Develop Reference

Android RecyclerView infinity scrolling: how to add more items to dataset

I have implemented my RecyclerView and even added an onscrolllistener to support infinity scrolling and now I'm stuck with a, hopefully, easy problem: How can I add the newly loaded data to the existing dataset?
My current approach: I create a new array with the length of the existing dataset + the length of the newly loaded data. I System.arraycopy my existing dataset and add the new content with a for-loop.
This works but the list is always reset (scrolls back to the top) and I assume my way to add additional content is overly complicated/wrong, though the tutorials I have looked at seem to pass over this "detail".
Update: I'm currently calling "scrollToPosition" on the UI-Thead after the data has been loaded, but I doubt this is the correct way of doing this or am I wrong?

You shouldn't be adding stuff to your dataset, you will sooner or later run out of memory. What you can do is return a big number (I used Short.MAX_VALUE) item in getItemCount inside your adapter and in the method that requests a view for postion you should do position % list.size();
It is not a truly endless RecyclerView this way, but good enough. I will paste some code tomorrow, I don't have it here now :/

I think you have to add items inside your adapter. Let`s say
class Adapter extends Recycler.Adapter<Recycler.ViewHolder>{
List<YourCustomObject> list;
public Adapter(){
list = new ArrayList<>();
}
public void addItem(YourCustomObject item){
list.add(item);
notifyItemDateSetChanged(); //This method for adapter to notice that list size have been changed
}
// Here your views
}
There is implementation of Your fragment or Activity where you retrieve data from internet.Let` say
class MainActivity extends AppCompactActivity{
Adapter adapter = new Adapter();
List<YourCustomObjects> objects;
public void onCreateView(){
//////// Something yours
}
public void onLoadMore(){
///// Your operation to retrieve data and init it to your list objects
for(YourCustomObject object : objects){
adapter.addItem(object);
}
}
}

Update custom listview item attribute

I am using listview in my app.I am adding items to list with this line:
conversationsAdapter.add(user);
and this initializes list
conversationsAdapter=new ArrayAdapter<JsonObject>(this,0) {
#Override
public View getView(int c_position,View c_convertView,ViewGroup c_parent) {
if (c_convertView == null) {
c_convertView=getLayoutInflater().inflate(R.layout.random_bars,null);
}
JsonObject user=getItem(c_position);
String name=user.get("name").getAsString();
String image_url="http://domain.com/photos/profile/thumb/"+user.get("photo").getAsString();
TextView nameView=(TextView)c_convertView.findViewById(R.id.tweet);
nameView.setText(name);
ImageView imageView=(ImageView)c_convertView.findViewById(R.id.image);
Ion.with(imageView)
.placeholder(R.drawable.twitter)
.load(image_url);
return c_convertView;
}
};
ListView conversationsListView = (ListView)findViewById(R.id.conversationList);
conversationsListView.setAdapter(conversationsAdapter);
conversationsListView.setOnItemClickListener(new AdapterView.OnItemClickListener() {
#Override
public void onItemClick(AdapterView<?> parent, View view,
int position, long id) {
startChat(conversationsAdapter.getItem(position));
}
});
My list view is looking like this:
I want to update an item in the list.How can I do this ?
Example:We can write a method like: changeName when this method calls,method sets name "Tolgay Toklar" to "Tolgay Toklar Test" so I want to update custom listview item attributes.

I totally disagree with tyczj. You never want to externally modify an ArrayAdapter's list and yes it's possible to update just an individual item. Lets start with updating an individual item.
You can just invoke getItem() and directly modify the object and call notifyDataSetChanged(). Example:
JSONObject object = conversationAdapter.getItem(position);
object.put("name", data);
conversationAdapter.notifyDataSetChanged();
Why does this work? Because the adapter will feed you the same object reference used internally, allowing you to modify it and update the adapter. No problem. Of course, I'd recommend instead building your own custom adapter to perform this directly on the adapter's internal list. As an alternative, I highly recommend using the ArrayBaseAdapter instead. It already provides that ability for you while fixing some other major bugs with Android's ArrayAdapter.
So why is tyczj wrong about modifying the external list? Simple. There's no guarantee that your external list is the same as the adapters. Once you perform a filter on the ArrayAdapter, your external list and the adapters are no longer the same. You can get into a dangerous scenario where (for example) index 5 no longer represents position 5 in the adapter because you later added an item to the adapter. I suggest reading Problems with ArrayAdapter's Constructors for a little more insight.
Update: How External List Fails
Lets say you create a List of objects to pass into an ArrayAdapter. Eg:
List<Data> mList = new ArrayList<Data>();
//...Load list with data
ArrayAdapter<Data> adapter = new ArrayAdapter<Data>(context, resource, mList);
mListView.setAdapter(adapter);
So far so good. You have your external list, you have an adapter instantiated with it and assigned to listview. Now lets say at some later point, the adapter is filtered and cleared.
adapter.filter("test");
//...later cleared
adapter.filter("");
Now at this point mList is NOT the same as the adapter. So if the adapter is modified:
adapter.add(newDataObject);
You'll find that mList does not contain that new data object. Hence why external lists like this can be dangerous as the filter creates a NEW ArrayList instance. It won't continue to use your mList referenced one. You could even try adding items to mList at this point and it won't be reflected in the adapter.

If you change the data in your list you need to call notifyDatasetCanged on the adapter to notify the list that the underlying data has changed needs to be updated and.
Example
List<MyData> data = new ArrayList<MyData>();
private void changeUserName(String name){
//find the one you need to change from the list here
.
.
.
data.set(myUpdatedData);
notifyDatasetChanged()
}

How to refresh an imageView inside an adapter for a GridView

I've been fooling around with Android but I'm stuck. I have a adapter class that
sets an image in an imageview for each item in my gridView. I use the observer pattern to notify the activity that calls the update method and it refreshes the adapter with notifydatasetchanged and that invalidates the gridview itself and the imageView.
The problem is that when i change an object (which has a reference to a drawable), and try to update, nothing happens in the imageview. I've tried to debug and the objects are changed so i don't understand why it doesn't update...
Also, with this it works perfect to make the images disappear, so that's pretty akward..: shape.setImage(android.R.color.transparent)
#Override
public void update(Observable arg0, Object arg1)
{
// Toast.makeText(this, "I am notified",0).show();
adapter.notifyDataSetChanged();
gridView.findViewById(R.id.picture).invalidate();
gridView.invalidate();
}
This is an example of an object that needs to be showed:
public class Square extends Shape
{
public Square()
{
setImage(R.drawable.square);
}
public String print()
{
return "s ";
}
public String getName()
{
return "Square";
}
}
my adapter:
public class GridAdapter extends ArrayAdapter<Shape>
{
Context context;
int layoutResourceId;
List<Shape> data = null;
public GridAdapter(Context context, int layoutResourceId, List<Shape> data)
{
super(context, layoutResourceId, data);
this.layoutResourceId = layoutResourceId;
this.context = context;
this.data = data;
}
public View getView(int position, View convertView,ViewGroup parent)
{
LayoutInflater inflater = (LayoutInflater) context.getSystemService(Context.LAYOUT_INFLATER_SERVICE);
View rowView = inflater.inflate(R.layout.list_item, parent, false);
rowView.setBackgroundResource(R.drawable.customshape);
ImageView imgView =(ImageView)rowView.findViewById(R.id.picture);
imgView.setImageResource(data.get(position).getImage());
return rowView;
}

I think the actual problem here is updating the drawable from outside the UI thread. notifyDataSetChanged() isn't the answer. The dataset didn't change, in my case at least. The dataset changed and I set the drawable to null (which worked). Later, when the image finished loading, I wasn't updating from the UI thread, so the GridView didn't know it was supposed to learn about the invalidation of the ImageView.
The answer in my case was to use AsyncTask which automatically uses the UI thread and thread syncs.
Note that notifyDatasetChanged() is for when the getViewId() changes in the adapter — which isn't the same as and actually has nothing to do with when the drawable changes. Also note that setImageDrawable() automatically invalidates the ImageView. There's actually no need to call ImageView.invalidate() or any other invalidate(). You just have to update the drawable in the UI thread, and AsyncTask comes with all the right baggage to not only do it at the right time, but in such a way that the GridView can learn about it before it updates the various canvases and drawable caches.
I learned most of the above from the Google Android dev pages and from source diving for hours on end. Curiously, source diving in Android Studio (ctrl-B) is actually easier than slogging through all the docs hoping to stumble on the right paragraph that explains the problem. Though, the problem is explained nicely here:
http://developer.android.com/training/displaying-bitmaps/process-bitmap.html

I cannot comment, so I am going to share my thoughts here, let me know in the comments if I'm missing something.
Seems like you need to add the following method in your GridAdapter:
public void setData(List<Shape> stuff) {
data.clear();
data.addAll(stuff);
notifyDataSetChanged();
}
Now, wherever in your activity you created the adapter, you should have a List that was passed to create the GridAdapter. Let's call that initial list 'data1'.
Assuming by 'refresh' you mean that you want to modify an existing item. You need to find that item in data1 and make the changes. Then you need to find the GridAdapter instance that you have already created and then use the setData method and pass data1.
Hope this is not totally useless info for you.
Note: it would be helpful if you posted more of your code.

The problem is that when i change an object (which has a reference to
a drawable), and try to update, nothing happens in the imageview.
Lets have a look at the doc:
public void notifyDataSetChanged()
Notifies the attached observers that the underlying data has been
changed and any View reflecting the data set should refresh itself.
the only reason that when you call notifyDataSetChanged() and it dose not update your gridview is that your adapter data set and your object are two different things. that means your gridview data has not changed. you must change those objects that you passed to your gridView constructor (List data). if you change any of them and then you call notifyDataSetChanged() it will work. The references of your object and the adapter data must be the same.
another things that I have seen on the net is you must change the data set of adapter by manipulating it with functions like (add(), insert(), remove(), clear(), etc.).

Problem solved lol, i just made a new adapter instead and it worked

ListView's header view destroyed on configuration changed

I have a ListFragment that uses a header view. Both the header's contents and the list's are fetched from a background task. In order to not re-fetch the data on configuration changes, I am calling setRetainInstance and keeping the data on the fragment.
When the the configuration changes, the view is recreated, so it removes the header view that I previously populated. Since now I already have the data, I should just re-add the header view to the list.
Unfortunately when I try doing this... boom!
java.lang.IllegalStateException: Cannot add header view to list -- setAdapter
has already been called.
Apparently, even tho the view is destroyed and onCreateView is called again, the list's adapter is already set (or the state is retained), making it impossible to add the header view again.
How can I keep the ListView's header or redraw it without recreating the fragment on orientation changes?

This is intended behaviour, take a look at the Android source code here for guidance on API 17, but really any will do. The relevant part is:
Add a fixed view to appear at the top of the list. If addHeaderView is
called more than once, the views will appear in the order they were
added. Views added using this call can take focus if they want. NOTE:
Call this before calling setAdapter. This is so ListView can wrap the
supplied cursor with one that will also account for header and footer
views.
public void addHeaderView(View v, Object data, boolean isSelectable) {
if (mAdapter != null && ! (mAdapter instanceof HeaderViewListAdapter)) {
throw new IllegalStateException(
"Cannot add header view to list -- setAdapter has already been" +
"called."); // Edit: SK9 wrapped this.
}
FixedViewInfo info = new FixedViewInfo();
info.view = v;
info.data = data;
info.isSelectable = isSelectable;
mHeaderViewInfos.add(info);
// in the case of re-adding a header view, or adding one later on,
// we need to notify the observer
if (mAdapter != null && mDataSetObserver != null) {
mDataSetObserver.onChanged();
}
}
The adapter is not null when you come to add the header again and an exception is being raised. To resolve your issue, something along the following lines will do just fine:
setListAdapter(null);
getListView().addHeaderView(mHeader);
setListAdapter(new MyAdapter(getActivity(), items));
I wouldn't even classify this as a workaround. I encountered the same problem and this worked for me.
Apparently footers are treated very differently, see here:
public void addFooterView(View v, Object data, boolean isSelectable) {
// NOTE: do not enforce the adapter being null here, since unlike in
// addHeaderView, it was never enforced here, and so existing apps are
// relying on being able to add a footer and then calling setAdapter to
// force creation of the HeaderViewListAdapter wrapper
FixedViewInfo info = new FixedViewInfo();
info.view = v;
info.data = data;
info.isSelectable = isSelectable;
mFooterViewInfos.add(info);
// in the case of re-adding a footer view, or adding one later on,
// we need to notify the observer
if (mAdapter != null && mDataSetObserver != null) {
mDataSetObserver.onChanged();
}
}

it's a know issue, but you can resolve it like this:
add header before the set adapter and remove him

Yes, it's a known issue, but can be avoided with the proper approach.
It seems that a solution similar to your problem exists.
These guys found a workaround: setSelected in OnItemClick in ListView
Hope it helps ;)

Obtaining the current Android view and forcing it to be redrawn

How can I get the current Android view when it displays data that has been updated, and force it to be redrawn? I worked through Android's Notepad tutorial and completed lesson three without any problems — the solution is provided, after all — but I'm stuck on my first non-trivial modification.
I added a new button to the menu, next to the Add note button. When pressed, that button adds a letter to the title of each note in the system. However, the new titles don't show up in the list of notes no matter how long I wait. I know the updater works because the changes do appear if I dismiss the app and bring it back up.
So far, I've discovered that I have to use some kind of invalidation method to make the program redraw itself with the new values. I know that invalidate() is used from the UI thread and postInvalidate() is used from non-UI threads 1, 2, but I don't even know which thread I'm in. Also, both of those methods have to be called from the View object that needs drawing, and I don't know how to obtain that object. Everything I try returns null.
My main class:
public boolean onMenuItemSelected(int featureId, MenuItem item) {
switch(item.getItemId()) {
case INSERT_ID:
createNote();
return true;
case NEW_BUTTON:
expandTitles();
return true;
default:
// Intentionally empty
}
return super.onMenuItemSelected(featureId, item);
}
private void expandTitles() {
View noteListView = null;
// noteListView = findViewById(R.layout.notes_list); // null
// noteListView =
// getWindow().getDecorView().findViewById(android.R.id.content);
// From SO question 4486034
noteListView = findViewById(R.id.body); // Fails
mDbHelper.expandNoteTitles(noteListView);
}
My DAO class:
public void expandNoteTitles(View noteListView) {
Cursor notes = fetchAllNotes();
for(int i = 1; i <= notes.getCount(); i++) {
expandNoteTitle(i);
}
// NPE here when attempt to redraw is not commented out
noteListView.invalidate(); // Analogous to AWT's repaint(). Not working.
// noteListView.postInvalidate(); // Like repaint(). Not working.
}
public void expandNoteTitle(int i) {
Cursor note = fetchNote(i);
long rowId =
note.getLong(note.getColumnIndexOrThrow(NotesDbAdapter.KEY_ROWID));
String title =
note.getString(note.getColumnIndexOrThrow(NotesDbAdapter.KEY_TITLE)) + "W";
String body =
note.getString(note.getColumnIndexOrThrow(NotesDbAdapter.KEY_BODY));
updateNote(rowId, title, body);
}
What do I have to do to get the updated note titles to show up as soon as I press the button?
Obviously, I'm a complete newbie to Android. I point this out to encourage you to use small words and explain even obvious things. I know this is the millionth "Android not redrawing" question, but I've read dozens of existing posts and they either don't apply or don't make sense to me.
1: What does postInvalidate() do?
2: What is the difference between Android's invalidate() and postInvalidate() methods?

According to the tutorial, the list of existing notes are presented in a ListView. That is an adapter based View, so the items it shows are sourced from an adapter extending theBaseAdapter class. In these cases, you should notify the adapter that the contents have changed by calling its notifyDatasetChanged method. This'll signal the ListView to update and redraw its rows.
Edit:
Sorry, I now realize that this example uses CursorAdapters. These source the items to show from a Cursor object that was obtained from a database query. Now, what the notifyDatasetChanged() tells the adapter is, that the data that backs the adapter has changed, so Views that show stuff based on this adapter need to redraw their contents. In the case of a CursorAdapter, this data is coming from a cursor. So you also need to requery that cursor, refreshing it from the DB, like this:
private void expandTitles() {
mDbHelper.expandNoteTitles();
CursorAdapter adapter = (CursorAdapter)getListAdapter();
adapter.getCursor().requery();
}
The requery() method automatically calls the notifyDatasetChanged() in this case, so you don't need to worry about that, the list will update itself. See this thread also: https://groups.google.com/forum/?fromgroups#!topic/android-developers/_FrDcy0KC-w%5B1-25%5D.