I know the recommended way of passing parameters to Fragments is to use a static method and create a bundle and call setArguments()/getArguments() on the Fragment:
public static MyFragment newInstance(int arg1, int arg2) {
Bundle b = new Bundle();
b.putInt("key1", arg1);
b.putInt("key2", arg2);
MyFragment frag = new MyFragment();
frag.setArguments(b);
}
public View onCreateView(....) {
Bundle b = getArguments();
memberVar1 = b.getInt("key1");
memberVar2 = b.getInt("key2");
.....
}
Correct me if I'm wrong, but it seems like the following approach works as well, which doesn't require calling getArguments() later on:
#Override
public void setArguments(Bundle args) {
memberVar1 = args.getInt("key1");
memberVar2 = args.getInt("key2");
}
This is based on the way that setArguments() is implemented in the Fragment class. If this allows mArguments to be accessed in the event of Fragment recreation, then shouldn't the same apply to other variables set within the setArguments() call?
659 public void setArguments(Bundle args) {
660 if (mIndex >= 0) {
661 throw new IllegalStateException("Fragment already active");
662 }
663 mArguments = args;
664 }
Aside from convention, does either scenario have an advantage over the other?
There shouldn't be a reason for you to have to override the setArguments(...) method... Here is a possible approach for you to take to 'cleanup' the arguments passing.
Create a new class, something like MyFragmentArguments.java.
public class MyFragmentArguments {
public long keyOne;
public long keyTwo;
public MyFragmentArguments(long keyOne, long keyTwo) {
this.keyOne = keyOne;
this.keyTwo = keyTwo;
}
public MyFragmentArguments(Bundle bundle) {
this.keyOne = bundle.getLong("KEY_ONE");
this.keyTwo = bundle.getLong("KEY_TWO");
}
public Bundle toBundle() {
Bundle args = new Bundle();
args.putLong("KEY_ONE", keyOne);
args.putLong("KEY_TWO", keyTwo);
return args;
}
}
Then, you can (very cleanly) call:
final MyFragmentArguments arguments = new MyFragmentArguments (getArguments());
if(arguments != null) {
// do something with arguments.keyOne & arguments.keyTwo
}
This way, you avoid unintended side effects of overriding something that's built into the android OS - something that you should strongly question whenever you do.
Once you call the super implementation of setArguments you maintain the integrity of the pattern.
I have a peculiar issue with Loaders. Currently I am unsure if this is a bug in my code or I misunderstand loaders.
The app
The issue arises with conversations (imagine something similar to Whatsapp).
The loaders I use are implemented based on the AsyncTaskLoader example. I am using the support library.
In OnCreate, I start a loader to retrieve cached messages.
When the CachedMessageLoader finishes, it starts a RefreshLoader to retrieve (online) the newest messages.
Each loader type as a distinct ID (say, offline:1 online:2)
This works very well, with the following exception.
Problem
When I open another fragment (and add the transaction to the backstack) and then use the Back-Key to go back to the conversationFragment, onLoadFinished is called again with both results from before.
This call happens before the fragment has had any chance to start a loader again...
This delivering of "old" results that I obtained before results in duplicated messages.
Question
Why are those results delivered again?
Do I use these loaders wrong?
Can I "invalidate" the results to ensure that I only get them delivered once or do I have to eliminate duplicates myself?
Stack trace of call
MyFragment.onLoadFinished(Loader, Result) line: 369
MyFragment.onLoadFinished(Loader, Object) line: 1
LoaderManagerImpl$LoaderInfo.callOnLoadFinished(Loader, Object) line: 427
LoaderManagerImpl$LoaderInfo.reportStart() line: 307
LoaderManagerImpl.doReportStart() line: 768
MyFragment(Fragment).performStart() line: 1511
FragmentManagerImpl.moveToState(Fragment, int, int, int, boolean) line: 957
FragmentManagerImpl.moveToState(int, int, int, boolean) line: 1104
BackStackRecord.popFromBackStack(boolean) line: 764
...
Update 1
The loaders mentioned here are initiated by the conversation fragment:
#Override
public void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setHasOptionsMenu(true);
Bundle args = getArguments();
m_profileId = args.getString(ArgumentConstants.ARG_USERID);
m_adapter = new MessageAdapter(this);
if (savedInstanceState != null) {
restoreInstanceState(savedInstanceState);
}
if (m_adapter.isEmpty()) {
Bundle bundle = new Bundle();
bundle.putString(ArgumentConstants.ARG_USERID, m_profileId);
getLoaderManager().restartLoader(R.id.loader_message_initial, bundle, this);
} else {
// Omitted: Some arguments passed in Bundle
Bundle b = new Bundle().
getLoaderManager().restartLoader(R.id.loader_message_refresh, b, this);
}
}
#Override
public void onResume() {
super.onResume();
// Omitted: setting up UI state / initiating other loaders that work fine
}
#Override
public AbstractMessageLoader onCreateLoader(final int type, final Bundle bundle) {
final SherlockFragmentActivity context = getSherlockActivity();
context.setProgressBarIndeterminateVisibility(true);
switch (type) {
case R.id.loader_message_empty:
return new EmptyOnlineLoader(context, bundle);
case R.id.loader_message_initial:
return new InitialDBMessageLoader(context, bundle);
case R.id.loader_message_moreoldDB:
return new OlderMessageDBLoader(context, bundle);
case R.id.loader_message_moreoldOnline:
return new OlderMessageOnlineLoader(context, bundle);
case R.id.loader_message_send:
sendPreActions();
return new SendMessageLoader(context, bundle);
case R.id.loader_message_refresh:
return new RefreshMessageLoader(context, bundle);
default:
throw new UnsupportedOperationException("Unknown loader");
}
}
#Override
public void onLoadFinished(Loader<Holder<MessageResult>> loader, Holder<MessageResult> holder) {
if (getSherlockActivity() != null) {
getSherlockActivity().setProgressBarIndeterminateVisibility(false);
}
// Omitted: Error handling of result (can contain exception)
List<PrivateMessage> unreadMessages = res.getUnreadMessages();
switch (type) {
case R.id.loader_message_moreoldDB: {
// Omitted error handling (no data)
if (unreadMessages.isEmpty()) {
m_hasNoMoreCached = true;
// Launch an online loader
Bundle b = new Bundle();
// Arguments omitted
getLoaderManager().restartLoader(R.id.loader_message_moreoldOnline, b, ConversationFragment.this);
}
// Omitted: Inserting results into adapter
}
case R.id.loader_message_empty: { // Online load when nothing in DB
// Omitted: error/result handling handling
break;
}
case R.id.loader_message_initial: { // Latest from DB, when opening
// Omitted: Error/result handling
// If we found nothing, request online
if (unreadMessages.isEmpty()) {
Bundle b = new Bundle();
// Omitted: arguments
getLoaderManager().restartLoader(R.id.loader_message_empty, b, this);
} else {
// Just get new stuff
Bundle b = new Bundle();
// Omitted: Arguments
getLoaderManager().restartLoader(R.id.loader_message_refresh, b, this);
}
break;
}
// Omitted: Loaders that do not start other loaders, but only add returned data to the adapter
default:
throw new IllegalArgumentException("Unknown loader type " + type);
}
// Omitted: Refreshing UI elements
}
#Override
public void onLoaderReset(Loader<Holder<MessageResult>> arg0) { }
Update 2
My MainActivity (which ultimatively hosts all fragments) subclasses SherlockFragmentActivity and basically launches fragments like this:
Fragment f = new ConversationFragment(); // Setup omitted
f.setRetainInstance(false);
// Omitted: Code related to navigation drawer
FragmentManager fragmentManager = getSupportFragmentManager();
fragmentManager.beginTransaction().replace(R.id.fragment_container_frame, f).commit();
The conversation fragment starts the "display profile" fragment like this:
DisplayProfileFragment f = new DisplayProfileFragment();
// Arguments omitted
FragmentManager manager = getSherlockActivity().getSupportFragmentManager();
FragmentTransaction transaction = manager.beginTransaction();
transaction.replace(R.id.fragment_container_frame, f).addToBackStack(null).commit();
There are other similar questions such as Android: LoaderCallbacks.OnLoadFinished called twice However the behavior of the loader manager hooks are what they are. You can either destroy the loader after getting the first set of results
public abstract void destroyLoader (int id)
or you can handle the onLoaderReset and tie your UI data more closely to the loader data
public abstract void onLoaderReset (Loader<D> loader)
Called when a previously created loader is being reset, and thus
making its data unavailable. The application should at this point
remove any references it has to the Loader's data.
Personally, I would use a ContentProvider and a CursorLoader for this (each row of data would need to have a unique _ID but for messages that should not be a problem).
use following this in onResume()
#Override
public void onResume() {
super.onResume();
getLoaderManager().initLoader(0, null, this);
}
i solved my problem from this
this is same type Q
In the Android documentation for Loaders found at http://developer.android.com/guide/components/loaders.html it says one of the properties of loaders is that:
They automatically reconnect to the last loader's cursor when being recreated after a configuration change. Thus, they don't need to re-query their data.
The following code does not seem to mirror that behaviour, a new Loader is created an finishes querying the ContentResolver, then I rotate the screen and the Loader is re-created!
public class ReportFragment extends Fragment implements LoaderCallbacks<Cursor> {
#Override
public void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
getLoaderManager().initLoader(1, null, this);
}
#Override
public View onCreateView(LayoutInflater inflater, ViewGroup container,
Bundle savedInstanceState) {
View v = inflater.inflate(R.layout.fragment_report, container, false);
return v;
}
public Loader<Cursor> onCreateLoader(int arg0, Bundle arg1) {
Log.d("TEST", "Creating loader");
return new CursorLoader(getActivity(), ResourcesContract.Reports.CONTENT_URI, null, null, null, null);
}
public void onLoadFinished(Loader<Cursor> arg0, Cursor arg1) {
Log.d("TEST", "Load finished");
}
public void onLoaderReset(Loader<Cursor> arg0) {
}
}
Here is the output from my logcat:
08-17 16:49:54.474: D/TEST(1833): Creating loader
08-17 16:49:55.074: D/TEST(1833): Load finished
*Here I rotate the screen*
08-17 16:50:38.115: D/TEST(1833): Creating loader
08-17 16:50:38.353: D/TEST(1833): Load finished
Any idea what I'm doing wrong here?
EDIT:
I should note that I'm building to Android Google API's version 8, and using the v4 support library.
2nd EDIT:
This is most likely due to a bug in the support library, take a look at this bug submission if you want further information:
http://code.google.com/p/android/issues/detail?id=20791&can=5&colspec=ID%20Type%20Status%20Owner%20Summary%20Stars
Though this is an old question, I've been experiencing the same issue as the OP. Using a loader, I need to have it restarting when navigating to a new Activity, and then back. But at the same time, I don't want the loader to restart when I rotate the phone's screen.
What I found is that it is possible to achieve this in onRestart(), if you restart the loader BEFORE calling its super.
public class MainActivity extends AppCompatActivity implements
LoaderManager.LoaderCallbacks<Cursor> {
...
#Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
...
//Initialize the loader.
getSupportLoaderManager().initLoader(0, null, this);
}
#Override
protected void onRestart() {
//Restart the loader before calling the super.
getSupportLoaderManager().restartLoader(LOADER_ID, null, this);
super.onRestart();
}
...
}
In my opinion you misunderstood what the documentation says. The documentations says, that they don't need to re-query their data, and it is not doing so.
Try to log/insert a breakpoint in your ContentProvider#query() method! The query will be called only on Activity startup, and not after orientation change.
But this is not true for the LoaderCallbacks#onCreateLoader() method. It will be called after every orientation change, but this not means re-querying, it just calls the method so you can change the CursorLoader if you want.
So far I found that retaining fragment Fragment.setRetainInstance(true) prevents recreating loader on orientation change using support library. The loader last results are nicely delivered in onLoadFinished(). It works at least when activity manages single fragment and the fragment is added to activity using FragmentTransaction.
Though this is a bit old question I would like to put my views here.
There is no need for storing additional info in onSaveInstanceState
The framework automatically reconnect to the last loader's cursor when being recreated after a configuration change. Thus, they don't need to re-query their data.
This means in the onCreate function you need to call loaderManager only if the savedInstanceState is null
Ex:
#Override
public void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
if(savedInstanceState == null) {
getLoaderManager().initLoader(1, null, this);
}
}
You can simply check to see if the loader already exists onCreate. Then you can either init or restart.
#Override
public void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
if (getLoaderManager().getLoader(LOADER_ID) == null) {
getLoaderManager().initLoader(LOADER_ID, null, this);
} else {
getLoaderManager().restartLoader(LOADER_ID);
}
}
You normally pass an ID to your loader so you can reference it later via the loader manager.
Hope this helps!
onCreate() gets called during screen orientation change since the activity gets destroyed and recreated.
Unless you're loading a lot of data then it doesn't hurt to do, but you can try the following if you want (I haven't tested it, but in theory I think it would work).
Declare a static boolean at the top global of the class. I think you'll also need a static cursor to reference
private static boolean dataDownloaded = false;
private static Cursor oldCursor;
Then on onLoadFinished set dataDownloaded = true
Override onSaveInstanceState to save the boolean value
#Override
protected void onSaveInstanceState(Bundle outSave) {
outSave.putBoolen("datadownloaded", dataDownloaded);
oldCursor = adapter.swapCursor(null);
}
and onCreate add the following
if (savedInstanceState != null) {
this.dataDownloaded = savedInstanceState.getBoolean("datadownloaded", false);
}
adjust your onCreateLoader
public Loader<Cursor> onCreateLoader(int id, Bundle args) {
CursorLoader cursorLoader;
if (dataDownloaded) {
cursorLoader = new CursorLoader(getActivity(),
null, projection, null, null, null);
cursorLoader.deliverResult(oldCursor);
} else {
CursorLoader cursorLoader = new CursorLoader(getActivity(),
URI_PATH, projection, null, null, null);
}
return cursorLoader;
}
I'd like to use a demo to show this:
enter code here
#Override
public void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.main);
Button button = (Button) findViewById(R.id.button1);
button.setOnClickListener(buttonClickListener);
}
private OnClickListener buttonClickListener = new OnClickListener() {
#Override
public void onClick(View v) {
// TODO Auto-generated method stub
startMyLoader();
}
};
private void startMyLoader() {
getLoaderManager().destroyLoader(0);
getLoaderManager().restartLoader(0, null, myLoaderListener);
}
/**
* The listener for the group metadata loader.
*/
private final LoaderManager.LoaderCallbacks<Cursor> myLoaderListener
= new LoaderCallbacks<Cursor>() {
#Override
public CursorLoader onCreateLoader(int id, Bundle args) {
return new CursorLoader(LoaderDemoActivity.this,
ContactsContract.Contacts.CONTENT_URI,
null, null, null, null);
}
#Override
public void onLoadFinished(Loader<Cursor> loader, Cursor cursor) {
cursor.moveToPosition(-1);
if (cursor.moveToNext()) {
Context context = getApplicationContext();
CharSequence text = "Load finished!";
int duration = Toast.LENGTH_SHORT;
Toast toast = Toast.makeText(context, text, duration);
toast.show();
}
}
#Override
public void onLoaderReset(Loader<Cursor> loader) {
}
};
enter code here
After orientation changed, I clicked the button,
the onCreateLoader can be called,
but onLoadFinished will not be called.
It seems strange.
Thanks for help in advance.
I faced the same problem. Please make a try call this.getSupportLoaderManager() in onCreate.
It solved my problem. Hope it will help you as well
I think I have found the reason.
In Activity onCreate, it will load all the LoaderMangers(of its own or its sub-Fragments)
from NonConfigurationInstances.
if (mLastNonConfigurationInstances != null) {
mAllLoaderManagers = mLastNonConfigurationInstances.loaders;
}
And in Activity onStart, it will try to start its own LoaderManger.
if (!mLoadersStarted) {
mLoadersStarted = true;
if (mLoaderManager != null) {
mLoaderManager.doStart();
} else if (!mCheckedForLoaderManager) {
mLoaderManager = getLoaderManager(-1, mLoadersStarted, false);
}
mCheckedForLoaderManager = true;
}
But after config changed, mLoaderManager == null, so it will not start it.
And here is the problem!
If you try to start loader belong to this loaderManager, it will fail.
void installLoader(LoaderInfo info) {
mLoaders.put(info.mId, info);
if (mStarted) {
// The activity will start all existing loaders in it's onStart(),
// so only start them here if we're past that point of the activitiy's
// life cycle
info.start();
}
}
note the mStarted value which will be set 'true' when LoaderManager started.
And there is two ways to solve this problem.
call getLoaderManger() in onCreate(), it will re-assign the mLoaderManager
and make it ready to be started in the subseuqent onStart().
public LoaderManager getLoaderManager() {
if (mLoaderManager != null) {
return mLoaderManager;
}
mCheckedForLoaderManager = true;
mLoaderManager = getLoaderManager(-1, mLoadersStarted, true);
return mLoaderManager;
}
have the loader located in fragments. Because in Fragments' onStart(),
it will start its own LoaderManager.
if (!mLoadersStarted) {
mLoadersStarted = true;
if (!mCheckedForLoaderManager) {
mCheckedForLoaderManager = true;
mLoaderManager = mActivity.getLoaderManager(mIndex, mLoadersStarted, false);
}
if (mLoaderManager != null) {
mLoaderManager.doStart();
}
}
You don't need to (neither ought to) destroy your Loader in order to reload it. Loader class is intended to be reusable.
Use initLoader instead. eg.:
getLoaderManager().initLoader(0, null, myLoaderListener);
If you want to force reloading allready registered loader:
getLoaderManager().getLoader(0).forceLoad();
If you are not sure if the Loader instance allready exists after configuration change event happened use initLoader instead of getLoader to retrieve your Loader instance on which you can call forceLoad().
getLoaderManager().initLoader(0, null, myLoaderListener).forceLoad();
If you use support library then use forceLoad even after first instantation - there is probably a bug - I remind myself there are some questions about it on this forum - try searching older posts.
Make sure you are not checking savedStateInfo while using fragments before you call your loader in activity onCreate
#Override
public void onCreate(Bundle savedInstanceState) {
// used to not overlap fragments
if (savedInstanceState != null) {
return null;
}
loadFragments();
getSupportLoaderManager().restartLoader(LISTS_LOADER, null, this);
}
If you need to check for savedInstanceState fragments anyway you can check for any class variable that should be created after loader finished loading, as activity gets destroyed when rotating, but raising from previous state when rotating back.
From the android's development site
"They automatically reconnect to the last loader's cursor when being
recreated after a configuration change. Thus, they don't need to
re-query their data."
As far as I understand even when we start the loader explicitly the loader won't start. Since the destroy which we are calling should actually call onLoaderReset() once it is destroyed. But that method is not called once the orientation is changed, but is called before.
Still I may be wrong in this. This is my assumption. Further discussion would be appreciated.
I'm working on a Smartphone / Tablet app, using only one APK, and loading resources as is needed depending on screen size, the best design choice seemed to be using Fragments via the ACL.
This app has been working fine until now being only activity based. This is a mock class of how I handle AsyncTasks and ProgressDialogs in the Activities in order to have them work even when the screen is rotated or a configuration change occurs mid communication.
I will not change the manifest to avoid recreation of the Activity, there are many reasons why I dont want to do it, but mainly because the official docs say it isnt recomended and I've managed without it this far, so please dont recomend that route.
public class Login extends Activity {
static ProgressDialog pd;
AsyncTask<String, Void, Boolean> asyncLoginThread;
#Override
public void onCreate(Bundle icicle) {
super.onCreate(icicle);
setContentView(R.layout.login);
//SETUP UI OBJECTS
restoreAsyncTask();
}
#Override
public Object onRetainNonConfigurationInstance() {
if (pd != null) pd.dismiss();
if (asyncLoginThread != null) return (asyncLoginThread);
return super.onRetainNonConfigurationInstance();
}
private void restoreAsyncTask();() {
pd = new ProgressDialog(Login.this);
if (getLastNonConfigurationInstance() != null) {
asyncLoginThread = (AsyncTask<String, Void, Boolean>) getLastNonConfigurationInstance();
if (asyncLoginThread != null) {
if (!(asyncLoginThread.getStatus()
.equals(AsyncTask.Status.FINISHED))) {
showProgressDialog();
}
}
}
}
public class LoginThread extends AsyncTask<String, Void, Boolean> {
#Override
protected Boolean doInBackground(String... args) {
try {
//Connect to WS, recieve a JSON/XML Response
//Place it somewhere I can use it.
} catch (Exception e) {
return true;
}
return true;
}
protected void onPostExecute(Boolean result) {
if (result) {
pd.dismiss();
//Handle the response. Either deny entry or launch new Login Succesful Activity
}
}
}
}
This code is working fine, I have around 10.000 users without complaint, so it seemed logical to just copy this logic into the new Fragment Based Design, but, of course, it isnt working.
Here is the LoginFragment:
public class LoginFragment extends Fragment {
FragmentActivity parentActivity;
static ProgressDialog pd;
AsyncTask<String, Void, Boolean> asyncLoginThread;
public interface OnLoginSuccessfulListener {
public void onLoginSuccessful(GlobalContainer globalContainer);
}
public void onSaveInstanceState(Bundle outState){
super.onSaveInstanceState(outState);
//Save some stuff for the UI State
}
#Override
public void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
//setRetainInstance(true);
//If I setRetainInstance(true), savedInstanceState is always null. Besides that, when loading UI State, a NPE is thrown when looking for UI Objects.
parentActivity = getActivity();
}
#Override
public void onAttach(Activity activity) {
super.onAttach(activity);
try {
loginSuccessfulListener = (OnLoginSuccessfulListener) activity;
} catch (ClassCastException e) {
throw new ClassCastException(activity.toString() + " must implement OnLoginSuccessfulListener");
}
}
#Override
public View onCreateView(LayoutInflater inflater, ViewGroup container,
Bundle savedInstanceState) {
RelativeLayout loginLayout = (RelativeLayout) inflater.inflate(R.layout.login, container, false);
return loginLayout;
}
#Override
public void onActivityCreated(Bundle savedInstanceState) {
super.onActivityCreated(savedInstanceState);
//SETUP UI OBJECTS
if(savedInstanceState != null){
//Reload UI state. Im doing this properly, keeping the content of the UI objects, not the object it self to avoid memory leaks.
}
}
public class LoginThread extends AsyncTask<String, Void, Boolean> {
#Override
protected Boolean doInBackground(String... args) {
try {
//Connect to WS, recieve a JSON/XML Response
//Place it somewhere I can use it.
} catch (Exception e) {
return true;
}
return true;
}
protected void onPostExecute(Boolean result) {
if (result) {
pd.dismiss();
//Handle the response. Either deny entry or launch new Login Succesful Activity
}
}
}
}
}
I cant use onRetainNonConfigurationInstance() since it has to be called from the Activity and not the Fragment, same goes with getLastNonConfigurationInstance(). I've read some similar questions here with no answer.
I understand that it might require some working around to get this stuff organized properly in fragments, that being said, I would like to maintain the same basic design logic.
What would be the proper way to retain the AsyncTask during a configuration change, and if its still runing, show a progressDialog, taking into consideration that the AsyncTask is a inner class to the Fragment and it is the Fragment itself who invokes the AsyncTask.execute()?
Fragments can actually make this a lot easier. Just use the method Fragment.setRetainInstance(boolean) to have your fragment instance retained across configuration changes. Note that this is the recommended replacement for Activity.onRetainnonConfigurationInstance() in the docs.
If for some reason you really don't want to use a retained fragment, there are other approaches you can take. Note that each fragment has a unique identifier returned by Fragment.getId(). You can also find out if a fragment is being torn down for a config change through Fragment.getActivity().isChangingConfigurations(). So, at the point where you would decide to stop your AsyncTask (in onStop() or onDestroy() most likely), you could for example check if the configuration is changing and if so stick it in a static SparseArray under the fragment's identifier, and then in your onCreate() or onStart() look to see if you have an AsyncTask in the sparse array available.
I think you will enjoy my extremely comprehensive and working example detailed below.
Rotation works, and the dialog survives.
You can cancel the task and dialog by pressing the back button (if you want this behaviour).
It uses fragments.
The layout of the fragment underneath the activity changes properly when the device rotates.
There is a complete source code download and a precompiled APK so you can see if the behaviour is what you want.
Edit
As requested by Brad Larson I have reproduced most of the linked solution below. Also since I posted it I have been pointed to AsyncTaskLoader. I'm not sure it is totally applicable to the same problems, but you should check it out anyway.
Using AsyncTask with progress dialogs and device rotation.
A working solution!
I have finally got everything to work. My code has the following features:
A Fragment whose layout changes with orientation.
An AsyncTask in which you can do some work.
A DialogFragment which shows the progress of the task in a progress bar (not just an indeterminate spinner).
Rotation works without interrupting the task or dismissing the dialog.
The back button dismisses the dialog and cancels the task (you can alter this behaviour fairly easily though).
I don't think that combination of workingness can be found anywhere else.
The basic idea is as follows. There is a MainActivity class which contains a single fragment - MainFragment. MainFragment has different layouts for horizontal and vertical orientation, and setRetainInstance() is false so that the layout can change. This means that when the device orientation is changed, both MainActivity and MainFragment are completely destroyed and recreated.
Separately we have MyTask (extended from AsyncTask) which does all the work. We can't store it in MainFragment because that will be destroyed, and Google has deprecated using anything like setRetainNonInstanceConfiguration(). That isn't always available anyway and is an ugly hack at best. Instead we will store MyTask in another fragment, a DialogFragment called TaskFragment. This fragment will have setRetainInstance() set to true, so as the device rotates this fragment isn't destroyed, and MyTask is retained.
Finally we need to tell the TaskFragment who to inform when it is finished, and we do that using setTargetFragment(<the MainFragment>) when we create it. When the device is rotated and the MainFragment is destroyed and a new instance is created, we use the FragmentManager to find the dialog (based on its tag) and do setTargetFragment(<the new MainFragment>). That's pretty much it.
There were two other things I needed to do: first cancel the task when the dialog is dismissed, and second set the dismiss message to null, otherwise the dialog is weirdly dismissed when the device is rotated.
The code
I won't list the layouts, they are pretty obvious and you can find them in the project download below.
MainActivity
This is pretty straightforward. I added a callback into this activity so it knows when the task is finished, but you might not need that. Mainly I just wanted to show the fragment-activity callback mechanism because it's quite neat and you might not have seen it before.
public class MainActivity extends Activity implements MainFragment.Callbacks
{
#Override
public void onCreate(Bundle savedInstanceState)
{
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
}
#Override
public void onTaskFinished()
{
// Hooray. A toast to our success.
Toast.makeText(this, "Task finished!", Toast.LENGTH_LONG).show();
// NB: I'm going to blow your mind again: the "int duration" parameter of makeText *isn't*
// the duration in milliseconds. ANDROID Y U NO ENUM?
}
}
MainFragment
It's long but worth it!
public class MainFragment extends Fragment implements OnClickListener
{
// This code up to onDetach() is all to get easy callbacks to the Activity.
private Callbacks mCallbacks = sDummyCallbacks;
public interface Callbacks
{
public void onTaskFinished();
}
private static Callbacks sDummyCallbacks = new Callbacks()
{
public void onTaskFinished() { }
};
#Override
public void onAttach(Activity activity)
{
super.onAttach(activity);
if (!(activity instanceof Callbacks))
{
throw new IllegalStateException("Activity must implement fragment's callbacks.");
}
mCallbacks = (Callbacks) activity;
}
#Override
public void onDetach()
{
super.onDetach();
mCallbacks = sDummyCallbacks;
}
// Save a reference to the fragment manager. This is initialised in onCreate().
private FragmentManager mFM;
// Code to identify the fragment that is calling onActivityResult(). We don't really need
// this since we only have one fragment to deal with.
static final int TASK_FRAGMENT = 0;
// Tag so we can find the task fragment again, in another instance of this fragment after rotation.
static final String TASK_FRAGMENT_TAG = "task";
#Override
public void onCreate(Bundle savedInstanceState)
{
super.onCreate(savedInstanceState);
// At this point the fragment may have been recreated due to a rotation,
// and there may be a TaskFragment lying around. So see if we can find it.
mFM = getFragmentManager();
// Check to see if we have retained the worker fragment.
TaskFragment taskFragment = (TaskFragment)mFM.findFragmentByTag(TASK_FRAGMENT_TAG);
if (taskFragment != null)
{
// Update the target fragment so it goes to this fragment instead of the old one.
// This will also allow the GC to reclaim the old MainFragment, which the TaskFragment
// keeps a reference to. Note that I looked in the code and setTargetFragment() doesn't
// use weak references. To be sure you aren't leaking, you may wish to make your own
// setTargetFragment() which does.
taskFragment.setTargetFragment(this, TASK_FRAGMENT);
}
}
#Override
public View onCreateView(LayoutInflater inflater, ViewGroup container,
Bundle savedInstanceState)
{
return inflater.inflate(R.layout.fragment_main, container, false);
}
#Override
public void onViewCreated(View view, Bundle savedInstanceState)
{
super.onViewCreated(view, savedInstanceState);
// Callback for the "start task" button. I originally used the XML onClick()
// but it goes to the Activity instead.
view.findViewById(R.id.taskButton).setOnClickListener(this);
}
#Override
public void onClick(View v)
{
// We only have one click listener so we know it is the "Start Task" button.
// We will create a new TaskFragment.
TaskFragment taskFragment = new TaskFragment();
// And create a task for it to monitor. In this implementation the taskFragment
// executes the task, but you could change it so that it is started here.
taskFragment.setTask(new MyTask());
// And tell it to call onActivityResult() on this fragment.
taskFragment.setTargetFragment(this, TASK_FRAGMENT);
// Show the fragment.
// I'm not sure which of the following two lines is best to use but this one works well.
taskFragment.show(mFM, TASK_FRAGMENT_TAG);
// mFM.beginTransaction().add(taskFragment, TASK_FRAGMENT_TAG).commit();
}
#Override
public void onActivityResult(int requestCode, int resultCode, Intent data)
{
if (requestCode == TASK_FRAGMENT && resultCode == Activity.RESULT_OK)
{
// Inform the activity.
mCallbacks.onTaskFinished();
}
}
TaskFragment
// This and the other inner class can be in separate files if you like.
// There's no reason they need to be inner classes other than keeping everything together.
public static class TaskFragment extends DialogFragment
{
// The task we are running.
MyTask mTask;
ProgressBar mProgressBar;
public void setTask(MyTask task)
{
mTask = task;
// Tell the AsyncTask to call updateProgress() and taskFinished() on this fragment.
mTask.setFragment(this);
}
#Override
public void onCreate(Bundle savedInstanceState)
{
super.onCreate(savedInstanceState);
// Retain this instance so it isn't destroyed when MainActivity and
// MainFragment change configuration.
setRetainInstance(true);
// Start the task! You could move this outside this activity if you want.
if (mTask != null)
mTask.execute();
}
#Override
public View onCreateView(LayoutInflater inflater, ViewGroup container,
Bundle savedInstanceState)
{
View view = inflater.inflate(R.layout.fragment_task, container);
mProgressBar = (ProgressBar)view.findViewById(R.id.progressBar);
getDialog().setTitle("Progress Dialog");
// If you're doing a long task, you probably don't want people to cancel
// it just by tapping the screen!
getDialog().setCanceledOnTouchOutside(false);
return view;
}
// This is to work around what is apparently a bug. If you don't have it
// here the dialog will be dismissed on rotation, so tell it not to dismiss.
#Override
public void onDestroyView()
{
if (getDialog() != null && getRetainInstance())
getDialog().setDismissMessage(null);
super.onDestroyView();
}
// Also when we are dismissed we need to cancel the task.
#Override
public void onDismiss(DialogInterface dialog)
{
super.onDismiss(dialog);
// If true, the thread is interrupted immediately, which may do bad things.
// If false, it guarantees a result is never returned (onPostExecute() isn't called)
// but you have to repeatedly call isCancelled() in your doInBackground()
// function to check if it should exit. For some tasks that might not be feasible.
if (mTask != null) {
mTask.cancel(false);
}
// You don't really need this if you don't want.
if (getTargetFragment() != null)
getTargetFragment().onActivityResult(TASK_FRAGMENT, Activity.RESULT_CANCELED, null);
}
#Override
public void onResume()
{
super.onResume();
// This is a little hacky, but we will see if the task has finished while we weren't
// in this activity, and then we can dismiss ourselves.
if (mTask == null)
dismiss();
}
// This is called by the AsyncTask.
public void updateProgress(int percent)
{
mProgressBar.setProgress(percent);
}
// This is also called by the AsyncTask.
public void taskFinished()
{
// Make sure we check if it is resumed because we will crash if trying to dismiss the dialog
// after the user has switched to another app.
if (isResumed())
dismiss();
// If we aren't resumed, setting the task to null will allow us to dimiss ourselves in
// onResume().
mTask = null;
// Tell the fragment that we are done.
if (getTargetFragment() != null)
getTargetFragment().onActivityResult(TASK_FRAGMENT, Activity.RESULT_OK, null);
}
}
MyTask
// This is a fairly standard AsyncTask that does some dummy work.
public static class MyTask extends AsyncTask<Void, Void, Void>
{
TaskFragment mFragment;
int mProgress = 0;
void setFragment(TaskFragment fragment)
{
mFragment = fragment;
}
#Override
protected Void doInBackground(Void... params)
{
// Do some longish task. This should be a task that we don't really
// care about continuing
// if the user exits the app.
// Examples of these things:
// * Logging in to an app.
// * Downloading something for the user to view.
// * Calculating something for the user to view.
// Examples of where you should probably use a service instead:
// * Downloading files for the user to save (like the browser does).
// * Sending messages to people.
// * Uploading data to a server.
for (int i = 0; i < 10; i++)
{
// Check if this has been cancelled, e.g. when the dialog is dismissed.
if (isCancelled())
return null;
SystemClock.sleep(500);
mProgress = i * 10;
publishProgress();
}
return null;
}
#Override
protected void onProgressUpdate(Void... unused)
{
if (mFragment == null)
return;
mFragment.updateProgress(mProgress);
}
#Override
protected void onPostExecute(Void unused)
{
if (mFragment == null)
return;
mFragment.taskFinished();
}
}
}
Download the example project
Here is the source code and the APK. Sorry, the ADT insisted on adding the support library before it would let me make a project. I'm sure you can remove it.
I've recently posted an article describing how to handle configuration changes using retained Fragments. It solves the problem of retaining an AsyncTask across a rotation change nicely.
The TL;DR is to use host your AsyncTask inside a Fragment, call setRetainInstance(true) on the Fragment, and report the AsyncTask's progress/results back to it's Activity (or it's target Fragment, if you choose to use the approach described by #Timmmm) through the retained Fragment.
My first suggestion is to avoid inner AsyncTasks, you can read a question that I asked about this and the answers: Android: AsyncTask recommendations: private class or public class?
After that i started using non-inner and... now i see A LOT of benefits.
The second is, keep a reference of your running AsyncTask in the Application Class - http://developer.android.com/reference/android/app/Application.html
Everytime you start an AsyncTask, set it on the Application and when it finishes it set it to null.
When a fragment/activity starts you can check if any AsyncTask is running (by checking if it's null or not on the Application) and then set the reference inside to whatever you want (activity, fragment etc so you can do callbacks).
This will solve your problem:
If you only have 1 AsyncTask running at any determined time you can add a simple reference:
AsyncTask<?,?,?> asyncTask = null;
Else, have in the Aplication a HashMap with references to them.
The progress dialog can follow the exact same principle.
I came up with a method of using AsyncTaskLoaders for this. It's pretty easy to use and requires less overhead IMO..
Basically you create an AsyncTaskLoader like this:
public class MyAsyncTaskLoader extends AsyncTaskLoader {
Result mResult;
public HttpAsyncTaskLoader(Context context) {
super(context);
}
protected void onStartLoading() {
super.onStartLoading();
if (mResult != null) {
deliverResult(mResult);
}
if (takeContentChanged() || mResult == null) {
forceLoad();
}
}
#Override
public Result loadInBackground() {
SystemClock.sleep(500);
mResult = new Result();
return mResult;
}
}
Then in your activity that uses the above AsyncTaskLoader when a button is clicked:
public class MyActivityWithBackgroundWork extends FragmentActivity implements LoaderManager.LoaderCallbacks<Result> {
private String username,password;
#Override
protected void onCreate(Bundle savedInstanceState) {
// TODO Auto-generated method stub
super.onCreate(savedInstanceState);
setContentView(R.layout.mylayout);
//this is only used to reconnect to the loader if it already started
//before the orientation changed
Loader loader = getSupportLoaderManager().getLoader(0);
if (loader != null) {
getSupportLoaderManager().initLoader(0, null, this);
}
}
public void doBackgroundWorkOnClick(View button) {
//might want to disable the button while you are doing work
//to prevent user from pressing it again.
//Call resetLoader because calling initLoader will return
//the previous result if there was one and we may want to do new work
//each time
getSupportLoaderManager().resetLoader(0, null, this);
}
#Override
public Loader<Result> onCreateLoader(int i, Bundle bundle) {
//might want to start a progress bar
return new MyAsyncTaskLoader(this);
}
#Override
public void onLoadFinished(Loader<LoginResponse> loginLoader,
LoginResponse loginResponse)
{
//handle result
}
#Override
public void onLoaderReset(Loader<LoginResponse> responseAndJsonHolderLoader)
{
//remove references to previous loader resources
}
}
This seems to handle orientation changes fine and your background task will continue during the rotation.
A few things to note:
If in onCreate you reattach to the asynctaskloader you will get called back in onLoadFinished() with the previous result (even if you had already been told the request was complete). This is actually good behavior most of the time but sometimes it can be tricky to handle. While I imagine there are lots of ways to handle this what I did was I called loader.abandon() in onLoadFinished. Then I added check in onCreate to only reattach to the loader if it wasn't already abandoned. If you need the resulting data again you won't want to do that. In most cases you want the data.
I have more details on using this for http calls here
I created a very tiny open-source background task library which is heavily based on the Marshmallow AsyncTask but with additional functionality such as:
Automatically retaining tasks across configuration changes;
UI callback (listeners);
Doesn't restart or cancel task when the device rotates (like Loaders would do);
The library internally uses a Fragment without any user interface, which is retained accross configuration changes (setRetainInstance(true)).
You can find it on GitHub: https://github.com/NeoTech-Software/Android-Retainable-Tasks
Most basic example (version 0.2.0):
This example fully retains the task, using a very limited amount of code.
Task:
private class ExampleTask extends Task<Integer, String> {
public ExampleTask(String tag){
super(tag);
}
protected String doInBackground() {
for(int i = 0; i < 100; i++) {
if(isCancelled()){
break;
}
SystemClock.sleep(50);
publishProgress(i);
}
return "Result";
}
}
Activity:
public class Main extends TaskActivityCompat implements Task.Callback {
#Override
public void onClick(View view){
ExampleTask task = new ExampleTask("activity-unique-tag");
getTaskManager().execute(task, this);
}
#Override
public Task.Callback onPreAttach(Task<?, ?> task) {
//Restore the user-interface based on the tasks state
return this; //This Activity implements Task.Callback
}
#Override
public void onPreExecute(Task<?, ?> task) {
//Task started
}
#Override
public void onPostExecute(Task<?, ?> task) {
//Task finished
Toast.makeText(this, "Task finished", Toast.LENGTH_SHORT).show();
}
}
My approach is to use delegation design pattern, in general, we can isolate the actual business logic (read data from internet or database or whatsoever) from AsyncTask (the delegator) to BusinessDAO (the delegate), in your AysncTask.doInBackground() method, delegate the actual task to BusinessDAO, then implement a singleton process mechanism in BusinessDAO, so that multiple call to BusinessDAO.doSomething() will just trigger one actual task running each time and waiting for the task result. The idea is retain the delegate (i.e. BusinessDAO) during the configuration change, instead of the delegator (i.e. AsyncTask).
Create/Implement our own Application, the purpose is to create/initialize BusinessDAO here, so that our BusinessDAO's lifecycle is application scoped, not activity scoped, note that you need change AndroidManifest.xml to use MyApplication:
public class MyApplication extends android.app.Application {
private BusinessDAO businessDAO;
#Override
public void onCreate() {
super.onCreate();
businessDAO = new BusinessDAO();
}
pubilc BusinessDAO getBusinessDAO() {
return businessDAO;
}
}
Our existing Activity/Fragement are mostly unchanged, still implement AsyncTask as an inner class and involve AsyncTask.execute() from Activity/Fragement, the difference now is AsyncTask will delegate the actual task to BusinessDAO, so during the configuration change, a second AsyncTask will be initialized and executed, and call BusinessDAO.doSomething() second time, however, second call to BusinessDAO.doSomething() will not trigger a new running task, instead, waiting for current running task to finish:
public class LoginFragment extends Fragment {
... ...
public class LoginAsyncTask extends AsyncTask<String, Void, Boolean> {
// get a reference of BusinessDAO from application scope.
BusinessDAO businessDAO = ((MyApplication) getApplication()).getBusinessDAO();
#Override
protected Boolean doInBackground(String... args) {
businessDAO.doSomething();
return true;
}
protected void onPostExecute(Boolean result) {
//Handle task result and update UI stuff.
}
}
... ...
}
Inside BusinessDAO, implement singleton process mechanism, for example:
public class BusinessDAO {
ExecutorCompletionService<MyTask> completionExecutor = new ExecutorCompletionService<MyTask(Executors.newFixedThreadPool(1));
Future<MyTask> myFutureTask = null;
public void doSomething() {
if (myFutureTask == null) {
// nothing running at the moment, submit a new callable task to run.
MyTask myTask = new MyTask();
myFutureTask = completionExecutor.submit(myTask);
}
// Task already submitted and running, waiting for the running task to finish.
myFutureTask.get();
}
// If you've never used this before, Callable is similar with Runnable, with ability to return result and throw exception.
private class MyTask extends Callable<MyTask> {
public MyAsyncTask call() {
// do your job here.
return this;
}
}
}
I am not 100% sure if this will work, moreover, the sample code snippet should be considered as pseudocode. I am just trying to give you some clue from design level. Any feedback or suggestions are welcome and appreciated.
You could make the AsyncTask a static field. If you need a context, you should ship your application context. This will avoid memory leaks, otherwise you'd keep a reference to your entire activity.
If anyone finds their way to this thread then I found a clean approach was to run the Async task from an app.Service (started with START_STICKY) and then on recreate iterate over the running services to find out whether the service (and hence async task) is still running;
public boolean isServiceRunning(String serviceClassName) {
final ActivityManager activityManager = (ActivityManager) Application.getContext().getSystemService(Context.ACTIVITY_SERVICE);
final List<RunningServiceInfo> services = activityManager.getRunningServices(Integer.MAX_VALUE);
for (RunningServiceInfo runningServiceInfo : services) {
if (runningServiceInfo.service.getClassName().equals(serviceClassName)){
return true;
}
}
return false;
}
If it is, re-add the DialogFragment (or whatever) and if it is not ensure the dialog has been dismissed.
This is particularly pertinent if you are using the v4.support.* libraries since (at the time of writing) they have know issues with the setRetainInstance method and view paging. Furthermore, by not retaining the instance you can recreate your activity using a different set of resources (i.e. a different view layout for the new orientation)
I write samepl code to solve this problem
First step is make Application class:
public class TheApp extends Application {
private static TheApp sTheApp;
private HashMap<String, AsyncTask<?,?,?>> tasks = new HashMap<String, AsyncTask<?,?,?>>();
#Override
public void onCreate() {
super.onCreate();
sTheApp = this;
}
public static TheApp get() {
return sTheApp;
}
public void registerTask(String tag, AsyncTask<?,?,?> task) {
tasks.put(tag, task);
}
public void unregisterTask(String tag) {
tasks.remove(tag);
}
public AsyncTask<?,?,?> getTask(String tag) {
return tasks.get(tag);
}
}
In AndroidManifest.xml
<application
android:allowBackup="true"
android:icon="#drawable/ic_launcher"
android:label="#string/app_name"
android:theme="#style/AppTheme"
android:name="com.example.tasktest.TheApp">
Code in activity:
public class MainActivity extends Activity {
private Task1 mTask1;
#Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
mTask1 = (Task1)TheApp.get().getTask("task1");
}
/*
* start task is not running jet
*/
public void handletask1(View v) {
if (mTask1 == null) {
mTask1 = new Task1();
TheApp.get().registerTask("task1", mTask1);
mTask1.execute();
} else
Toast.makeText(this, "Task is running...", Toast.LENGTH_SHORT).show();
}
/*
* cancel task if is not finished
*/
public void handelCancel(View v) {
if (mTask1 != null)
mTask1.cancel(false);
}
public class Task1 extends AsyncTask<Void, Void, Void>{
#Override
protected Void doInBackground(Void... params) {
try {
for(int i=0; i<120; i++) {
Thread.sleep(1000);
Log.i("tests", "loop=" + i);
if (this.isCancelled()) {
Log.e("tests", "tssk cancelled");
break;
}
}
} catch (InterruptedException e) {
e.printStackTrace();
}
return null;
}
#Override
protected void onCancelled(Void result) {
TheApp.get().unregisterTask("task1");
mTask1 = null;
}
#Override
protected void onPostExecute(Void result) {
TheApp.get().unregisterTask("task1");
mTask1 = null;
}
}
}
When activity orientation changes variable mTask is inited from app context. When task is finished variable is set to null and remove from memory.
For me its enough.
Have a look at below example , how to use retained fragment to retain background task:
public class NetworkRequestFragment extends Fragment {
// Declare some sort of interface that your AsyncTask will use to communicate with the Activity
public interface NetworkRequestListener {
void onRequestStarted();
void onRequestProgressUpdate(int progress);
void onRequestFinished(SomeObject result);
}
private NetworkTask mTask;
private NetworkRequestListener mListener;
private SomeObject mResult;
#Override
public void onAttach(Activity activity) {
super.onAttach(activity);
// Try to use the Activity as a listener
if (activity instanceof NetworkRequestListener) {
mListener = (NetworkRequestListener) activity;
} else {
// You can decide if you want to mandate that the Activity implements your callback interface
// in which case you should throw an exception if it doesn't:
throw new IllegalStateException("Parent activity must implement NetworkRequestListener");
// or you could just swallow it and allow a state where nobody is listening
}
}
#Override
public void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
// Retain this Fragment so that it will not be destroyed when an orientation
// change happens and we can keep our AsyncTask running
setRetainInstance(true);
}
/**
* The Activity can call this when it wants to start the task
*/
public void startTask(String url) {
mTask = new NetworkTask(url);
mTask.execute();
}
#Override
public void onActivityCreated(Bundle savedInstanceState) {
super.onActivityCreated(savedInstanceState);
// If the AsyncTask finished when we didn't have a listener we can
// deliver the result here
if ((mResult != null) && (mListener != null)) {
mListener.onRequestFinished(mResult);
mResult = null;
}
}
#Override
public void onDestroy() {
super.onDestroy();
// We still have to cancel the task in onDestroy because if the user exits the app or
// finishes the Activity, we don't want the task to keep running
// Since we are retaining the Fragment, onDestroy won't be called for an orientation change
// so this won't affect our ability to keep the task running when the user rotates the device
if ((mTask != null) && (mTask.getStatus == AsyncTask.Status.RUNNING)) {
mTask.cancel(true);
}
}
#Override
public void onDetach() {
super.onDetach();
// This is VERY important to avoid a memory leak (because mListener is really a reference to an Activity)
// When the orientation change occurs, onDetach will be called and since the Activity is being destroyed
// we don't want to keep any references to it
// When the Activity is being re-created, onAttach will be called and we will get our listener back
mListener = null;
}
private class NetworkTask extends AsyncTask<String, Integer, SomeObject> {
#Override
protected void onPreExecute() {
if (mListener != null) {
mListener.onRequestStarted();
}
}
#Override
protected SomeObject doInBackground(String... urls) {
// Make the network request
...
// Whenever we want to update our progress:
publishProgress(progress);
...
return result;
}
#Override
protected void onProgressUpdate(Integer... progress) {
if (mListener != null) {
mListener.onRequestProgressUpdate(progress[0]);
}
}
#Override
protected void onPostExecute(SomeObject result) {
if (mListener != null) {
mListener.onRequestFinished(result);
} else {
// If the task finishes while the orientation change is happening and while
// the Fragment is not attached to an Activity, our mListener might be null
// If you need to make sure that the result eventually gets to the Activity
// you could save the result here, then in onActivityCreated you can pass it back
// to the Activity
mResult = result;
}
}
}
}
Have a look here.
There is a solution based on Timmmm's solution.
But I improved it:
Now the solution is extendable - you only need to extend FragmentAbleToStartTask
You able to keep running several tasks at the same time.
And in my opinion it's as easy as startActivityForResult and receive result
You also can stop a running task and check whether particular task is running
Sorry for my English