Develop Reference

How can you change ViewPager2 position inside the ViewPager2Adapter?

I programmed a Vocabulary Trainer with Vocabulary Cards. The Vocabulary Cards are Entries in a Room Database created from an asset. I am displaying these Vocabulary Cards with ViewPager2 in an Activity. I have a 'correct' and a 'false' button and when the user clicks on either, I want to update the Vocabulary Card (-> The entry in the sqlite database) and automatically swipe to the next item of the ViewPager2.
If I implement the buttons in the ViewPager2Adapter, I can't find a way to change the position of the ViewPager2. If I implement the buttons in the activity the sqlite entry does not update properly (After it updates the entry, the activity is constantly refreshed, it seems like it never the leaves the OnClick methode of the button).
So is it possible to change the position of ViewPager2 from inside the ViewPager2Adpater?
Thanks for your help!
That is the relevant code if I have the buttons in my ViewPager2Adapter. Here I don't know how to change the position of the ViewPager2
public void onBindViewHolder(#NonNull #NotNull ViewHolder holder, int position) {
VocabularyCard vocabularyCard = currentCards.get(position);
holder.btn_correct.setOnClickListener(view -> {
vocabularyViewModel.updateSingleVocabularyCard(vocabularyCard);
});
holder.btn_false.setOnClickListener(v15 -> {
vocabularyViewModel.updateSingleVocabularyCard(vocabularyCard);
});
That is the relevant code if I have the buttons in the Activity. Here the update function triggers an infinite updating of the Activity:
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_test);
initAll();
btn_correct_2.setOnClickListener(view -> {
int currentPos = viewpager2.getCurrentItem();
vocabularyViewModel.getCurrentCards().observe(this, vocabularyCards -> {
if (vocabularyCards.size() == currentPos){
Intent intent = new Intent(TestActivity.this, MainActivity.class);
startActivity(intent);
}else {
viewpager2.setCurrentItem(currentPos + 1);
}
VocabularyCard vocabularyCard = vocabularyCards.get(currentPos);
vocabularyViewModel.updateSingleVocabularyCard(vocabularyCard);
});
});
btn_false_2.setOnClickListener(view -> {
int currentPos = viewpager2.getCurrentItem();
vocabularyViewModel.getCurrentCards().observe(this, vocabularyCards -> {
if (vocabularyCards.size() == currentPos){
Intent intent = new Intent(TestActivity.this, MainActivity.class);
startActivity(intent);
}else {
viewpager2.setCurrentItem(currentPos + 1);
}
VocabularyCard vocabularyCard = vocabularyCards.get(currentPos);
vocabularyViewModel.updateSingleVocabularyCard(vocabularyCard);
});
});
Objects.requireNonNull(getSupportActionBar()).setTitle(getResources().getString(R.string.learn_new_words));
LiveData<List<VocabularyCard>> allNewCards = vocabularyViewModel.getAllNewCards(goal);
allNewCards.observe(this, vocabularyCards -> vocabularyViewModel.setCurrentCards(vocabularyCards));
vocabularyViewModel.getCurrentCards().observe(this, vocabularyCards -> {
viewPager2Adapter.setCurrentCards(vocabularyCards);
viewpager2.setAdapter(viewPager2Adapter);
viewpager2.registerOnPageChangeCallback(new ViewPager2.OnPageChangeCallback() {
#Override
public void onPageScrolled(int position, float positionOffset, int positionOffsetPixels) {
super.onPageScrolled(position, positionOffset, positionOffsetPixels);
}
#Override
public void onPageSelected(int position) {
super.onPageSelected(position);
}
#Override
public void onPageScrollStateChanged(int state) {
super.onPageScrollStateChanged(state);
}
});
});
The update function in the Room DAO is straightforward:
#Update
void updateSingleVocabularyCard(VocabularyCard vocabularyCard);
I left out all the code that is not relevant.

There are several ways to propagate an event from the adapter to the activity where you manage your cards using ViewPager2. Let's have a look how it can be done either using an interface or using the same view model. But in any case I strongly recommend you to update your database in a background thread to prevent any possible UI lags.
1. Using an interface
This option is more flexible since you can propagate events as well as pass data as parameters. You can also reuse this interface for other cases. As far as I See you have a holder that has 2 buttons for the users to make choices. So our event here would be something like ChoiceEventListener, let's call this interface like so. Then you'd have to add a method to handle this event from within anywhere you wanna hear this event, and let's call its handle method onChoice(). Finally we would need a variable to indicate what the choice is. Now that ready to implement, let's write the new interface...
ChoiceEventListener.java
public interface ChoiceEventListener {
void onChoice(VocabularyCard vocabularyCard, boolean choice);
}
The next thing to do is to implement this interface where you want to listen to this event. In this case it is in your activity. There are 2 ways to do this:
You make your activity to inherit its methods using the implements keyword
YourActivity.java
public class YourActivity extends AppCompatActivity implements ChoiceEventListener {
// Use a background thread for database operations
private Executor executor = Executors.newSingleThreadExecutor();
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_test);
initAll();
// You must construct your adapter class with the listener
ViewPager2Adapter adapter = new ViewPager2Adapter(/* Other params... */, this);
}
#Override
public void onChoice(VocabularyCard vocabularyCard, boolean choice) {
if(choice) {
// User pressed the correct button
}
else {
// User pressed the false button
}
// Update card in the background
executor.execute(()-> vocabularyViewModel.updateSingleVocabularyCard(vocabularyCard));
}
}
You can implement it as an anonymous function
YourActivity.java
public class YourActivity extends AppCompatActivity {
// Use a background thread for database operations
private Executor executor = Executors.newSingleThreadExecutor();
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_test);
initAll();
// You must construct your adapter class with the listener
ViewPager2Adapter adapter = new ViewPager2Adapter(/* Other params... */, (vocabularyCard, choice) -> {
if(choice) {
// User pressed the correct button
}
else {
// User pressed the false button
}
// Update card in the background
executor.execute(()-> vocabularyViewModel.updateSingleVocabularyCard(vocabularyCard));
});
}
}
Finally the ViewPager2Adapter class implementation would be something like this:
ViewPager2Adapter.java
public class ViewPager2Adapter extends RecyclerView.Adapter<ViewPager2ViewHolder> {
// Here is your listener to deliver the choice event to it
private final ChoiceEventListener listener;
// Constructor
public ViewPager2Adapter(/* Other params... */, ChoiceEventListener listener) {
/* Other inits */
this.listener = listener;
}
public void onBindViewHolder(#NonNull #NotNull ViewHolder holder, int position) {
VocabularyCard vocabularyCard = currentCards.get(position);
holder.btn_correct.setOnClickListener(view -> {
listener.onChoice(vocabularyCard, true); // true for correct
});
holder.btn_false.setOnClickListener(v15 -> {
listener.onChoice(vocabularyCard, false); // false for false :)
});
}
}
2. Use the ViewModel for inter-communication
In this option we use a LiveData object to make page switching. The only thing you need to know in your activity is the current position which you get it from the adapter class. Once you update it in the adapter, set the current position value in live data so that you can switch the page in your activity.
VocabularyViewModel.java
public class VocabularyViewModel extends ViewModel {
public MutableLiveData<Integer> mldCurrentPosition = new MutableLiveData<>(0);
}
YourActivity.java
public class YourActivity extends AppCompatActivity {
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_test);
initAll();
vocabularyViewModel.mldCurrentPosition().observe(this, currentPosition -> {
if(currenPosition == null) return; // ignore when null
viewpager2.setCurrentItem(currentPosition + 1);
}
}
}
Finally the ViewPager2Adapter class implementation would be something like this:
ViewPager2Adapter.java
public class ViewPager2Adapter extends RecyclerView.Adapter<ViewPager2ViewHolder> {
// Use a background thread for database operations
private Executor executor = Executors.newSingleThreadExecutor();
public void onBindViewHolder(#NonNull #NotNull ViewHolder holder, int position) {
VocabularyCard vocabularyCard = currentCards.get(position);
holder.btn_correct.setOnClickListener(view -> {
// Update card in the background
executor.execute(()-> vocabularyViewModel.updateSingleVocabularyCard(vocabularyCard));
// Then invoke switching to the next card
vocabularyViewModel.mldCurrentPosition.setValue(position + 1);
});
holder.btn_false.setOnClickListener(v15 -> {
// Update card in the background
executor.execute(()-> vocabularyViewModel.updateSingleVocabularyCard(vocabularyCard));
// Then invoke switching to the next card
vocabularyViewModel.mldCurrentPosition.setValue(position + 1);
});
}
}

How to detect each RecyclerView item after it is displayed

I want to detect each item in my RecylerView after it is displayed to the user.
Basically, I am trying to play a sound on each item after it is loaded on the screen.
But I am not able to detect whenever each item is loaded on the screen! Is there any method I have to call to detect each item rendered
E.g 1st RecyclerView item displayed -> play sound
2st RecyclerView item displayed -> play sound...
My Adapter class looks like this -
public class AdapterListAnimation extends RecyclerView.Adapter<RecyclerView.ViewHolder> {
private List<Multiples> items = new ArrayList<>();
private Context ctx;
private OnItemClickListener mOnItemClickListener;
private int animation_type = 0;
.........
.........
I am calling this initComponent() method from onCreated() method. Can you give advice on what should I do to achieve my goal as described above?
private void initComponent() {
recyclerView = (RecyclerView) findViewById(R.id.recyclerView);
recyclerView.setLayoutManager(new LinearLayoutManager(this));
recyclerView.setHasFixedSize(true);
items = DataGenerator.getPeopleData(this,of,value);
setAdapter();
/* MediaPlayer mp=MediaPlayer.create(this, R.raw.sword);
if (mp.isPlaying()) {
mp.stop();
mp.release();
mp = MediaPlayer.create(this, R.raw.sword);
} mp.start();*/
}
private void setAdapter() {
// Set data and list adapter
mAdapter = new AdapterListAnimation(this, items, animation_type);
recyclerView.setAdapter(mAdapter);
// on item list clicked
mAdapter.setOnItemClickListener(new AdapterListAnimation.OnItemClickListener() {
#Override
public void onItemClick(View view, com.math.multiplication.model.Multiples obj, int position) {
Snackbar.make(parent_view, "Item " + obj.first + " clicked", Snackbar.LENGTH_SHORT).show();
}
});
}

you need to override onViewAttachedToWindow and onViewDetachedFromWindow. but for detecting holer type you need getItemViewType() just like that:
public class PostAdapter extends RecyclerView.Adapter {
#Override
public int getItemViewType(int position) {
switch (types.get(position)){
case 1:
return 1;
case 2:
return 2;
default:
return position;
}
}
#Override
public void onViewAttachedToWindow(#NonNull RecyclerView.ViewHolder holder) {
super.onViewAttachedToWindow(holder);
if (holder.getItemViewType() == 1){
//play song
}
}
#Override
public void onViewDetachedFromWindow(#NonNull RecyclerView.ViewHolder holder) {
super.onViewDetachedFromWindow(holder);
if (holder.getItemViewType() == 1){
//pause song
}
}

You need to add listener for TTS. Then update your RecyclerView to show right image when speech starts and ends.
I've created a test project to show how it can be implemented. Here you can see how it works. Here is my github repository.
Here is main part of my MainActivity class.
private void initTts() {
tts = new TextToSpeech(this, this);
tts.setLanguage(Locale.US);
tts.setOnUtteranceProgressListener(new MyListener());
}
#Override
public void onInit(int status) {
playSound(0);
}
private void playSound(int index) {
HashMap<String, String> hashMap = new HashMap<>();
hashMap.put(TextToSpeech.Engine.KEY_PARAM_UTTERANCE_ID, String.valueOf(index));
tts.speak(data.get(index), TextToSpeech.QUEUE_ADD, hashMap);
}
class MyListener extends UtteranceProgressListener {
#Override
public void onStart(String utteranceId) {
int currentIndex = Integer.parseInt(utteranceId);
mMainAdapter.setCurrentPosition(currentIndex);
handler.post(new Runnable() {
#Override
public void run() {
mMainAdapter.notifyDataSetChanged();
}
});
}
#Override
public void onDone(String utteranceId) {
int currentIndex = Integer.parseInt(utteranceId);
mMainAdapter.setCurrentPosition(-1);
handler.post(new Runnable() {
#Override
public void run() {
mMainAdapter.notifyDataSetChanged();
}
});
if (currentIndex < data.size() - 1) {
playSound(currentIndex + 1);
}
}
#Override
public void onError(String utteranceId) {
}
}

You can simply use onViewAttachedToWindow(VH) in your adapter.
See https://developer.android.com/reference/android/support/v7/widget/RecyclerView.Adapter.html#onViewAttachedToWindow(VH)
Update:
As you know RecyclerView will be call OnBindViewHolder only once for each item.
RecyclerView will not call this method again if the position of the item changes in the data set unless the item itself is invalidated or the new position cannot be determined.
So you can use onViewAttachedToWindow
(onViewAttachedToWindow) Called when a view created by this adapter has been attached to a window.
This can be used as a reasonable signal that the view is about to be seen by the user. If the adapter previously freed any resources in onViewDetachedFromWindow those resources should be restored here.
You can use it like this:
public class Adapter extends RecyclerView.Adapter<MyViewHolder> {
// rest of your code
#Override
public void onViewAttachedToWindow(MyViewHolder holder) {
super.onViewAttachedToWindow(holder);
// play your sound
}
}
Hope it helps!

If I understood your problem correctly, you want to play a sound when an item in the RecyclerView is loaded.
Hence I would like to think of a workaround here. You might consider having an item added after the sound for the previous item has been played.
I would like to provide a pseudo code for what I am trying to explain. You might consider using the MediaPlayer.OnCompletionListener for listening if your sound has stopped playing and then add the next item to the RecyclerView and then call notifyDataSetChanged on your adapter to load the next item in your RecyclerView.
Hence you might want the following modification in your adapter first.
// Declare a function in your adapter which adds new item in the RecyclerView
public void addItem(Multiples item) {
items.add(item);
notifyItemInserted(items.size());
}
Now in your Activity where you set up the RecyclerView, you need to have the following arrays.
ArrayList<Multiples> allItems = new ArrayList<Multiples>(); // This list will contain all the data
int soundToBePlayed = -1; // Initialize the value with -1
ArrayList<Integer> soundList = getAllSoundsInAList();
Modify your functions as follows.
private void initComponent() {
recyclerView = (RecyclerView) findViewById(R.id.recyclerView);
recyclerView.setLayoutManager(new LinearLayoutManager(this));
recyclerView.setHasFixedSize(true);
allItems = DataGenerator.getPeopleData(this,of,value);
setAdapter();
}
private void setAdapter() {
mAdapter = new AdapterListAnimation(this, animation_type); // Do not pass the items list
recyclerView.setAdapter(mAdapter);
// .. Your item click listener goes here
}
As the items was removed from the constructor, you need to modify the constructor of your adapter as well. Just remove the variable which takes the items. Because we will be adding elements to the items later.
Then you need to write another function which needs to be called in your onCreate function of your activity after your call your initComponent function. The function should look like the following.
private void addItemInRecyclerViewAndPlaySound() {
soundToBePlayed++;
adapter.addItem(allItems.get(soundToBePlayed));
recyclerView.scrollToPosition(adapter.getItemCount() - 1);
Integer soundId = soundList.get(soundToBePlayed);
MediaPlayer mp = MediaPlayer.create(this, soundId);
if (mp.isPlaying()) {
mp.stop();
mp.release();
mp = MediaPlayer.create(this, soundId);
mp.setOnCompletionListener(new MediaPlayer.OnCompletionListener() {
public void onCompletion(MediaPlayer mp) {
// Call this function again to add next item
// and play the next sound at the same time
if(soundToBePlayed < allItems.size() - 1)
addItemInRecyclerViewAndPlaySound();
}
});
}
mp.start();
}
I have not tested the code, however, I think you get the idea already. This can be implemented gracefully using a interface inside the adapter as well. The imeplemntation technique is your choice.

Android: How to refresh online data when Internet is available without creating a new Fragment?

When my MainFragment first loads, it checks for internet connectivity. If there is internet, it loads the content from online. However, if there is no internet connectivity, I replace the existing fragment MainFragment within the container main_browse_fragment with InternetConnectivityFragment.
Within InternetConnectivtyFragment, I have a Retry button that re-checks the internet, and if there is internet connectivity, I remove InternetConnectivityFragment and call popbackstack.
The problem is, when popbackstack is called, I dont know how to reload the online data from within MainFragment. The online data loading and internet check is done within onActivityCreated of MainFragment.java, but when popbackstack is called within InternetConnectivityFragment.java, the view of MainFragment is blank
Obviously this is because the fragment is not "created" and onActivityCreated is not called again, so no data is reloaded.
Here's the relevant code:
MainFragment.java:
public class MainFragment extends DetailsFragment
{
#Override
public void onActivityCreated(Bundle savedInstanceState)
{
super.onActivityCreated(savedInstanceState);
setupUIElements();
if (isConnectingToInternet() == true)
{
// Call methods to load online data
}
else
{
InternetConnectivityFragment internetError = new InternetConnectivityFragment();
getFragmentManager().beginTransaction().add(R.id.main_browse_fragment, internetError).commit();
}
}
...
}
InternetConnectivityFragment.java:
public class InternetConnectivityFragment extends ErrorFragment
{
private static final String TAG = "InternetFragment";
private static final boolean TRANSLUCENT = true;
#Override
public void onCreate(Bundle savedInstanceState)
{
super.onCreate(savedInstanceState);
setErrorContent();
}
private void setErrorContent()
{
setImageDrawable(getResources().getDrawable(R.drawable.lb_ic_sad_cloud, null));
setMessage(getResources().getString(R.string.no_internet_message));
setDefaultBackground(TRANSLUCENT);
setButtonText(getResources().getString(R.string.retry_connection));
setButtonClickListener(new View.OnClickListener()
{
#Override
public void onClick(View arg0)
{
if (Singleton.getInstance().isConnectedToInternet() == true)
{
getFragmentManager().beginTransaction().remove(InternetConnectivityFragment.this).commit();
getFragmentManager().popBackStack();
// How to call methods to re-load online data from MainFragment????
}
}
});
}
}
I have tried to perform the following within the onClick method:
MainFragment mainFragment = new MainFragment();
getFragmentManager().beginTransaction().remove(InternetConnectivityFragment.this).commit();
getFragmentManager().popBackStack();
getFragmentManager().beginTransaction().add(R.id.main_browse_fragment, mainFragment).commit();
However, I believe this is wrong as it just adds a new fragment MainFragment on top of an already existing fragment.
Can someone point me in the right direction?
Thanks

How do I save application state between Activities with embedded fragments in the backstack?

I've found a ton of conflicting information regarding the proper way to restore application state when using Fragments embedded in Activities. Please let me know if my architecture is the problem because that is totally possible. My test Weather app is architected as follows.
The main activity "ReportsActivity" contains the fragment "ReportsFragment" (This is a list of the next 10 days of Weather Reports)
ReportsFragment has an onItemClickListener that launches a new Activity "WeatherDetailActivity" and passes it an intent which contains some JSON Data that I use to populate the Weather Detail UI. This data is then presented on a fragment that the WeatherDetailActivity manages.
My problem is, when the user presses the back button, the ReportsFragment has been destroyed so it runs through its full lifecycle. I've tried a number of techniques I've found online to load the activity's data from a bundle, but no matter what I've tried so far the Intents' Extras are null in the ReportsActivity's onCreate method. (Note: the reason I need to do this is to avoid firing off an API Call each time I open my main Activity which fetches weather data from Weather Underground).
I'm struggling determining what would be the best way to construct this app: Should I have a single activity that pushes and pops Fragments that it manages? Or are multiple activities that each manage their own fragments the standard practice?
At the moment here is how I'm attempting to save my application state onto the intent. I'm trying to save the state in onPostExecute from my AsyncTask so i'm on the main thread after i've fetched my results from the API Call:
#Override
protected void onPostExecute(Report[] result){
if (result != null){
ArrayList<String>reportsArrayList = new ArrayList<String>();
Gson jsonArray = new GsonBuilder().setPrettyPrinting().create();
for (int x = 0; x < result.length; x++){
reportsArrayList.add(jsonArray.toJson(result[x], Report.class));
}
mExtras.putStringArrayList(ReportsActivity.ReportsActivityState.KEY_ACTIVITY_REPORTS,reportsArrayList);
}
}
I then attempt to restore state from the ReportsActivity's onCreate Method:
#Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_reports);
if (savedInstanceState == null) {
Intent intent = getIntent();
mFragment = ReportsFragment.newInstance(intent
.getStringArrayListExtra(ReportsActivityState.KEY_ACTIVITY_REPORTS));
getFragmentManager().beginTransaction()
.add(R.id.container, mFragment).commit();
}
}
In all cases the StringArrayListExtra I'm trying to get from the intent return null.
This could very well be me trying to solve an Android problem with an iOS mindset, but is there not an easy way to just restore the main activity to what it was before I pushed the detail view?

I think it would be worth your while taking a look at EventBus.
Basically you can define a object holder of any kind, for example:
class WeatherData {
List<String> reports;
public WeatherData(List<String> reports) {
this.reports = reports;
}
}
Now, in an Activity or Fragment in which you wish to remember the state, or pass some state to another Activity or Fragment do:
// this removes all the hazzle of creating bundles etc
EventBus.getDefault().postSticky(new WeatherData(reports));
And any where in your code your wish to know the most recent WeatherData:
WeatherData weatherData = EventBus.getDefault().getSticky(WeatherData.class);
EventBus also has nice methods for event handling (button clicks, completion of long running processes, etc..)
The library can be found here: https://github.com/greenrobot/EventBus
And some more examples here: http://awalkingcity.com/blog/2013/02/26/productive-android-eventbus/
Some suggestions without using 3. part library:
1) Calling setRetainInstance(true) in your fragments onCreate method, what this should do is to persist public variables between instances.
Though it seems it does not work for fragments on the back stack: Understanding Fragment's setRetainInstance(boolean)
2) Hand the fragment data to your Activity, something like reading/updating ((YourActivity)getActivity()).someFragmentBundle, possibly save it in onSaveInstanceState of the Activity and retrieve it in onCreate. That is, having your Activity hold the data in-between instances.
3) You could also persist the data, saving it to a file or using SharedPreferences http://developer.android.com/training/basics/data-storage/index.html
This method has the advantage that it will enable restoring the data even after a complete kill of your app.
The Architectural question
Disclaimer: subjective opinion
I would generally say keep the Activity as 'slim' as possible, holding a range of related fragments.
Thus, having multiple Activities is fine but they should each manage a set of (or a single) related fragments that are relevant for the current Activity.

It just occurred to me to check out one of the Android Studio templates that Google Provides that I often overlooked. From Google's own templates, it appears clear that the preferred method for Master Detail Activities/Fragments is to have each Fragment Managed by their own activities (as I was attempting to achieve above).
(I should note that I was able to successfully achieve to flow that I wanted using a single Activitiy with multiple fragments and customizing the Animation and forcefully showing and hiding the up button.)
PersonListActivity.java
public class PersonListActivity extends Activity
implements PersonListFragment.Callbacks {
/**
* Whether or not the activity is in two-pane mode, i.e. running on a tablet
* device.
*/
private boolean mTwoPane;
#Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_person_list);
if (findViewById(R.id.person_detail_container) != null) {
// The detail container view will be present only in the
// large-screen layouts (res/values-large and
// res/values-sw600dp). If this view is present, then the
// activity should be in two-pane mode.
mTwoPane = true;
// In two-pane mode, list items should be given the
// 'activated' state when touched.
((PersonListFragment) getFragmentManager()
.findFragmentById(R.id.person_list))
.setActivateOnItemClick(true);
}
// TODO: If exposing deep links into your app, handle intents here.
}
/**
* Callback method from {#link PersonListFragment.Callbacks}
* indicating that the item with the given ID was selected.
*/
#Override
public void onItemSelected(String id) {
if (mTwoPane) {
// In two-pane mode, show the detail view in this activity by
// adding or replacing the detail fragment using a
// fragment transaction.
Bundle arguments = new Bundle();
arguments.putString(PersonDetailFragment.ARG_ITEM_ID, id);
PersonDetailFragment fragment = new PersonDetailFragment();
fragment.setArguments(arguments);
getFragmentManager().beginTransaction()
.replace(R.id.person_detail_container, fragment)
.commit();
} else {
// In single-pane mode, simply start the detail activity
// for the selected item ID.
Intent detailIntent = new Intent(this, PersonDetailActivity.class);
detailIntent.putExtra(PersonDetailFragment.ARG_ITEM_ID, id);
startActivity(detailIntent);
}
}
}
PersonListFragment.java
public class PersonListFragment extends ListFragment {
/**
* The serialization (saved instance state) Bundle key representing the
* activated item position. Only used on tablets.
*/
private static final String STATE_ACTIVATED_POSITION = "activated_position";
/**
* The fragment's current callback object, which is notified of list item
* clicks.
*/
private Callbacks mCallbacks = sDummyCallbacks;
/**
* The current activated item position. Only used on tablets.
*/
private int mActivatedPosition = ListView.INVALID_POSITION;
/**
* A callback interface that all activities containing this fragment must
* implement. This mechanism allows activities to be notified of item
* selections.
*/
public interface Callbacks {
/**
* Callback for when an item has been selected.
*/
public void onItemSelected(String id);
}
/**
* A dummy implementation of the {#link Callbacks} interface that does
* nothing. Used only when this fragment is not attached to an activity.
*/
private static Callbacks sDummyCallbacks = new Callbacks() {
#Override
public void onItemSelected(String id) {
}
};
/**
* Mandatory empty constructor for the fragment manager to instantiate the
* fragment (e.g. upon screen orientation changes).
*/
public PersonListFragment() {
}
#Override
public void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
// TODO: replace with a real list adapter.
setListAdapter(new ArrayAdapter<DummyContent.DummyItem>(
getActivity(),
android.R.layout.simple_list_item_activated_1,
android.R.id.text1,
DummyContent.ITEMS));
}
#Override
public void onViewCreated(View view, Bundle savedInstanceState) {
super.onViewCreated(view, savedInstanceState);
// Restore the previously serialized activated item position.
if (savedInstanceState != null
&& savedInstanceState.containsKey(STATE_ACTIVATED_POSITION)) {
setActivatedPosition(savedInstanceState.getInt(STATE_ACTIVATED_POSITION));
}
}
#Override
public void onAttach(Activity activity) {
super.onAttach(activity);
// Activities containing this fragment must implement its callbacks.
if (!(activity instanceof Callbacks)) {
throw new IllegalStateException("Activity must implement fragment's callbacks.");
}
mCallbacks = (Callbacks) activity;
}
#Override
public void onDetach() {
super.onDetach();
// Reset the active callbacks interface to the dummy implementation.
mCallbacks = sDummyCallbacks;
}
#Override
public void onListItemClick(ListView listView, View view, int position, long id) {
super.onListItemClick(listView, view, position, id);
// Notify the active callbacks interface (the activity, if the
// fragment is attached to one) that an item has been selected.
mCallbacks.onItemSelected(DummyContent.ITEMS.get(position).id);
}
#Override
public void onSaveInstanceState(Bundle outState) {
super.onSaveInstanceState(outState);
if (mActivatedPosition != ListView.INVALID_POSITION) {
// Serialize and persist the activated item position.
outState.putInt(STATE_ACTIVATED_POSITION, mActivatedPosition);
}
}
/**
* Turns on activate-on-click mode. When this mode is on, list items will be
* given the 'activated' state when touched.
*/
public void setActivateOnItemClick(boolean activateOnItemClick) {
// When setting CHOICE_MODE_SINGLE, ListView will automatically
// give items the 'activated' state when touched.
getListView().setChoiceMode(activateOnItemClick
? ListView.CHOICE_MODE_SINGLE
: ListView.CHOICE_MODE_NONE);
}
private void setActivatedPosition(int position) {
if (position == ListView.INVALID_POSITION) {
getListView().setItemChecked(mActivatedPosition, false);
} else {
getListView().setItemChecked(position, true);
}
mActivatedPosition = position;
}
}
PersonDetailActivity.java
public class PersonDetailActivity extends Activity {
#Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_person_detail);
// Show the Up button in the action bar.
getActionBar().setDisplayHomeAsUpEnabled(true);
// savedInstanceState is non-null when there is fragment state
// saved from previous configurations of this activity
// (e.g. when rotating the screen from portrait to landscape).
// In this case, the fragment will automatically be re-added
// to its container so we don't need to manually add it.
// For more information, see the Fragments API guide at:
//
// http://developer.android.com/guide/components/fragments.html
//
if (savedInstanceState == null) {
// Create the detail fragment and add it to the activity
// using a fragment transaction.
Bundle arguments = new Bundle();
arguments.putString(PersonDetailFragment.ARG_ITEM_ID,
getIntent().getStringExtra(PersonDetailFragment.ARG_ITEM_ID));
PersonDetailFragment fragment = new PersonDetailFragment();
fragment.setArguments(arguments);
getFragmentManager().beginTransaction()
.add(R.id.person_detail_container, fragment)
.commit();
}
}
#Override
public boolean onOptionsItemSelected(MenuItem item) {
int id = item.getItemId();
if (id == android.R.id.home) {
// This ID represents the Home or Up button. In the case of this
// activity, the Up button is shown. For
// more details, see the Navigation pattern on Android Design:
//
// http://developer.android.com/design/patterns/navigation.html#up-vs-back
//
navigateUpTo(new Intent(this, PersonListActivity.class));
return true;
}
return super.onOptionsItemSelected(item);
}
}
PersonDetailFragment.java
public class PersonDetailFragment extends Fragment {
/**
* The fragment argument representing the item ID that this fragment
* represents.
*/
public static final String ARG_ITEM_ID = "item_id";
/**
* The dummy content this fragment is presenting.
*/
private DummyContent.DummyItem mItem;
/**
* Mandatory empty constructor for the fragment manager to instantiate the
* fragment (e.g. upon screen orientation changes).
*/
public PersonDetailFragment() {
}
#Override
public void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
if (getArguments().containsKey(ARG_ITEM_ID)) {
// Load the dummy content specified by the fragment
// arguments. In a real-world scenario, use a Loader
// to load content from a content provider.
mItem = DummyContent.ITEM_MAP.get(getArguments().getString(ARG_ITEM_ID));
}
}
#Override
public View onCreateView(LayoutInflater inflater, ViewGroup container,
Bundle savedInstanceState) {
View rootView = inflater.inflate(R.layout.fragment_person_detail, container, false);
// Show the dummy content as text in a TextView.
if (mItem != null) {
((TextView) rootView.findViewById(R.id.person_detail)).setText(mItem.content);
}
return rootView;
}
}

Sensormanager on fragment (add if visible; remove if invisible)

I want to have a sensormanager on a fragment, which is only active when the fragment is active. If the user changes the fragment, the listener should be removed.
Adding and removing the listener is pretty simple. I'm not aware of any listeners / function on the fragment side, when the fragment appears / disappears. Also a problem was, that on almost all functions, this.getActivity() returned a null pointer.

That's my solution. I tried to cut it out of my Fragment. If there is anything wrong / syntax issues, please let me know.
public class MyFragment extends Fragment implements SensorEventListener {
private SensorManager mSensorManager;
#Override
public void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
mSensorManager = (SensorManager) this.getActivity().getSystemService(Activity.SENSOR_SERVICE);
}
#Override
public View onCreateView(LayoutInflater inflater, ViewGroup container, Bundle savedInstanceState) {
View rootView = inflater.inflate(R.layout.mylayout, container, false);
return rootView;
}
#Override
public void onSensorChanged(SensorEvent event) {
float x = event.values[0], y = event.values[1];
}
#Override public void onAccuracyChanged(Sensor sensor, int accuracy) { }
#Override
public void setMenuVisibility(boolean menuVisible) {
super.setMenuVisibility(menuVisible);
// First starts (gets called before everything else)
if(mSensorManager == null) {
return;
}
if(menuVisible) {
this.registerSensorListener();
} else {
this.unregisterSensorListener();
}
}
#Override
public void onStart() {
super.onStart();
if(this.getUserVisibleHint()) {
this.registerSensorListener();
}
}
#Override
public void onStop() {
super.onStop();
this.unregisterSensorListener();
}
private void registerSensorListener() {
mSensorManager.registerListener(this, mSensorManager.getSensorList(Sensor.TYPE_ACCELEROMETER).get(0), SensorManager.SENSOR_DELAY_FASTEST);
}
private void unregisterSensorListener() {
mSensorManager.unregisterListener(this);
}
}

Hold a reference of Activity in your fragment to handle that nullpointerexception.
Here is an example of a fragment.
public class YourFragment extends Fragment {
private Activity mActivity;
#Override
public void onAttach(Activity activity) {
super.onAttach(activity);
mActivity = activity;
}
#Override
public void onResume() {
super.onResume();
// BIND sensor here with mActivity,
// could also be done in other fragment lifecycle events,
// depends on how you handle configChanges
}
#Override
public void onPause() {
super.onPause();
// UNBIND sensor here from mActivity,
// could also be done in other fragment lifecycle events,
// depends on how you handle configChanges
}
}
Debug that code do determine if you should handle the binding there or in another method e.g. onCreate of a fragment. I have not tested this code for your purpose.
Edit:
This is indeed as commented below a dirty fix and could easily resolve into exceptions in some cases. I just wanted to show how you can use fragment lifecycle methods to bind and unbind sensors with a reference to activity. I'm currently learning fragments for quite some time but still don't understand them thoroughly. I advice you to take a look at the source of Fragment and other components involved. This is the only place were fragments are documented thoroughly hence the documentation on reference in my opinion isn't that explanatory.
Some of the options regarding null value Activity:
If you want to be completely sure that getActivity doesn't return null you should wait for onActivityCreated to be called. This method tells the fragment that its activity has
completed its own Activity.onCreate(). After this getActivity() will not return null until initState() gets called by the FragmentManager.
// Called by the fragment manager once this fragment has been removed,
// so that we don't have any left-over state if the application decides
// to re-use the instance. This only clears state that the framework
// internally manages, not things the application sets.
void initState() {
mIndex = -1;
mWho = null;
mAdded = false;
mRemoving = false;
mResumed = false;
mFromLayout = false;
mInLayout = false;
mRestored = false;
mBackStackNesting = 0;
mFragmentManager = null;
mActivity = null;
mFragmentId = 0;
mContainerId = 0;
mTag = null;
mHidden = false;
mDetached = false;
mRetaining = false;
mLoaderManager = null;
mLoadersStarted = false;
mCheckedForLoaderManager = false;
}
Before you call getActivity you can always check if activity isn't null by calling isAdded() method. As you can see below this method checks if mActivity isn't null. Optionally you can create a recursive function with Handler.postDelayed that tries to return an non null Activity in intervalls (you should add a max try counter). But this is also a dirty trick.
//Return true if the fragment is currently added to its activity.
final public boolean isAdded() {
return mActivity != null && mAdded;
}