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);
});
}
}

SavedIntanceState.getBoolean() is making my app crash (i think)

My Goal:
So I need help putting a boolean primitives into a bundle and retrieving it from the bundle when there is a screen orientation change in Android. I am using that boolean value in a conditional statement that helps decide if 2 Button views (mTrueButton, mFalseButton) should be enabled or not. What i have so far is causing the app to shut down (aka crash) when there is a screen rotation. I think I am not retrieving or writing my boolean from my bundle or into my bundle correctly, and it is causing the app to crash.
How The App Should Works:
When a user touches the mTrueButton or mFalseButton button to answer a question, both buttons become disabled so the user is not allowed to answer again. I want those buttons to keep being disabled when the user answers and then rotates the screen.**
I know that when a user rotates their Android device, onDestroy() is called because runtime configuration changes take place, causing the app to be relaunched without having knowledge of it's previous state, (unless store my necessary data onto a bundle and pass it onto my onCreate method).
These are SOME global variables in my activity class
private int index = 0;
priavate Button mTrueButton,mFalseButton;
private static final String KEY_INDEX = "index";
private static final String BTTN_ENABLED = "bttnEnabled";
private boolean trueFalseButtonsEnabled = true;
These are SOME statements in my onCreate() method of the same activity class
#Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
Log.d(TAG, "onCreate(Bundle) called");
setContentView(R.layout.activity_quiz);
if(savedInstanceState != null) {
index = savedInstanceState.getInt(KEY_INDEX, 0);
changeButtonEnableStatus(savedInstanceState.getBoolean(BTTN_ENABLED,true));
}
mTrueButton = (Button)findViewById(R.id.true_button);
mFalseButton = (Button)findViewById(R.id.false_button);
mTrueButton.setOnClickListener(new View.OnClickListener(){
#Override
public void onClick(View v){
checkAnswer(true);
changeButtonEnableStatus(false);
}
});
mFalseButton.setOnClickListener(new View.OnClickListener() {
#Override
public void onClick(View v) {
checkAnswer(false);
changeButtonEnableStatus(false);
}
});
}
These are SOME methods in the same activity class but not in my onCreate()
private void changeButtonEnableStatus(boolean bool){
trueFalseButtonsEnabled = bool;
mTrueButton.setEnabled(bool);
mFalseButton.setEnabled(bool);
}
#Override
public void onSaveInstanceState(Bundle savedInstanceState){
super.onSaveInstanceState(savedInstanceState);
Log.d(TAG,"onSavedInstanceState() called");
savedInstanceState.putInt(KEY_INDEX,index);
savedInstanceState.putBoolean(BTTN_ENABLED, trueFalseButtonsEnabled);
}
Note that:
index = savedInstanceState.getInt(KEY_INDEX, 0);
works properly. It is setting global variable "index" to equal to the int primitive what was stored in into keywork "KEY_INDEX".
However I don't think: changeButtonEnableStatus(savedInstanceState.getBoolean(BTTN_ENABLED,true)); is working properly. My app seems to crash when I include that statement and run the app, and then rotate the device.

How to disable a Button in ListView rows connected to ArrayAdapter?

My class ProductAdapter extends ArrayAdapter
on getView i'm inflating rows with 2 buttons in the each row for (+) and (-)
and set anonim OnClickListener for each button , like this :
viewHolder.removeButton.setOnClickListener(new View.OnClickListener() {
#Override
public void onClick(View v) {
BigDecimal count = product.getCount().subtract(BigDecimal.ONE);
if (count.signum() < 0) count = BigDecimal.ZERO;
product.setCount(count);
viewHolder.countView.setText(formatValue(count, product.getUnit()));
mListener.onCardClick(v);
}
});
On activity i need to do some AsyncTask when i'm using integer value from each row.
The problem is when AsyncTask executing user still can change product adapter(Buttons are working).
I need to disable them while AsyncTask is working and then reenable after completing.
I was trying to disable ListView with no luck.
Also i was trying to override ArrayAdapter methods isAllEnadled and isEnabled also with no luck.

Interesting problem, you need a State and a place to save this, this State can be used to control the click behavior. You can save this either in the product itself or some other place like a list corresponding to that index
Something like
protected void onPreExecute(Void result) {
product.setFetching(true)
}
protected void onPostExecute(Void result) {
product.setFetching(false)
}
in onClick() you can check same and return like this
if (product.isFetching())
return;

You need to set a callback method for enable/disable buttons. You can use an interface for that.
pseudocode:
public interface ButtonsHandler {
void enableButtons();
void disableButtons();
}
Then you have to implement that interface in your viewHolder
public class CustomViewHolder extends RecyclerView.ViewHolder implements ButtonsHandler {
...
void enableButtons() {
yourButton1.setEnable(true);
yourButton2.setEnable(true);
}
void disableButtons() () {
yourButton1.setEnable(false);
yourButton2.setEnable(false);
}
...
}
Third, when you call the listener from the Holder to start the task, pass the object itself to manage buttons handling.
mListener.onCardClick(View v, ButtonsHandler buttonsHandler);
so the call will be:
mListener.onCardClick(v,viewHolder);
And finally, while your asynktask is working, you can call
buttonsHandler.disableButtons();
...
buttonsHandler.enableButtons();

Fragments and ViewPager in Android

I am having a ViewPager to allow the user to slide between 5 different views where each "view" extends Fragment.
I have my own adapter which extend FragmentPagerAdapter and which implement getItem() as
#Override public Fragment getItem(int position) {
switch(position) {
case 0:
return new TextDescriptionFragment();
// Handle the 4 other cases the same way
}
}
This works fine and the user can swipe between the 5 different views. But here comes the problem: Each of the first 4 views contains Views such as Button and EditText which the user can interact with.
And I then want the last page(Page number 5) to show all the user input values from all the views from the 4 previous pages(fragments). How do I do that?
I can't find any way to read the user input values from the previous fragments. The views may not even exist anymore(But will be recreated if the user goes back).
And I can't seem to get the existing fragments.

I would consider having a custom object that keeps the data each fragment fills. Something like:
public class FillerData implements Parcelable {
private String page0$data0;
private String page0$data1;
private String page0$data2;
// getters and setters if you wish
// implement Parcelable interface as this object will be managed by host activity
}
You'll have only one such object managed by parent activity and the parent activity will implement an interface for exposing this object:
public static interface FillerDataExposer {
public FillerData exposeFiller();
}
public class MyFragmentHostActivity extends FragmentActivity implements FillerDataExposer {
private static final String FILLER_KEY = "FILLER_KEY";
private FillerData myFillerData;
protected void onCreate(Bundle savedInstance) {
.......
if(savedInstance != null) {
myFillerData = (FillerData) savedInstance.getParcelable(FILLER_KEY);
} else {
myFillerData = new FillerData();
}
}
protected void onSaveInstanceState(Bundle savedInstance) {
super.onSaveInstanceState();
savedInstance.putExtra(FILLER_KEY, myFillerData);
}
public FillerData exposeFiller() {
return this.myFillerData;
}
}
Now, each of your fragments will have access to that centralized data filler object through parent activity. To reduce the weight of your code, all your fragments could extend from a base fragment class that provides access to FillerDataExposer implementation (actually, the parent activity):
public abstract class AbstractFillerFragment extends Fragment {
protected FillerDataExposer dataExposer;
public void onAttach(Activity act) {
super.onAttach(act);
// make sure no ClassCastExceptions
this.dataExposer = (FillerDataExposer) act;
}
}
Fragments that should only record the data filled could look like this:
public class Page1Fragment extends AbstractFillerFragment {
public View onCreateView(LayoutInflater inflater, ViewGroup container, Bundle savedInstanceState) {
View view = // inflate XML resource ...
yourEditText = (EditText) view.findViewById(...);
// other relevant code ....
}
public void onViewCreated(View view, Bundle savedInstanceState) {
yourEditText.setText(dataExposer.exposeFiller.setPageX$DataY());
// some code for EditText.addTextChangedListener(new TextWatcher() could look like:
yourEditText.addTextChangedListener(new TextWatcher() {
public void afterTextChanged(Editable s) {
dataExposer.exposeFiller().setPage1$Data0(s.toString());
}
public void beforeTextChanged(CharSequence s, int start, int count, int after) {}
public void onTextChanged(CharSequence s, int start, int before, int count) {}
});
}
}
While the fragment that needs to have access to all data stored could look like this:
public class FinalFragment extends AbstractFillerFragment {
public void collectAllData() {
DataFiller allDataCollectedObject = dataExposer.exposeFiller();
// by calling get...() you should have access to collected data.
}
}
This only a sketch, but you'll get the picture. The idea is to keep a single object in your activity managed across activity restarts and to make it accessible through interfaces so you will respect the fragment to activity patterns.
Hope it makes sense ...

2 solutions come to my mind.
The first one is to save user input data when your first 4 fragment's onpause() methods are called. You may save the data to a preference and then retrieve it from your 5th fragment.
The second approach is to persist your fragments while swiping.This way the swiping will be faster and cleaner and they want be recreated everytime a swipe happens:
yourcustomviewpager.setOffscreenPageLimit(5);
About setOffscreenPageLimit from the android doc:
Set the number of pages that should be retained to either side of the current page in the view hierarchy in an idle state. Pages beyond this limit will be recreated from the adapter when needed.
This is offered as an optimization. If you know in advance the number of pages you will need to support or have lazy-loading mechanisms in place on your pages, tweaking this setting can have benefits in perceived smoothness of paging animations and interaction. If you have a small number of pages (3-4) that you can keep active all at once, less time will be spent in layout for newly created view subtrees as the user pages back and forth.
You should keep this limit low, especially if your pages have complex layouts. This setting defaults to 1.

Spinner onItemSelected() executes inappropriately [duplicate]

This question already has answers here:
Closed 11 years ago.
Possible Duplicate:
Android Spinner OnItemSelected Called Erroneously (without user action on opening spinner)
Does anyone know how to prevent the onItemSelected() (OnItemSelectedListener interface) method from running when the layout is instantiated? I need to know if there is a way to do this because I want to keep how I instantiate my layout separate from this listener.
I have tried creating an if statement initially set to false around all the code inside of the overridden method, but there is no way of knowing when to set it to true because the overridden method runs after the onCreate(), onStart(), and onResume() methods everytime.
I have not found any clear cut answers on this. Any clear cut solutions would be greatly appreciated.

David, here is a tutorial I wrote up for this problem...
Problem Statement
an undesirable onItemSelected() is triggered whilst the Gallery (or Spinner) is initializing.
This means that code is prematurely executed; code which is intended to execute ONLY when a user physically makes a selection.
Solution
in onCreate(), count how many Gallery (or Spinner) widgets you have in the view. (mGalleryCount)
in onItemSelected(), count how often it has triggered. (mGalleryInitializedCount)
when (mGalleryInitializedCount < mGalleryCount) == false, then execute the code meant for the user
Code Example
public class myActivity extends Activity implements OnItemSelectedListener
{
//this counts how many Gallery's are on the UI
private int mGalleryCount=0;
//this counts how many Gallery's have been initialized
private int mGalleryInitializedCount=0;
//UI reference
private Gallery mGallery;
#Override
public void onCreate(Bundle savedInstanceState)
{
super.onCreate(savedInstanceState);
setContentView(R.layout.myxmllayout);
//get references to UI components
mGallery = (Gallery) findViewById(R.id.mygallery);
//trap selection events from gallery
mGallery.setOnItemSelectedListener(this);
//trap only selection when no flinging is taking place
mGallery.setCallbackDuringFling(false);
//
//do other stuff like load images, setAdapter(), etc
//
//define how many Gallery's are in this view
//note: this could be counted dynamically if you are programmatically creating the view
mGalleryCount=1;
}
public void onItemSelected(AdapterView<?> parent, View view, int position, long id)
{
if (mGalleryInitializedCount < mGalleryCount)
{
mGalleryInitializedCount++;
}
else
{
//only detect selection events that are not done whilst initializing
Log.i(TAG, "selected item position = " + String.valueOf(position) );
}
}
}
Why this works
this solution works because the Gallery finishes initialization long before a user is physically able to make a selection.

Here is a modified version of "Someone Somewhere" code. You can use it if you have a single view.
public class myActivity extends Activity implements OnItemSelectedListener
{
// Set view initialization to false while the it is being built
private boolean initializedView = false;
//UI reference
private Gallery mGallery;
#Override
public void onCreate(Bundle savedInstanceState)
{
super.onCreate(savedInstanceState);
setContentView(R.layout.myxmllayout);
//get references to UI components
mGallery = (Gallery) findViewById(R.id.mygallery);
//trap selection events from gallery
mGallery.setOnItemSelectedListener(this);
//trap only selection when no flinging is taking place
mGallery.setCallbackDuringFling(false);
//
//do other stuff like load images, setAdapter(), etc
//
}
public void onItemSelected(AdapterView<?> parent, View view, int position, long id)
{
if (initializedView == false)
{
initializedView = true;
}
else
{
//only detect selection events that are not done whilst initializing
Log.i(TAG, "selected item position = " + String.valueOf(position) );
}
}
}

Same solution:
private int m_intSpinnerInitiCount = 0;
private static final int NO_OF_EVENTS = 1;
...
m_spnTemplates.setOnItemSelectedListener(new Spinner.OnItemSelectedListener() {
#Override
public void onItemSelected(AdapterView<?> parentView,
View selectedItemView, int position, long id) {
//trying to avoid undesired spinner selection changed event, a known problem
if (m_intSpinnerInitiCount < NO_OF_EVENTS) {
m_intSpinnerInitiCount++;
} else {
//YOUR CODE HERE
}
}

I ran into this problem yesterday with an OnCheckedChangedListener. I ended up adding a boolean instance variable initialized to true inside of my adapter class with an accessor method isListenerEnabled(). I then set the variable to false in my layout code and set it to true again at the end of the layout code. In my listener, I inspect the value of the variable to decide whether to execute the listener code or not.

You can set your variable back to false everytime on onPause is called.
As for when to set it to true, you could do this for the first motion/key/trackball event after onResume.

I also looked for a good solution on the internet but didn't find any that satisfied my needs.
So I've written this extension on the Spinner class so you can set a simple OnItemClickListener, which has the same behaviour as a ListView.
Only when an item gets 'selected', the onItemClickListener is called.
Have fun with it!
public class MySpinner extends Spinner
{
private OnItemClickListener onItemClickListener;
public MySpinner(Context context)
{
super(context);
}
public MySpinner(Context context, AttributeSet attrs)
{
super(context, attrs);
}
public MySpinner(Context context, AttributeSet attrs, int defStyle)
{
super(context, attrs, defStyle);
}
#Override
public void setOnItemClickListener(android.widget.AdapterView.OnItemClickListener inOnItemClickListener)
{
this.onItemClickListener = inOnItemClickListener;
}
#Override
public void onClick(DialogInterface dialog, int which)
{
super.onClick(dialog, which);
if (this.onItemClickListener != null)
{
this.onItemClickListener.onItemClick(this, this.getSelectedView(), which, this.getSelectedItemId());
}
}
}