Develop Reference

Why does LinearLayoutManager.childAt() returns null?

I have a recylcerview where the method shown below gets called on each selection of an item. The method only runs sucessfully when I do not scroll the recyclerview, so the firstvisible is 0. The moment I scroll down a bit and select something the first and last visible are set correctly in the method, but as can be seen in the screenshot, for reasons I do not understand the childAt returns null instead of the view I can see on my app screen. In the screenshot for position 7, the first 4-6 returned a child.
Can someone explain to me how this can happen? From my pov, getChildAt() should always return a view in this scenario.

first and last are going to be the adapter positions of the data and not the position as laid out in the layout manager. See LinearLayoutManager#findFirstVisibleItemPosition. The children will always start with zero and increase from there.
That is why it works before your scroll since the child at the zeroth index in the layout manager is also the zeroth item in the adapter.
Here is a discussion about the various positions in RecyclerView.
It looks like you want to make changes to all visible items. Your first and last variables will have the correct start/end adapter positions that correspond to what is visible on the screen. You need the adapter positions to call the various "notify" methods.
So, given the adapter positions, we need a map to the views that are represented on the screen. As an example, the following code loops through every visible view and changes the background color of each view.
LinearLayoutManager lm = (LinearLayoutManager) Recycler.getLayoutManager();
// Get adapter positions for first and last visible items on screen.
int firstVisible = lm.findFirstVisibleItemPosition();
int lastVisible = lm.findLastVisibleItemPosition();
for (int i = firstVisible; i <= lastVisible; i++) {
// Find the view that corresponds to this position in the adapter.
View visibleView = lm.findViewByPosition(i);
visibleView.setBackgroundColor(getResources().getColor(android.R.color.holo_red_light));
}
If you use the child methods of the layout manager, you will need to loop from zero to LayoutManager.getChildCount() - 1 to make the changes. You will see each attached view which, I believe, can exceed the number of visible views.

Find if the first visible item in the recycler view is the first item of the list or not

I have a recycler view with 13 data items. I want to find out whether the first item of my list is visible or not?
I am aware about the methods like findFirstVisibleItemPosition and findLastVisibleItemPosition but they did not tell whether the first visible item is actually the first item of the list or not.
The problem that I am trying to solve is this, I have a view pager as the first item of my recycler view and I want to stop auto scroll when user scrolls down and it becomes completely invisible.
Please help if anyone has any idea about how to do this.

You can do it by checking if findFirstVisibleItemPosition is 0 or not, like this :
// layoutManager is your recycler view's layout manager
int position = layoutManager.findFirstVisibleItemPosition();
if(position != 0){
stopAutoScroll();
}else{
startAutoScroll();
}
i.e, if findFirstVisibleItemPosition returns 0 we should start auto scroll and if it is not 0, stop swiping.

Scrolling GridView only on Button Click

I want to achieve the following layout -
In this layout, I want to show all the available items to the user. Pressing right arrow will show the next 6 available items. This should happen via horizontal scrolling type motion.
Since items can be many, I want to use GridView for this because of its recycling optimisation. Optimisation is necessary because every item is a Linear Layout in itself.
Now, I can't use GridView because I want to show next 6 items using the arrows only. Also, it will be good if this happens in smooth scrolling type motion.
Can I make any tweak to Gridview so that I can use arrows to scroll GridView to show next 6 items or Is there any other ViewGroup available to achieve this along with the recycling optimisation.

This example is for listview you can refer for gridview also.
For a SmoothScroll with Scroll duration:
getListView().smoothScrollToPositionFromTop(position,offset,duration);
Parameters
position -> Position to scroll to
offset ---->Desired distance in pixels of position from the top of the view when scrolling is finished
duration-> Number of milliseconds to use for the scroll
Note: From API 11.
listview is quite lengthy and also with alphabet scroller. Then I found that the same function can take other parameters as well :)
To position the current selection:
int h1 = mListView.getHeight();
int h2 = v.getHeight();
mListView.smoothScrollToPositionFromTop(position, h1/2 - h2/2, duration);
Or
You can use RecyclerView :
You have to make use of LayoutManager for that. Follow the below steps.
1). First of all, declare LayoutManager in your Activity/Fragment. For example, I have taken LinearLayoutManager
private LinearLayoutManager mLinearLayoutManager;
2). Initialise the LinearLayoutManager and set that to your RecyclerView
mLinearLayoutManager = new LinearLayoutManager(this);
recyclerView.setLayoutManager(mLinearLayoutManager);
3). On your Button onClick, do this to scroll to the bottom of your RecyclerView.
mLinearLayoutManager.scrollToPosition(yourList.size() - 1); // yourList is the ArrayList that you are passing to your RecyclerView Adapter.
Hope this will help..!!

You could try giving corresponding list input to adapter at each click. And then call the notifydatasetchanged to reflect the changes on the View

How to programmatically snap to position on Recycler view with LinearSnapHelper

I have implemented a horizontal recyclerView with LinearSnapHelper, to implement a UI input that selects a particular configuration. Kinda like the old school number picker/selector or spinner. The item in the center is the selected position.
it works fine and all, but here's the problem. On initial start up, I need to programmatically set the position of the recycler view such that the selected item (the index of which was loaded from disk) is position in the center.
.scrollToPosition() wont work becuase it places the selected item in the begining.
now I know I can do all the math and calculate the x coordinate and manually set it, but thats a lot of redundant work because LinearSnapHelper is already doing this, and I feel like there should be a way to just reuse that logic, but with actually initiating a fling.
I need something like LinearSnapHelper.snapToPosition()

More general solution:
First scroll RecyclerView to make target item visible.
Than, take the object of target View and use SnapHelper to determine
distance for the final snap.
Finally scroll to target position.
NOTE: This works only because programmatically you are scrolling at the exact position & covering the missing distance by exact value using scrollBy instead of doing smooth scrolling
Code snippet:
mRecyclerView.scrollToPosition(selectedPosition);
mRecyclerView.post(() -> {
View view = mLayoutManager.findViewByPosition(selectedPosition);
if (view == null) {
Log.e(WingPickerView.class.getSimpleName(), "Cant find target View for initial Snap");
return;
}
int[] snapDistance = mSnapHelper.calculateDistanceToFinalSnap(mLayoutManager, view);
if (snapDistance[0] != 0 || snapDistance[1] != 0) {
mRecyclerView.scrollBy(snapDistance[0], snapDistance[1]);
}
}
});

Try calling smoothScrollToPosition on the RecyclerView object, and passing the position index (int)
mRecyclerView.smoothScrollToPosition(position);
Worked for me with a LinearLayoutManager and LinearSnapHelper. It animates the initial scroll, but at least snaps the item in position.
This is my first post on the stack, hope it helps :)

I have a recyclerView which I have added padding at the left and right with dummy views in the adapter. So that the first "actual" item can be snapped to.
I couldn't get smoothScrollToPosition(0) to work though for the initial snap. I used the following
recycler.scrollBy(snapHelper.calculateDistanceToFinalSnap(binding.recycler.getLayoutManager(), recycler.getChildAt(1))[0], 0);
Isn't the nicest looking way, but seems to work!

RecycleView: how to go to a row without scrolling

If the recyclerview has a list of items and I want to go to a specific row immediately when user enters the UI, so I don't want the user to see it scroll to reach that row. Is it achievable?
Thanks.

You can use method scrollToPositionWithOffset (int position, int offset)
Scroll to the specified adapter position with the given offset from
resolved layout start.
See documentation
Example:
//Scroll to item position 2 with offset 0
RECYCLERVIEW_LAYOUT_MANAGER.scrollToPositionWithOffset(2, 0);
Hope this will help~

I would say it is impossible.
RecyclerView#scrollToPosition causes sort of animation because it does not know the height of all rows initially. Not all items are loaded immediatly but one by one as you scroll down the list.

I use ListView and I solved the issue by calling setSelection(position)