viewPager Adapter not being updated

viewPager Adapter not being updated - android

I am trying to update adapter(MyCollegePagerAdapter) from CollegeHTMLParse.class. However, even though I created an object and called the notifyDataSetChanged() method on it, noting is showing up on the fragments. Can you tell me if there is anything wrong with my code as I have googled for solution for weeks now and after trying for hours upon hours, I am not able to figure out the issue.
Here is the CollegeHTMLParse class:
public class CollegeHTMLParse extends AppCompatActivity {
public static ArrayList<String> colleges = new ArrayList<>();
public static ArrayList<String> accepted = new ArrayList<>();
public static ArrayList<String> link = new ArrayList<>();
public static ArrayList<String> location = new ArrayList<>();
public static ArrayList<String> tuition = new ArrayList<>();
FirebaseDatabase mRef;
DatabaseReference database;
public static ArrayList<String> data = new ArrayList<>();
MyCollegePagerAdapter adapter = new MyCollegePagerAdapter(getSupportFragmentManager());
public void onCreate(String CollegeCode) {
colleges.clear();
accepted.clear();
link.clear();
location.clear();
tuition.clear();
mRef = FirebaseDatabase.getInstance("https://successway18.firebaseio.com/");
database = mRef.getReference().child("CollegeCodes").child("30");
database.addChildEventListener(new ChildEventListener() {
#Override
public void onChildAdded(DataSnapshot dataSnapshot, String s) {
setCollegeValues(dataSnapshot.getKey());
}
#Override
public void onChildChanged(DataSnapshot dataSnapshot, String s) {
}
#Override
public void onChildRemoved(DataSnapshot dataSnapshot) {
}
#Override
public void onChildMoved(DataSnapshot dataSnapshot, String s) {
}
#Override
public void onCancelled(DatabaseError databaseError) {
}
});
}
public void setCollegeValues(String values) {
colleges.add(values);
int i = 0;
while (i < colleges.size()) {
database.child(colleges.get(i)).addChildEventListener(new ChildEventListener() {
#Override
public void onChildAdded(DataSnapshot dataSnapshot, String s) {
setDetailsValues(dataSnapshot.getValue().toString());
}
#Override
public void onChildChanged(DataSnapshot dataSnapshot, String s) {
}
#Override
public void onChildRemoved(DataSnapshot dataSnapshot) {
}
#Override
public void onChildMoved(DataSnapshot dataSnapshot, String s) {
}
#Override
public void onCancelled(DatabaseError databaseError) {
}
});
i++;
}
}
private void setDetailsValues(String details) {
data.add(details);
if (data.size() == 4) {
accepted.add(data.get(0));
link.add(data.get(1));
location.add(data.get(2));
tuition.add(data.get(3));
data.clear();
}
update();
}
private void update() {
adapter.notifyDataSetChanged();
}
}
Thank you very much in advance!

Add this in your pager adapter
#Override
public int getItemPosition(Object object) {
return POSITION_NONE;
}

Firebase query runs in the worker thread, so you have to run notifyDataSetChanged in the main thread.
Try to use the below code in the function update()
runOnUiThread(new Runnable() {
#Override
public void run() {
}
});

Related

Informing observers of LiveData Firebase Realtime Database elements are deleted

As I know the ChildEventListener is the only way to detect that data in Firebase Realtime Database was deleted. That's why I extend LiveData to handle the events of ChildEventListener.
public class UserRatingsLiveData extends LiveData<DataSnapshot> {
private static final String LOG_TAG = "UserRatingsLiveData";
private final Query query;
private final UserRatingsChildEventListener listener = new UserRatingsChildEventListener();
public UserRatingsLiveData(DatabaseReference ref) {
this.query = ref;
}
#Override
protected void onActive() {
Log.d(LOG_TAG, "onActive");
query.addChildEventListener(listener);
}
#Override
protected void onInactive() {
Log.d(LOG_TAG, "onInactive");
query.removeEventListener(listener);
}
private class UserRatingsChildEventListener implements ChildEventListener {
#Override
public void onChildAdded(#NonNull DataSnapshot dataSnapshot, #Nullable String s) {
setValue(dataSnapshot);
}
#Override
public void onChildChanged(#NonNull DataSnapshot dataSnapshot, #Nullable String s) {
setValue(dataSnapshot);
}
#Override
public void onChildRemoved(#NonNull DataSnapshot dataSnapshot) {
// What to do here to signale that the element was removed?
}
#Override
public void onChildMoved(#NonNull DataSnapshot dataSnapshot, #Nullable String s) {
}
#Override
public void onCancelled(#NonNull DatabaseError databaseError) {
Log.d(LOG_TAG, "entering onCancelled(...)");
}
}
}
Delegating newly created elements in onChildAdded(...) with setValue(dataSnapshot) is straight forward. But what's with signaling that an element was removed?
Should I use a special LiveData object only for the remove event?
Any other idea?

public void onCancelled(FirebaseError firebaseError) gives error

I'm using Firebase and this line of code results in an error.
#Override
public void onCancelled(FirebaseError firebaseError) {
}
The errors are as below,
Method does not override method from its superclass
Method 'onCancelled(com.firebase.client.FirebaseError)' is never used
How to resolve these errors?
My code -
public class FirebaseClient {
Context c;
String DB_URL;
ListView listView;
Firebase firebase;
ArrayList<Dog> dogies= new ArrayList<>();
CustomAdapter customAdapter;
public FirebaseClient(Context c, String DB_URL, ListView listView)
{
this.c= c;
this.DB_URL= DB_URL;
this.listView= listView;
Firebase.setAndroidContext(c);
firebase=new Firebase(DB_URL);
}
public void refreshdata()
{
firebase.addChildEventListener(new ChildEventListener() {
#Override
public void onChildAdded(DataSnapshot dataSnapshot, String s) {
getupdates(dataSnapshot);
}
#Override
public void onChildChanged(DataSnapshot dataSnapshot, String s) {
getupdates(dataSnapshot);
}
#Override
public void onChildRemoved(DataSnapshot dataSnapshot) {
}
#Override
public void onChildMoved(DataSnapshot dataSnapshot, String s) {
}
#Override
public void onCancelled(DatabaseError databaseError) {
}
#Override
public void onCancelled(FirebaseError firebaseError) {
}
});
}
public void getupdates(DataSnapshot dataSnapshot){
dogies.clear();
for(DataSnapshot ds :dataSnapshot.getChildren()){
Dog d= new Dog();
d.setTimestamp(ds.getValue(Dog.class).getTimestamp());
d.setInstituteNotice(ds.getValue(Dog.class).getInstituteNotice());
dogies.add(d);
}
if(dogies.size()>0)
{
customAdapter=new CustomAdapter(c, dogies);
listView.setAdapter((ListAdapter) customAdapter);
} else
{
Toast.makeText(c, "No data", Toast.LENGTH_SHORT).show();
}
}
}

You are using the legacy Firebase SDK (which has been deprecated for more than a year) and mixing in classes from the new SDK. You should consider migrating to the new SDK. The Upgrade Guide outlines the steps.
In the code you posted, DatabaseError is from the new SDK, not the legacy SDK, so this override is not valid and should be removed:
#Override
public void onCancelled(DatabaseError databaseError) {
}

In order to solve this, you need to remove the following method from your code:
#Override
public void onCancelled(FirebaseError firebaseError) {}
You are already overriding public void onCancelled(DatabaseError databaseError) from its superclass. The code that i adviced you to be removed was used in ealier version of SDK. You need to use the new one.

Firebase RecyclerView Adapter - Join Query

I am trying to get UserID from one table(homeaccount) and get the user's info from another table(userinfo) using the user id. I tried to do it by the following way. But, it is overwriting the cardviews one on top of another.
databaseReference.child("homeaccount/"+globalSharedPrefs.getUserDetail("currenthome")+"/Users")
.addChildEventListener(new ChildEventListener() {
#Override
public void onChildAdded(DataSnapshot dataSnapshot, String s) {
String str_iteration_uid = dataSnapshot.getKey();
userQuery = databaseReference.child("userinfo").orderByChild("uid").equalTo(str_iteration_uid);
firebaseRecyclerAdapter = new FirebaseRecyclerAdapter<HomeUsers, HomeUsersViewHolder>(
HomeUsers.class,
R.layout.cardview_manage_user,
HomeUsersViewHolder.class,
userQuery
){
#Override
protected void populateViewHolder(final HomeUsersViewHolder viewHolder, final HomeUsers model, int position) {
//viewHolder.setFirstName(model.getFirstName());
//viewHolder.setProfilePicture(model.getProfilePicture());
}
};
recyclerView.setAdapter(firebaseRecyclerAdapter);
}
#Override
public void onChildChanged(DataSnapshot dataSnapshot, String s) {
}
#Override
public void onChildRemoved(DataSnapshot dataSnapshot) {
}
#Override
public void onChildMoved(DataSnapshot dataSnapshot, String s) {
}
#Override
public void onCancelled(DatabaseError databaseError) {
}
});

Why I'm getting duplicated data from firebase database?

public class Post {
private String title;
private String massage;
public Post() {
}
public String getTitle() {
return title;
}
public void setTitle(String title) {
this.title = title;
}
public String getMassage() {
return massage;
}
public void setMassage(String massage) {
this.massage = massage;
}
}
public class Posts extends AppCompatActivity {
private DatabaseReference databaseReference, barRoomsCoordinates;
private FirebaseDatabase firebaseDatabase;
private List<Post> postList;
private List<String> list;
#Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_posts);
postList = new ArrayList<>();
list = new ArrayList<>();
showAllMyPosts();
}
private void getPosts(List<String> posts) {
for (int i = 0; i < posts.size(); i++) {
databaseReference = firebaseDatabase.getReference().child("Posts").child(posts.get(i));
databaseReference.addValueEventListener(new ValueEventListener() {
#Override
public void onDataChange(DataSnapshot dataSnapshot) {
Post posts = dataSnapshot.getValue(Post.class);
Post post = new Post();
post.setTitle(posts.getTitle());
post.setMassage(posts.getMassage());
postList.add(post);
}
#Override
public void onCancelled(DatabaseError databaseError) {
}
});
}
}
private void showAllMyPosts() {
FirebaseApp.initializeApp(Posts.this);
Handler handler = new Handler();
Runnable runnable = new Runnable() {
#Override
public void run() {
barRoomsCoordinates = firebaseDatabase.getReference().child("Posts");
barRoomsCoordinates.addValueEventListener(new ValueEventListener() {
#Override
public void onDataChange(DataSnapshot dataSnapshot) {
Iterator iterator = dataSnapshot.getChildren().iterator();
while (iterator.hasNext()) {
list.add(((DataSnapshot) iterator.next()).getKey());
}
getPosts(list);
}
#Override
public void onCancelled(DatabaseError databaseError) {
}
});
}
};
handler.post(runnable);
}
}
This is my database structure
Posts
Post1
message1:
"abc + something"
title1:
"abc"
Post2
message2:
"def + something"
title2:
"def"
Post3
message3:
"ghi + something"
title3:
"ghi"
I'm getting duplicated array and can't understand why
please help me.
I also tried delete one reference and make a call but in this way I can't access the children of the JSON.

This is happening because of your for loop. You are creating a new databaseReference every time you loop. Take the databaseReference out of your loop and it will solve your problem or try to change the logic of getting the data from your Firebase database.
Hope it helps.

public class Posts extends AppCompatActivity {
private DatabaseReference databaseReference, barRoomsCoordinates;
private FirebaseDatabase firebaseDatabase;
private List<Post> postList;
private List<String> list;
#Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_posts);
postList = new ArrayList<>();
list = new ArrayList<>();
getPosts();
}
private void getPosts() {
databaseReference = firebaseDatabase.getReference().child("Posts");
databaseReference.addChildEventListener(new ChildEventListener() {
#Override
public void onChildAdded(DataSnapshot dataSnapshot, String s) {
}
#Override
public void onChildChanged(DataSnapshot dataSnapshot, String s) {
Iterator iterator = dataSnapshot.getChildren().iterator();
while (iterator.hasNext()) {
Post post = new Post();
post.setTitle((String) ((DataSnapshot)iterator.next()).getValue());
post.setMassage((String) ((DataSnapshot)iterator.next()).getValue());
postList.add(post);
}
}
#Override
public void onChildRemoved(DataSnapshot dataSnapshot) {
}
#Override
public void onChildMoved(DataSnapshot dataSnapshot, String s) {
}
#Override
public void onCancelled(DatabaseError databaseError) {
}
});
}
}
Hello all,
I solved my problem with one reference to the database instead of two calls as i did before and with addChildEventListener.
Thanks

Firebase android pagination

I'm building an app which will show videos stored on firebase. The list of videos needs to be paginated fetching most recent 20 videos at a time.
Here is the code I thought would work
private void getVideos() {
Query videosQuery = FirebaseUtil.getVideosRef();
videosQuery.startAt(0);
videosQuery.endAt(1);
ChildEventListener videosChildEventListener = new ChildEventListener() {
#Override
public void onChildAdded(DataSnapshot dataSnapshot, String s) {
String date = dataSnapshot.getKey();
String temp = date;
}
#Override
public void onChildChanged(DataSnapshot dataSnapshot, String s) {
}
#Override
public void onChildRemoved(DataSnapshot dataSnapshot) {
}
#Override
public void onChildMoved(DataSnapshot dataSnapshot, String s) {
}
#Override
public void onCancelled(DatabaseError databaseError) {
Log.d(tag, "database error");
}
};
ValueEventListener videoValueEventListener = new ValueEventListener() {
#Override
public void onDataChange(DataSnapshot dataSnapshot) {
String date = dataSnapshot.getKey();
String temp = date;
long count = dataSnapshot.getChildrenCount();
String value = dataSnapshot.getValue().toString();
temp = value;
}
#Override
public void onCancelled(DatabaseError databaseError) {
Log.d(tag, "database error");
}
};
// videosQuery.addChildEventListener(videosChildEventListener);
videosQuery.addValueEventListener(videoValueEventListener);
}
But above code retrieves entire list of videos instead of limited videos. How can pagination be implemented.

Below is the code I'm using for pagination which shows the latest node first.
public void getImages() {
Query imagesQuery = FirebaseDatabase.getInstance().getReference().child("englishDps").child(mChildName).orderByKey().limitToLast(21);
ChildEventListener childEventListener = new ChildEventListener() {
#Override
public void onChildAdded(DataSnapshot dataSnapshot, String s) {
Image image = dataSnapshot.getValue(Image.class);
image.setNodeKey(dataSnapshot.getKey());
mTempImages.add(image);
if (mTempImages.size() == 21) {
mLastKey = mTempImages.get(0).getNodeKey();
Collections.reverse(mTempImages);
mTempImages.remove(mTempImages.size() - 1);
mImages.addAll(mTempImages);
setAdapter();
}
}
#Override
public void onChildChanged(DataSnapshot dataSnapshot, String s) {
}
#Override
public void onChildRemoved(DataSnapshot dataSnapshot) {
}
#Override
public void onChildMoved(DataSnapshot dataSnapshot, String s) {
}
#Override
public void onCancelled(DatabaseError databaseError) {
if (isAdded()) {
Toast.makeText(getActivity(), "Problem loading more images...", Toast.LENGTH_LONG).show();
}
}
};
imagesQuery.addChildEventListener(childEventListener);
}
#Override
public void getMoreImages() {
if (!mGettingMoreImages) {
mGettingMoreImages = true;
Query imagesQuery = FirebaseDatabase.getInstance().getReference("englishDps").child(mChildName).orderByKey().endAt(mLastKey).limitToLast(21);
ChildEventListener childEventListener = new ChildEventListener() {
#Override
public void onChildAdded(DataSnapshot dataSnapshot, String s) {
Image image = dataSnapshot.getValue(Image.class);
image.setNodeKey(dataSnapshot.getKey());
mMoreImages.add(image);
if (mMoreImages.size() == 21) {
mLastKey = mMoreImages.get(0).getNodeKey();
Collections.reverse(mMoreImages);
mMoreImages.remove(mMoreImages.size() - 1);
mImages.addAll(mMoreImages);
mMoreImages.clear();
mGettingMoreImages = false;
mImagesAdapter.notifyDataSetChanged();
return;
}
if (mLastKey.equalsIgnoreCase(image.getNodeKey())) {
Collections.reverse(mMoreImages);
mImages.addAll(mMoreImages);
mMoreImages.clear();
mGettingMoreImages = false;
mImagesAdapter.onNoMoreImages();
;
mImagesAdapter.notifyDataSetChanged();
}
}
#Override
public void onChildChanged(DataSnapshot dataSnapshot, String s) {
}
#Override
public void onChildRemoved(DataSnapshot dataSnapshot) {
}
#Override
public void onChildMoved(DataSnapshot dataSnapshot, String s) {
}
#Override
public void onCancelled(DatabaseError databaseError) {
if (isAdded()) {
Toast.makeText(getActivity(), "Problem loading more images...", Toast.LENGTH_LONG).show();
}
}
};
imagesQuery.addChildEventListener(childEventListener);
}
}

I have following method to paginate through a firebase realtime database node:
private void getUsers(String nodeId) {
Query query;
if (nodeId == null)
query = FirebaseDatabase.getInstance().getReference()
.child(Consts.FIREBASE_DATABASE_LOCATION_USERS)
.orderByKey()
.limitToFirst(mPostsPerPage);
else
query = FirebaseDatabase.getInstance().getReference()
.child(Consts.FIREBASE_DATABASE_LOCATION_USERS)
.orderByKey()
.startAt(nodeId)
.limitToFirst(mPostsPerPage);
query.addListenerForSingleValueEvent(new ValueEventListener() {
#Override
public void onDataChange(DataSnapshot dataSnapshot) {
UserModel user;
List<UserModel> userModels = new ArrayList<>();
for (DataSnapshot userSnapshot : dataSnapshot.getChildren()) {
userModels.add(userSnapshot.getValue(UserModel.class));
}
mAdapter.addAll(userModels);
mIsLoading = false;
}
#Override
public void onCancelled(DatabaseError databaseError) {
mIsLoading = false;
}
});
}
Every time I reach the bottom of the paginated data, I call the getUsers(mAdapter.getLastItemId()); and then it brings the next set of records.
I have written a complete guide with open source sample app on this that you can check at https://blog.shajeelafzal.com/2017/12/13/firebase-realtime-database-pagination-guide-using-recyclerview/

You want to be using the limitToFirst/limitToLast methods to retrieve a limited number of results.
videosQuery.orderByKey().limitToFirst(20)
https://www.firebase.com/docs/web/api/query/limittofirst.html
You should really consider changing the naming convention of your videos to include leading 0s (i.e. video01, video02... video10, video11) because the above code will display them exactly as you have them above (which I assume is out of order?)
Alternatively, if you're adding the videos via Java, you could just let firebase create the uniqueids via push(). The uniqueid are generated in a way that they'll sort chronilogically, which sounds like it'll suit your need to grab the most recent(ly added?) videos.
https://www.firebase.com/docs/web/api/firebase/push.html

mPageEndOffset = 0;
mPageLimit = 10;
mPageEndOffset += mPageLimit;
deviceListQuery = mDatabase.child("users")
.orderByChild("id").limitToFirst(mPageLimit).startAt(mPageEndOffset);
deviceListQuery.addValueEventListener(YourActivity.this);

You can achieve it by using Firebase database paging library as below code...
In this code, I have shown Post as an data model item and PostViewHolder as ViewHolder
//Initialize RecyclerView
mRecyclerView = findViewById(R.id.recycler_view);
mRecyclerView.setHasFixedSize(true);
LinearLayoutManager mManager = new LinearLayoutManager(this);
mRecyclerView.setLayoutManager(mManager);
//Initialize Database
mDatabase = FirebaseDatabase.getInstance().getReference().child("posts");
//Initialize PagedList Configuration
PagedList.Config config = new PagedList.Config.Builder()
.setEnablePlaceholders(false)
.setPrefetchDistance(5)
.setPageSize(10)
.build();
//Initialize FirebasePagingOptions
DatabasePagingOptions<Post> options = new DatabasePagingOptions.Builder<Post>()
.setLifecycleOwner(this)
.setQuery(mDatabase, config, Post.class)
.build();
//Initialize Adapter
mAdapter = new FirebaseRecyclerPagingAdapter<Post, PostViewHolder>(options) {
#NonNull
#Override
public PostViewHolder onCreateViewHolder(#NonNull ViewGroup parent, int viewType) {
return new PostViewHolder(LayoutInflater.from(parent.getContext()).inflate(R.layout.item_list, parent, false));
}
#Override
protected void onBindViewHolder(#NonNull PostViewHolder holder,
int position,
#NonNull Post model) {
holder.setItem(model);
}
#Override
protected void onLoadingStateChanged(#NonNull LoadingState state) {
switch (state) {
case LOADING_INITIAL:
case LOADING_MORE:
// Do your loading animation
mSwipeRefreshLayout.setRefreshing(true);
break;
case LOADED:
// Stop Animation
mSwipeRefreshLayout.setRefreshing(false);
break;
case FINISHED:
//Reached end of Data set
mSwipeRefreshLayout.setRefreshing(false);
break;
case ERROR:
retry();
break;
}
}
#Override
protected void onError(#NonNull DatabaseError databaseError) {
super.onError(databaseError);
mSwipeRefreshLayout.setRefreshing(false);
databaseError.toException().printStackTrace();
}
};
//Set Adapter to RecyclerView
mRecyclerView.setAdapter(mAdapter);
Just visit this below URL for reference
https://firebaseopensource.com/projects/patilshreyas/firebaserecyclerpagination/app/readme.md/
Here you will find implementation of library which helps to implement Pagination of firebase data in RecyclerView
I hope this will help you!

This is how I implemented pagination from firebase database. Consider a case if we have 100 messages. So I first load last 20 messages from firebase i.e. 81 to 100. Then on scroll up I call getMoreMessages() function to load next 20 messages which are 61 to 80 and so on.
public class ChatActivity extends Activity {
String chat_id = "";
long mTotalChildren = 0;
boolean loading = false;
ChildEventListener childEventListenerMain, childEventListenerPager;
ChatActivity mContext = ChatActivity.this;
MessageAdapter messageAdapter;
#BindView(R.id.pb_messages)
ProgressBar pb_messages;
#BindView(R.id.ll_audio)
LinearLayout llAudio;
#BindView(R.id.tv_record_time)
AppCompatTextView tvRecordTime;
#BindView(R.id.tv_cancel)
AppCompatTextView tvCancel;
#BindView(R.id.iv_send)
AppCompatImageView ivSend;
#BindView(R.id.iv_record)
AppCompatImageView ivRecord;
#BindView(R.id.ab_layout)
AppBarLayout abLayout;
#BindView(R.id.messages)
RecyclerView rvMessages;
#BindView(R.id.iv_chat_icon)
SimpleDraweeView ivChatIcon;
#BindView(R.id.iv_camera)
AppCompatImageView ivCamera;
#BindView(R.id.iv_gallery)
AppCompatImageView ivGallery;
#BindView(R.id.message_input)
AppCompatEditText messageInput;
#BindView(R.id.tv_send)
AppCompatTextView tvSend;
#BindView(R.id.toolbar_title)
AppCompatTextView toolbarTitle;
#BindView(R.id.toolbar_status)
AppCompatTextView toolbarStatus;
#BindView(R.id.ll_send)
LinearLayout llSend;
int categoryId, senderId, receiverId, offerId;
String mLastKey;
LinearLayoutManager layoutManager;
private ArrayList<MessageModel> messageArrayList = new ArrayList<>();
private ArrayList<MessageModel> tempMessageArrayList = new ArrayList<>();
#Override
public int getLayoutId() {
return R.layout.activity_chat;
}
#Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
messageAdapter = new MessageAdapter(mContext, messageArrayList) {
};
layoutManager = new LinearLayoutManager(mContext);
rvMessages.setLayoutManager(layoutManager);
rvMessages.setItemAnimator(new DefaultItemAnimator());
rvMessages.setAdapter(messageAdapter);
rvMessages.setHasFixedSize(true);
rvMessages.addOnScrollListener(new RecyclerView.OnScrollListener() {
#Override
public void onScrollStateChanged(RecyclerView recyclerView, int newState) {
super.onScrollStateChanged(recyclerView, newState);
}
#Override
public void onScrolled(RecyclerView recyclerView, int dx, int dy) {
super.onScrolled(recyclerView, dx, dy);
if (messageArrayList.size() >= 20 &&
!loading && layoutManager.findFirstVisibleItemPosition() == 0 && messageArrayList.size() < mTotalChildren) {
loading = true;
getMoreMessages();
}
}
});
messageAdapter.notifyDataSetChanged();
//used to scroll up recyclerview when keyboard pops up
rvMessages.addOnLayoutChangeListener(new View.OnLayoutChangeListener() {
#Override
public void onLayoutChange(View view, int left, int top, int right, int bottom,
int oldLeft, int oldTop, int oldRight, int oldBottom) {
if (bottom < oldBottom) {
rvMessages.postDelayed(new Runnable() {
#Override
public void run() {
rvMessages.scrollToPosition(messageArrayList.size() - 1);
}
}, 100);
}
}
});
loading = true;
getMessages();
}
public void getMessages() {
FirebaseDatabase.getInstance().getReference().child(pathtomsgs).addListenerForSingleValueEvent(new ValueEventListener() {
#Override
public void onDataChange(DataSnapshot dataSnapshot) {
Log.e("childrenCount", String.valueOf(+dataSnapshot.getChildrenCount()));
mTotalChildren = dataSnapshot.getChildrenCount();
}
#Override
public void onCancelled(DatabaseError databaseError) {
}
});
Query messageQuery = FirebaseDatabase.getInstance().getReference().child(pathtomsgs).limitToLast(20);
childEventListenerMain = new ChildEventListener() {
#Override
public void onChildAdded(DataSnapshot dataSnapshot, String s) {
loading = true;
MessageModel messageModel = dataSnapshot.getValue(MessageModel.class);
if (messageModel != null) {
messageModel.chat_id = chat_id;
messageModel.message_id = dataSnapshot.getKey();
messageArrayList.add(messageModel);
messageAdapter.notifyDataSetChanged();
mLastKey = messageArrayList.get(0).message_id;
rvMessages.scrollToPosition(messageArrayList.size() - 1);
}
}
#Override
public void onChildChanged(DataSnapshot dataSnapshot, String s) {
}
#Override
public void onChildRemoved(DataSnapshot dataSnapshot) {
}
#Override
public void onChildMoved(DataSnapshot dataSnapshot, String s) {
}
#Override
public void onCancelled(DatabaseError databaseError) {
Toast.makeText(mContext, "Problem loading more images...", Toast.LENGTH_LONG).show();
}
};
messageQuery.addChildEventListener(childEventListenerMain);
ValueEventListener messageChildSINGLEValueEventListener = new ValueEventListener() {
#Override
public void onDataChange(DataSnapshot dataSnapshot) {
pb_messages.setVisibility(View.GONE);
System.out.println("We're done loading messages " + dataSnapshot.getChildrenCount() + " items");
loading = false;
}
#Override
public void onCancelled(DatabaseError databaseError) {
}
};
messageQuery.addListenerForSingleValueEvent(messageChildSINGLEValueEventListener);
}
public void getMoreMessages() {
tempMessageArrayList.clear();
Query messageQuery = FirebaseDatabase.getInstance().getReference().child(pathtomsgs).orderByKey().endAt(mLastKey).limitToLast(20);
childEventListenerPager = new ChildEventListener() {
#Override
public void onChildAdded(DataSnapshot dataSnapshot, String s) {
loading = true;
MessageModel messageModel = dataSnapshot.getValue(MessageModel.class);
if (messageModel != null) {
messageModel.chat_id = chat_id;
messageModel.message_id = dataSnapshot.getKey();
tempMessageArrayList.add(messageModel);
}
}
#Override
public void onChildChanged(DataSnapshot dataSnapshot, String s) {
}
#Override
public void onChildRemoved(DataSnapshot dataSnapshot) {
}
#Override
public void onChildMoved(DataSnapshot dataSnapshot, String s) {
}
#Override
public void onCancelled(DatabaseError databaseError) {
Toast.makeText(mContext, "Problem loading more images...", Toast.LENGTH_LONG).show();
}
};
messageQuery.addChildEventListener(childEventListenerPager);
ValueEventListener messageChildSINGLEValueEventListener = new ValueEventListener() {
#Override
public void onDataChange(DataSnapshot dataSnapshot) {
System.out.println("We're done loading messages " + dataSnapshot.getChildrenCount() + " items");
tempMessageArrayList.remove(tempMessageArrayList.size() - 1);
messageArrayList.addAll(0, tempMessageArrayList);
mLastKey = messageArrayList.get(0).message_id;
messageAdapter.notifyDataSetChanged();
//rvMessages.scrollToPosition(20);
layoutManager.scrollToPositionWithOffset(19, 0);
loading = false;
}
#Override
public void onCancelled(DatabaseError databaseError) {
}
};
messageQuery.addListenerForSingleValueEvent(messageChildSINGLEValueEventListener);
}
#Override
protected void onDestroy() {
FirebaseDatabase.getInstance().getReference().child(pathtomsgs).removeEventListener(childEventListenerPager);
FirebaseDatabase.getInstance().getReference().child(pathtomsgs).removeEventListener(childEventListenerMain);
super.onDestroy();
}
}

Android paging library can be used to implement pagination for data fetched from firebase database. Data is displayed in recycler view and paging component fetches pages in response to user scroll events.
Paging data source calls your firebase DAO object passing query parameters for the page to be displayed and results are passed back to paging component using callback provided to it.
Here is a complete example for reference http://www.zoftino.com/firebase-pagination-using-android-paging-library

accepted answear dos not work when less then the max. itmes (21) in database.
this is my solution build on the accepted answer:
0)set variables
private String lastMessageKey;
private int totalItemCount;
private int lastVisibleItem;
private boolean isLoadingMoreFollowers = false;
private boolean hasMoreItems = true;
private boolean hasShownNoMoreItemsToast = false;
private boolean initialLoad = true;
private final int MAX_LOAD_ITEMS = 11;
private final int VISIBLE_TRESHOLD = 1;
1) set a listener:
private void setMessageListener() {
if (childEventListener == null) {
mmessageArrayList.clear();
final ArrayList<Mmessage> tempMmessages = new ArrayList<>();
query = mDatabaseInstace.getReference().child(chat1).child(F.CHATS).child(F.MESSAGES).orderByKey().limitToLast(MAX_LOAD_ITEMS-1);
childEventListener = new ChildEventListener() {
#Override
public void onChildAdded(#NonNull DataSnapshot dataSnapshot, #Nullable String s) {
Log.d(TAG, "onChildAdded: -----------------> " + dataSnapshot);
Log.d(TAG, "onChildAdded: -----------------> " + s);
tempMmessages.add(dataSnapshot.getValue(Mmessage.class));
preloadMessagePics(tempMmessages);
if (initialLoad) {
lastMessageKey = tempMmessages.get(0).getMessagePushKey();
initialLoad = false;
}
mmessageArrayList.add(tempMmessages.size()-1, tempMmessages.get(0));
messageAdapter.notifyDataSetChanged();
tempMmessages.clear();
}
#Override
public void onChildChanged(#NonNull DataSnapshot dataSnapshot, #Nullable String s) {
Log.d(TAG, "onChildChanged: --------------------------------->");
}
#Override
public void onChildRemoved(#NonNull DataSnapshot dataSnapshot) {
Log.d(TAG, "onChildRemoved: ---------------------------------->");
}
#Override
public void onChildMoved(#NonNull DataSnapshot dataSnapshot, #Nullable String s) {
Log.d(TAG, "onChildMoved: ------------------------------------>");
}
#Override
public void onCancelled(#NonNull DatabaseError databaseError) {
Log.d(TAG, "onCancelled: ------------------------------------->");
}
};
query.addChildEventListener(childEventListener);
}
}
2) set load more listener:
private void setLoadMoreListener(){
setup.RV_messages.addOnScrollListener(new RecyclerView.OnScrollListener() {
#Override
public void onScrolled(#NonNull RecyclerView recyclerView, int dx, int dy) {
super.onScrolled(recyclerView, dx, dy);
totalItemCount = linearLayoutManager.getItemCount();
lastVisibleItem = linearLayoutManager.findLastVisibleItemPosition();
if (!isLoadingMoreFollowers && totalItemCount <= (lastVisibleItem+VISIBLE_TRESHOLD) && (totalItemCount >= MAX_LOAD_ITEMS-1)){
if (hasMoreItems) {
getMoreMessages();
Toast.makeText(homeActivity, "load more items", Toast.LENGTH_SHORT).show();
}else {
if (!hasShownNoMoreItemsToast) {
Toast.makeText(homeActivity, "has no more items", Toast.LENGTH_SHORT).show();
hasShownNoMoreItemsToast = true;
}
}
}
}
});
}
3) get more items:
private void getMoreMessages(){
final ArrayList<Mmessage> tempMmessages = new ArrayList<>();
isLoadingMoreFollowers = true;
loadMoreQuery = mDatabaseInstace.getReference().child(chat1).child(F.CHATS).child(F.MESSAGES).orderByKey().endAt(lastMessageKey).limitToLast(MAX_LOAD_ITEMS);
loadMoreChildEventListener = new ChildEventListener() {
#Override
public void onChildAdded(#NonNull DataSnapshot dataSnapshot, #Nullable String s) {
tempMmessages.add(dataSnapshot.getValue(Mmessage.class));
preloadMessagePics(tempMmessages);
if (tempMmessages.size() == MAX_LOAD_ITEMS){
lastMessageKey = tempMmessages.get(0).getMessagePushKey();
Collections.reverse(tempMmessages);
tempMmessages.remove(0);
mmessageArrayList.addAll(tempMmessages);
isLoadingMoreFollowers = false;
messageAdapter.notifyDataSetChanged();
tempMmessages.clear();
return;
}
if (lastMessageKey.equalsIgnoreCase(tempMmessages.get(tempMmessages.size()-1).getMessagePushKey())){
Collections.reverse(tempMmessages);
tempMmessages.remove(0);
mmessageArrayList.addAll(tempMmessages);
isLoadingMoreFollowers = false;
hasMoreItems = false;
messageAdapter.notifyDataSetChanged();
tempMmessages.clear();
}
}
#Override
public void onChildChanged(#NonNull DataSnapshot dataSnapshot, #Nullable String s) {
}
#Override
public void onChildRemoved(#NonNull DataSnapshot dataSnapshot) {
}
#Override
public void onChildMoved(#NonNull DataSnapshot dataSnapshot, #Nullable String s) {
}
#Override
public void onCancelled(#NonNull DatabaseError databaseError) {
}
};
loadMoreQuery.addChildEventListener(loadMoreChildEventListener);
}
have fun!!

If you want to get list from firebase realtime database descending with pagination you need use smth like this:
Query query = myRef.child("stories").orderByChild("takenAt").endAt(1600957136000L).limitToLast(30);
Full code, two requests
List<Story> storyList = new ArrayList<>();
FirebaseDatabase database = FirebaseDatabase.getInstance();
DatabaseReference myRef = database.getReference();
//first request will be look like this
Query query = myRef.child("stories").orderByChild("takenAt").endAt(1600957136000L).limitToLast(30);
query.addValueEventListener(new ValueEventListener() {
#Override
public void onDataChange(#NonNull DataSnapshot dataSnapshot) {
if (dataSnapshot.exists()) {
for (DataSnapshot snapshot : dataSnapshot.getChildren()) {
Story story = snapshot.getValue(Story.class);
storyList.add(story);
}
}
Collections.reverse(storyList);
}
#Override
public void onCancelled(#NonNull DatabaseError error) {
Log.e("ptg", "onCancelled: ", error.toException());
}
});
//second request
long lastTakenAtInTheStoryList = storyList.get(storyList.size()-1).getTakenAt();
Query query2 = myRef.child("stories").orderByChild("takenAt").endAt(lastTakenAtInTheStoryList).limitToLast(30);
query2.addValueEventListener(new ValueEventListener() {
#Override
public void onDataChange(#NonNull DataSnapshot dataSnapshot) {
if (dataSnapshot.exists()) {
for (DataSnapshot snapshot : dataSnapshot.getChildren()) {
Story story = snapshot.getValue(Story.class);
storyList.add(story);
}
}
Collections.reverse(storyList);
}
#Override
public void onCancelled(#NonNull DatabaseError error) {
Log.e("ptg", "onCancelled: ", error.toException());
}
});

In the past (but no longer) a question regarding implementing Pagination to a query on Firestore was marked as a duplicate of this question I will answer for FireStore here.
Paginate a query on android Firestore
Query query = db.collection("cities")
.orderBy("population")
.limit(25);
Query next = query;
private void loadCities() {
query = next;
query.get()
.addOnSuccessListener(new OnSuccessListener<QuerySnapshot>() {
#Override
public void onSuccess(QuerySnapshot documentSnapshots) {
// Get the last visible document
DocumentSnapshot lastVisible =
documentSnapshots.getDocuments()
.get(documentSnapshots.size() -1);
// Construct a new query starting at this document,
// get the next 25 cities.
next = db.collection("cities")
.orderBy("population")
.startAfter(lastVisible)
.limit(25);
}
});
}
Now just call the loadCities() method whenever you need to load more cities.

Develop Reference

The Android operating system is a mobile operating system that was developed by Google (GOOGL?) to be primarily used for touchscreen devices, cell phones, and tablets.

viewPager Adapter not being updated - android

Add this in your pager adapter #Override public int getItemPosition(Object object) { return POSITION_NONE; }

Firebase query runs in the worker thread, so you have to run notifyDataSetChanged in the main thread. Try to use the below code in the function update() runOnUiThread(new Runnable() { #Override public void run() { } });

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