Develop Reference

How do I check when my RecycleView notifyDataSetChanged() has finished?

I have a RecycleView . I updated its adapter, and call
notifydatasetchanged()
. I want to wait until the list finishes drawing

just add this:
recycler.post(() -> {
do something...
});
done.

try this one.
recyclerView.getViewTreeObserver().addOnGlobalLayoutListener(new ViewTreeObserver.OnGlobalLayoutListener() {
#Override
public void onGlobalLayout() {
//At this point the layout is complete and the
//dimensions of recyclerView and any child views are known.
}
});

RecyclerView scroll in incorrect direction when new element is inserted

I use smoothScrollToPosition to scroll RecyclerView. It does scroll every time a new entry is inserted; but to the top, not to the bottom, which is the direction i want.
list_chat = (RecyclerView) findViewById(R.id.list_chat);
//Set up Layout Manager
linearLayoutManager = new LinearLayoutManager(this);
linearLayoutManager.setStackFromEnd(true);
list_chat.setLayoutManager(linearLayoutManager);
//set adapter
list_chat.setAdapter(adapter);
//set scroll
list_chat.post(new Runnable() {
#Override
public void run() {
list_chat.smoothScrollToPosition(adapter.getItemCount());
}
});
The adapter is from Firebase
adapter = new FirebaseRecyclerAdapter<ChatItem, ChatRecylerViewHolder>(ChatItem.class,R.layout.chat_item
,ChatRecylerViewHolder.class,queryChat ) {
#Override
protected void populateViewHolder(ChatRecylerViewHolder viewHolder, ChatItem model, int position) {
viewHolder.tvAuthorChat.setText(model.chatAuthor);
viewHolder.tvContentChat.setText(model.chatContent);
}
};

You do notice you are using linearLayoutManager.setStackFromEnd(true); this mean you first position is at the bottom. I suggest you a better option.
RecycleView didn't work the way listView work, you can scroll it with your layout manager something like this
linearLayoutManager.scrollToPositionWithOffset(position,offset);
Which position is the position you want to scroll to, offset is the offset within the current position. You could just use with one parameter as well.
linearLayoutManager.scrollToPosition(position);

Ok. I found the answer.
First, the old problem with my question: i thought list_chat.post is called whenever an item is inserted (turn out that is wrong). The reason for it keeps scrolling top is linearLayoutManager.setStackFromEnd(true);
Thus, the question comes down to Where to call the scrolling ?
The answer is : Since adapter manages data, it makes sense to guess that adapter will notify the insertion.
Here is the code
adapter.registerAdapterDataObserver(new RecyclerView.AdapterDataObserver() {
#Override
public void onItemRangeInserted(int positionStart, int itemCount) {
super.onItemRangeInserted(positionStart, itemCount);
list_chat.smoothScrollToPosition(adapter.getItemCount());
}
});

Sync al horizontal RecyclerView inside vertical list view

Hi I have vertical list (Custom ArrayAdapter) where each element has a horizontal RecyclerView, to give me a table like format.
What I want to do is, if one row is scrolled horizontally, rest rows too should be. I tried putting them just inside horizontal scroll view and various tricks available, but got no luck.
If possible, I would also like this synchronization keeping first element fixed.

Make an instance of RecyclerView.OnScrollListener. This instance will be common to all your horizontal recycler views. When onScrolled( ) gets called, scroll all the views except the one on which this got called.

Adding more to Little Child's answer, it was trickier than expected, I had to create a custom listener that was in my main Activity that was called by onScroll. This is because onScroll was in a getView method and did not get access to other views in listview, but my mainactivty could access them.
In my listadapter, I added
public interface RecycleListener {
public void onScroll(int scroll_x);
}
Then in getView
mRecyclerView.setOnScrollListener(new RecyclerView.OnScrollListener() {
#Override
public void onScrolled(RecyclerView recyclerView, int dx, int dy) {
listener.onScroll(recyclerView.computeHorizontalScrollOffset());
}
});
and finally in MainActivity
dayAdapter.setRecycleListener(new DayAdapter.RecycleListener() {
int busy = 0;
#Override
public void onScroll(int scroll_x)
{
if(busy == 1) return;
busy = 1;
for(int i = 0;i<days.size();i++)
{
View view = lv.getChildAt(i);
if(view == null) continue;
RecyclerView r = (RecyclerView) view.findViewById(R.id.timetable_row_recycler);
LinearLayoutManager layoutManager = ( LinearLayoutManager) r.getLayoutManager();
layoutManager.scrollToPositionWithOffset(0,0-scroll_x);
}
busy = 0;
}
});
Note I found out that you can easily get scroll position by using int scroll_x = recyclerView.computeHorizontalScrollOffset() however there is no direct method to appy it back. The method layoutManager.scrollToPosition(i) scrolls to ith element and not the scroll position, so the only correct possible way is
layoutManager.scrollToPositionWithOffset(0,0-scroll_x);
or (in case of custom layoutmanager)
((LinearLayoutManager) layoutManager).scrollToPositionWithOffset(0,0-scroll_x);
We basically apply negative offset to the 0th position, so instead of scrolling to offset before 0th, it scrolls to our desired scroll_x.
UPDATE
this worked but caused a lot of lag, so later I ended up switching to android.widget.TableLayout, and inserted rows (android.widget.TableRow) like I was trying with recyclerview.

How to use RecyclerView.scrollToPosition() to move the position to the top of current view?

The RecyclerView.scrollToPosition() is extremely strange.
for example, supposed a RecyclerView named "rv".
if now item 10 is under the current RecyclerView, call rv.scrollToPosition(10), the RecyclerView will scroll item 10 to bottom.
if now item 10 is in the current RecyclerView, call rv.scrollToPosition(10), there won't be any scrolling, nothing will be done.
if now item 10 is on the top of the current RecyclerView, call rv.scrollToPosition(10), the RecyclerView will scroll item 10 to top.
To help understanding, look at this picture
But what i need is that, whenever i call it, the RecyclerView will scroll the supposed position to the top of current view just like case 3. How to do that?

If I understand the question, you want to scroll to a specific position but that position is the adapter's position and not the RecyclerView's item position.
You can only achieve this through the LayoutManager.
Do something like:
rv.getLayoutManager().scrollToPosition(youPositionInTheAdapter).

Below link might solve your problem:
https://stackoverflow.com/a/43505830/4849554
Just create a SmoothScroller with the preference SNAP_TO_START:
RecyclerView.SmoothScroller smoothScroller = new
LinearSmoothScroller(context) {
#Override protected int getVerticalSnapPreference() {
return LinearSmoothScroller.SNAP_TO_START;
}
};
Now you set the position where you want to scroll to:
smoothScroller.setTargetPosition(position);
And pass that SmoothScroller to the LayoutManager:
layoutManager.startSmoothScroll(smoothScroller);

If you want to scroll to a specific position and that position is the adapter's position, then you can use StaggeredGridLayoutManager scrollToPosition
StaggeredGridLayoutManager staggeredGridLayoutManager = new StaggeredGridLayoutManager(1, StaggeredGridLayoutManager.VERTICAL);
staggeredGridLayoutManager.scrollToPosition(10);
recyclerView.setLayoutManager(staggeredGridLayoutManager);

This is the Kotlin code snippet but you can just get the point for scrolling to the item by position properly. The point is to declare the member variable for the layout manager and use its method to scroll.
lateinit var layoutManager: LinearLayoutManager
fun setupView() {
...
layoutManager = LinearLayoutManager(applicationContext)
mainRecyclerView.layoutManager = layoutManager
...
}
fun moveToPosition(position: Int) {
layoutManager.scrollToPositionWithOffset(position, 0)
}

try recycler_view.smoothScrollBy(0, 100); here 0 represents x-coordinate and 100 represents y-coordinate I used this in vertical recyclerView where I wanted to scroll to next position in the vertical list and for coming to the previous position I simply replaced 100 with -100

getChildCount() and getChildAt() return zero for RecyclerView

I am trying to get a count of RecyclerView children. I've tried this:
myRView.getChildCount();
And this:
LinearLayoutManager lm = ((LinearLayoutManager)myRViews.getLayoutManager());
lm.getChildCount();
I need to get a child View at certain position so that's why I need this. If I call getChildAt(index) I never get any child View.
Both of these methods return 0. How else to get a child View from RecyclerView? My items appear correctly and everything is working fine. I am calling these methods after I create my adapter and set it to RecyclerView. Thank you for your help.

I assume you are calling those methods before layout is calculated.
When you set the adapter, the layout will happen in the next view tree traversal. You can consider adding a ViewTreeObserver.

You could use your adapter getItemCount() function.
myRView.getAdapter.getItemCount();

The function is called when the view is not ready that's why it returns zero, use post() method will help.
myRView.post(new Runnable() {
#Override
public void run() {
Log.show("myRView.post " + myRView.getChildCount());
View childView = myRView.getChildAt(0);
}
});

getChildCount() may return 0 if you forget to set LayoutManager to RecyclerView.
myRView.setLayoutManager(mLayoutManager);