
apps = new ArrayList<ApplistItem>();
loadAppsToService();
} else {
//if Service already holds the data
manageViews();
}
}
private void loadAppsToService() {
LoadApplications loadApps;
loadApps = new LoadApplications(this);
loadApps.setOnLoadApplicationsFinishedListener(this);
loadApps.execute();
}
private void manageViews() {
FragmentManager fragmentManager = getSupportFragmentManager();
myViewPager = (ViewPager) findViewById(R.id.view_pager);
myViewPager.setAdapter(new ViewPagerAdapter(fragmentManager, this));
}
#Override
public void OnLoadApplicationsComplete(ArrayList<ApplistItem> apps) {
manageViews();
}
LoadApplications:
#Override
protected void onPostExecute(Object o) {
AppDrawerService.setApps(apps);
listener.OnLoadApplicationsComplete(apps);
super.onPostExecute(o);
}
ViewPagerAdapter:
#Override
public Fragment getItem(int position) {
Fragment category = null;
category = new AppListFragment()
return category;
}
#Override
public int getCount() {
pages = 5;
return pages;
}
AppListFragment:
#Override
public void onViewCreated(View view, Bundle savedInstanceState) {
super.onViewCreated(view, savedInstanceState);
if ((AppDrawerService.getApps()) == null) {
apps = new ArrayList<ApplistItem>();
} else {
apps = AppDrawerService.getApps();
}
v = getActivity().findViewById(android.R.id.list);
myApplist = (ListView) v;
applistAdapter = new ApplistAdapter(context, 0, apps);
myApplist.setAdapter(applistAdapter);
myApplist.setOnItemClickListener(this);
}
The problem which I have with this code is, that it alway populates only one Fragment with the applist. On first start the first fragment is populated with the apps but if you swipe two fragments to right and then one back to left, this fragment (the second) is populated. The impression which I have is that the
return new ApplistFragment();
from the ViewPager does actually not create an independent instance of the fragment. (At least I think so)
What I want is to show an undefined amount of Fragments which can all show different categories of the same list. So if possible I want to load the list only once and I want to reuse the Fragment code for every fragment since I don't want to restrict the max amount of categories.
My idea was to use the ApplistAdapter to filter the unwanted apps for every category but I really don't know.
Help is really really appreciated since I have no idea how to go along.
Thanks in advance.
I would change how this works all together. A few suggestions of the top of my head:
Download the data in the Service instead of the Activity and
persist it somewhere for example in a database.
You can use an Intent to tell the Service what you want to download and when to do it.
You can also use an IntentService instead of a Service.
IntentServices already handle each Intent in a separate worker
thread so you don't need an AsyncTask or anything like that in the
IntentService to perform the downloading.
Each Fragment should load the data from the database in onResume(). You
can use local broadcasts to inform the Fragments when the data
changed while they are being displayed.
If you don't give us more information it will be difficult to give you very specific advice.
I have 3 Tabs like in the android development tutorial
Now what I want to do is very simple I use Fragments on each page. I want to show different content from a rss feed on each page.
The problem is when I go to the next tab it runs AsyncTask (which is in onCreateView) of the previous Fragment.
So you start on Page 1 it loads the content fine. Then when you go to Page 2 is runs the onCreateView of the Fragment of Page 1 again. And obviously gives an NullException. The point is it should not be running AsyncTask of Page 1 at all at that Page 2.
I don't think there is any example code needed if so tell me which part you need to see. Then I will edit my question.
AsyncTask inside a ListFragment :
public class MyAsyncTask extends AsyncTask<List<String>, Void, List<String>>
{
// List of messages of the rss feed
private List<Message> messages;
private volatile boolean running = true;
#SuppressWarnings("unused")
private WeakReference<NieuwsSectionFragment> fragmentWeakRef;
private MyAsyncTask(NieuwsSectionFragment fragment)
{
this.fragmentWeakRef = new WeakReference<NieuwsSectionFragment>(fragment);
}
#Override
protected void onPreExecute()
{
super.onPreExecute();
mProgress.show();
// progress.setVisibility(View.VISIBLE); //<< set here
}
#Override
protected void onCancelled()
{
Log.w("onCancelled", "now cancelled");
running = false;
}
#Override
protected List<String> doInBackground(List<String>... urls)
{
FeedParser parser = FeedParserFactory.getParser();
messages = parser.parse();
List<String> titles = new ArrayList<String>(messages.size());
for (Message msg : messages)
{
titles.add(msg.getTitle());
// Log.w("doInBackground", msg.getTitle());
}
return titles;
}
#Override
protected void onPostExecute(List<String> result)
{
super.onPostExecute(result);
mProgress.dismiss();
if (result != null)
{
PostData data = null;
listData = new PostData[result.size()];
for (int i = 0; i < result.size(); i++)
{
data = new PostData();
data.postTitle = result.get(i);
data.postThumbUrl = "http://igo.nl/foto/app_thumb/28991-Taxi-vast-na-poging-tot-nemen-van-sluiproute.jpg";
listData[i] = data;
Log.w("onPostExecute", "" + listData[i].postTitle);
}
adapter = new PostItemAdapter (getActivity(), android.R.layout.simple_list_item_1, listData);
setListAdapter(adapter);
adapter.notifyDataSetChanged();
}
}
}
It's called inside a method and that method is executed inside the onCreateView of the ListFragment :
#Override
public View onCreateView(LayoutInflater inflater, ViewGroup container, Bundle savedInstanceState)
{
startNewAsyncTask();
View rootView = inflater.inflate(R.layout.fragment_section_nieuws, container, false);
return rootView;
}
#SuppressWarnings("unchecked")
public void startNewAsyncTask()
{
MyAsyncTask asyncTask = new MyAsyncTask(this);
this.asyncTaskWeakRef = new WeakReference<MyAsyncTask>(asyncTask);
asyncTask.execute();
}
The LogCat :
Try using isAdded() before onPostExecute(). isAdded() returns true if the fragment is currently added to its activity.
http://developer.android.com/reference/android/app/Fragment.html#isAdded()
#Override
protected void postExecute(){
if(isAdded()){
//perform Display Changes
}
}
Move your
startNewAsyncTask();
to onActivityCreated()
I'm assuming your using FragmentPagerAdapter with your ViewPager.
To enable smooth animations, the ViewPager by default keeps the current fragment and the two neighbors in resumed state. So onCreateView is not the best place to start the AsyncTask.
Instead you need to create a custom listener interface. The fragments in the ViewPager should implement it, and call the new interface from the ViewPager's OnPageChangeListener.
Check out my answer to this question or you can read the whole tutorial here.
You're getting that exception because you're calling getActivity() too early. You should do it after onActivityCreated() (see this diagram)
Executing of onCreateView() in background is fine and actually is default behaviour. The thing is, ViewPager is optimised to load a content of neighbour non-visible pages in background to improve UX. You can do this:
mViewPager.setOffscreenPageLimit(2); (default value is 1) to load all 3 pages at once (1 is loading as currently visible and other 2 as optimisation). Or set it to 0 to disable this behaviour, but it's not the best idea.
In general, you should cash your loaded data and do not load it again by making your fragment's lifecycle methods as light as possible. Page limit of 2 is fine for 3 pages, but if you'll have for example 10 pages, limit of 9 is too much.
If I've understood your question right, I think you need unique content with each Fragment right?
Try using the varible arguments of the execute method. For example:
yourTask.execute(<some-unique-URL>, parameter, one-more-parameter);
In this way you can pass a unique URL per fragment form which you can get your content.
I feel you already have this. The doInBackground method has the List of URLs. You just need to pass that information in the execute method and utilize it in doInBackground.
Hope this helps!
It is normal that it runs the AsyncTask from the adjacent Fragments, since the ViewPager + PagerAdapter combo, works loading the current, previous and next Fragment.
You should focus the problem not to stop AsyncTask from running, but to let it run w/o throwing a NullPointerException.
The following should be called inside onCreateView()
adapter = new PostItemAdapter (getActivity(), android.R.layout.simple_list_item_1, myList);
setListAdapter(adapter);
And then, onPostExecute()
myList.clear();
myList.addAll(listData);
adapter.notifyDataSetChanged();
The ViewPager will create views for fragments in adjacent pages and destroy views for fragments which are not adjacent to current page. Thus, the onCreateView of page 1 will get called if you navigate from page1->page2->page3->page2. You can have the viewpager keep more pages in memory by using ViewPager.setOffscreenPageLimit.
The fragmentPagerAdapter retains the fragment objects. Only the views are destroyed. Thus, when viewpage recreates page1's view, the fragment object is the same. Hence, all the fields in the fragment will get retained.
As in most applications where there's no realtime data, it is not required not load the data every time the view of the fragment is created, you can store the data in the fragment object after loading. Then, before starting the AsyncTask in onCreateView/onActivityCreated, check if the data has been previously loaded or not.
class PageFragment {
private List<String> mData;
...
void onActivityCreated() {
if (data == null) { // OR if the data is expired
startAsyncTask();
} else {
updateViews();
}
}
void updateViews() {
// Display mData in views
}
class LoadDataTask extends AsyncTask<List<String>, ..., ...> {
...
void onPostExecute(List<String> result) {
PageFragment.this.mData = result;
PageFragment.this.updateViews();
}
}
I recommend that you use loaders for loading data for a fragment. For your purpose, you can configure a loader to load data only once.
This is a great tutorial on Loaders.
http://www.androiddesignpatterns.com/2012/07/loaders-and-loadermanager-background.html
In the tutorial, the loader is configured to return previous data immediately if available, and then fetch data in background and return it after fetching completes. Thus, the UI will get updated after fresh data gets downloaded but at the same time, it will show the previous data initially while the download happens.
You can use another activity - this activity will run asynctask and then move to your fragment related activity. In this way it should call only once.
In case you need to update Fragment UI using this AsyncTask then use a static method to call through AsyncTask.
I am trying to retrieve some image URLs and image texts by scraping a website when my app starts up. I then want to use arrays I've created of imageTexts and imageURLs to create a ViewPager so I can display the images as a scrollable gallery.
I execute the network task of fetching the webpage and parsing the HTML as an AsyncTask (downloadTask() in the code below).
My problem is that as this happens in the background it doesn't have time to fetch and parse the data before the code tries to display the images. I get a null pointer error when the code hits
new ViewPagerAdapter(this, getimageText(), getimageURLS())
I can deduce that the array that should eventually store the image URLs and the image Texts hasn't been created yet. This is at the point my app crashes.
Here's my code. I'm new to both Android and Java so clearly I could do with a lot of help!
public void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
// Get the view from viewpager_main.xml
setContentView(R.layout.viewpager_main);
// Fetch the list of URLs using a background network task
new downloadTask().execute(BASE_URL);
// Locate the ViewPager in viewpager_main.xml
viewPager = (ViewPager) findViewById(R.id.pager);
// Pass results to ViewPagerAdapter Class
adapter = new ViewPagerAdapter(this, getimageText(), getimageURLS());
// Binds the Adapter to the ViewPager
viewPager.setAdapter(adapter);
}
You can initially set a default image and you can modify your asyncTask to take in a callback method. This callback method would be a method that initializes your ViewPager. In your AsyncTask you would get the images and override the onPostExecute() method to call that callback method.
Example
class PictureGrabber extends AsyncTask<URL, VOID, VOID>{
IPicCallBack callBack;
PictureGrabber(IPicCallBack callback)
{
this.callback = callback;
}
protected Void doInBackGround(URL... urls){
}
protected void onPostExecute(VOID result)
{
callback.Init();
}
public interface IPicCallBack{
public void Init();
}
Then in your code you need to implement that IPicCallback and set it on the asyncTask and when the task is complete it'll call that method