Develop Reference

How to tell RecyclerView to start at specific item position

I want my RecyclerView with LinearLayoutManager to show up with scroll position at specific item after adapter got updated. (not first/last position)
Means the at first (re-)layout, this given position should be in visible area.
It should not layout with position 0 on top and scroll afterwards to target position.
My Adapter starts with itemCount=0, loads its data in thread and notifies its real count later. But the start position must be set already while count is still 0!
As of now I used some kind of post Runnable containingscrollToPosition but this has side effects (starts at first pos and jumps immediately to target position (0 -> target) and seems not to work well with DiffUtil (0 -> target -> 0))
Edit: To clearify: I need alternative to layoutManager.setStackFromEnd(true);, something like setStackFrom(position). ScrollToPosition does not work, if I call it when itemCount is still 0, so it gets ignored. If I call it when I notify that itemCount is now >0, it will layout from 0 and jumps short after to target position. And it fails completely if I use DiffUtil.DiffResult.dispatchUpdatesTo(adapter)`. (shows from 0, then scrolls to target position and then again back to position 0)

I found a solution myself:
I extended the LayoutManager:
class MyLayoutManager extends LinearLayoutManager {
private int mPendingTargetPos = -1;
private int mPendingPosOffset = -1;
#Override
public void onLayoutChildren(Recycler recycler, State state) {
if (mPendingTargetPos != -1 && state.getItemCount() > 0) {
/*
Data is present now, we can set the real scroll position
*/
scrollToPositionWithOffset(mPendingTargetPos, mPendingPosOffset);
mPendingTargetPos = -1;
mPendingPosOffset = -1;
}
super.onLayoutChildren(recycler, state);
}
#Override
public void onRestoreInstanceState(Parcelable state) {
/*
May be needed depending on your implementation.
Ignore target start position if InstanceState is available (page existed before already, keep position that user scrolled to)
*/
mPendingTargetPos = -1;
mPendingPosOffset = -1;
super.onRestoreInstanceState(state);
}
/**
* Sets a start position that will be used <b>as soon as data is available</b>.
* May be used if your Adapter starts with itemCount=0 (async data loading) but you need to
* set the start position already at this time. As soon as itemCount > 0,
* it will set the scrollPosition, so that given itemPosition is visible.
* #param position
* #param offset
*/
public void setTargetStartPos(int position, int offset) {
mPendingTargetPos = position;
mPendingPosOffset = offset;
}
}
It stores my target position. If onLayoutChildren is called by RecyclerView, it checks if adapters itemCount is already > 0. If true, it calls scrollToPositionWithOffset().
So I can tell immediately what position should be visible, but it will not be told to LayoutManager before position exists in Adapter.

You can try this, it will scroll to a position you want:
rv.getLayoutManager().scrollToPosition(positionInTheAdapter).

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);
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.
rv.getLayoutManager().scrollToPosition(youPositionInTheAdapter);

None of the methods above worked for me. I did the following using ViewTreeObserver that is triggered once its children have been added/changed visibility.
recyclerView.apply {
adapter = ...
layoutManager = ...
val itemCount = adapter?.itemCount ?: 0
if(itemCount > 1) {
viewTreeObserver.addOnGlobalLayoutListener(object: ViewTreeObserver.OnGlobalLayoutListener {
override fun onGlobalLayout() {
viewTreeObserver.removeOnGlobalLayoutListener(this)
(layoutManager as? LinearLayoutManager)?.scrollToPosition(#PositionToStartAt)
}
}
}
Go ahead and set #PositionToStartAt to a specific value. You can also ensure that the RecyclerView initial position setting gets triggered once a specific number of children have been laid out to ensure it is set correctly.
if(recyclerView.childCount() > #PositionCheck) {
viewTreeObserver.removeOnGlobalLayoutListener(this)
(layoutManager as? LinearLayoutManager)?.scrollToPosition(#PositionToStartAt)
}

If your only motive is to start the recyclerView from a specific position without any scroll-like animation I'll suggest using StaggeredGridLayoutManager
val staggeredGridLayoutManager = StaggeredGridLayoutManager(1, StaggeredGridLayoutManager.HORIZONTAL)//or VERTICAL
staggeredGridLayoutManager.scrollToPosition(specificPosition)
recyclerView.apply{
layoutManager = staggeredGridLayoutManager
}

Another contribution to a long running question...
As mentioned, layoutManager.scrollToPosition/WithOffset() does work to get the RecyclerView positioned, but timing this can be tricky. For example with variable length item views the RecyclerView has to work hard to get all the prior item views measured.
The approach below simply delays telling the RecyclerView about the prior items, then calls notifyItemRangeInserted(0, offset). This enabled the view to appear in the right place, with no visible scrolling at the start, and ready to scroll back.
private List<Bitmap> bitmaps = new ArrayList<>();
...
private class ViewAdapter extends RecyclerView.Adapter<ViewHolder> {
private volatile int offset;
private boolean offsetCancelled;
ViewAdapter(int initialOffset) {
this.offset = initialOffset;
this.offsetCancelled = initialOffset > 0;
}
#Override
public int getItemCount() {
return bitmaps.size() - offset;
}
#NonNull
#Override
public ViewHolder onCreateViewHolder(#NonNull ViewGroup parent, int viewType) {
ImageView imageView = new ImageView(MyActivity.this); // Or getContext() from a Fragment
RecyclerView.LayoutParams lp = new RecyclerView.LayoutParams(ViewGroup.LayoutParams.MATCH_PARENT, ViewGroup.LayoutParams.WRAP_CONTENT);
imageView.setLayoutParams(lp);
return new ViewHolder(imageView);
}
#Override
public void onBindViewHolder(#NonNull ViewHolder holder, int position) {
holder.imageView.setImageBitmap(bitmaps.get(position + offset));
if (!offsetCancelled) {
offsetCancelled = true;
recyclerView.post(() -> {
int previousOffset = offset;
offset = 0;
notifyItemRangeInserted(0, previousOffset);
Log.i(TAG, "notifyItemRangeInserted(0, " + previousOffset + ")");
});
}
}
}
private class ViewHolder extends RecyclerView.ViewHolder {
private final ImageView imageView;
ViewHolder(#NonNull ImageView itemView) {
super(itemView);
this.imageView = itemView;
}
}
For context, this is a full example, based around ImageView's and Bitmap's. The key bit is the use of the offset in getItemCount() and onBindViewHolder().

RecyclerView notifyDataSetChange scroling all the way up, not working as i want it to

I am writing a photo picker for Facebook inside my app, i have a recyclerview with a grid layout and i want to prevent for scrolling up, i was able to do this by using scrollToPosition and this works but not the way i want
Problem
When i click in a photo on the 2 row that row jumps to the top and becomes the number 1 visible row, if i click the 3 row the samething happens.
I don't want the recycler to move if the view is visible it should remain the same, so if i click on a a photo that is on the last visible row i want the scroll to stay the same, i don't want it to make the last row the first.
Tries to solve it
I tried several things to fix this, i tried calling setNestedScrollingEnabled i followed this How to disable RecyclerView scrolling?
public static void onItemClick(int position){
//picker.setNestedScrollingEnabled(false);
for(int k = 0; k<photoBag.size();k++) {
if(k == position)
photoBag.set(position, new PhotoBag(photoBag.get(position).getPhoto(), true)); //Here im marking the photo to selected
else
photoBag.set(k, new PhotoBag(photoBag.get(k).getPhoto(), false));//Here im setting unselecting all the other photos
}
picker.setAdapter(adapter);
adapter.notifyDataSetChanged();
picker.scrollToPosition(position);
//Log.d("FacebookPicker", "position " + grid.findFirstCompletelyVisibleItemPosition());
//picker.setNestedScrollingEnabled(true);
}
I thought that maybe disabling the scroll would lock the recyclerview on the corrent position but it didn't jumps right up.
I also tried getting the Vertical offset and set it after calling notifyDataSetChange but i can't find a way to set the offset programmatically
EDIT
Adapter
class PickerAdapter extends RecyclerView.Adapter<PickerAdapter.PickerAdapterHolder> {
public final String TAG = "PickerAdapter";
private ArrayList<PhotoBag> photoBag;
private Context context;
private OnClickListener onClickListener;
class PickerAdapterHolder extends RecyclerView.ViewHolder implements View.OnClickListener {
ImageView photo;
ImageView imageBorder;
PickerAdapterHolder(View view) {
super(view);
photo = (ImageView) view.findViewById(R.id.photoItem);
photo.setOnClickListener(this);
}
#Override
public void onClick(View v) {
switch (v.getId()) {
case R.id.photoItem:
FacebookPhotoPicker.onItemClick(getAdapterPosition()); //i know that there are better ways to get the clicked item from other class but since im still debuging i don't need to worry about performace i just need it to work
break;
}
}
}
PickerAdapter(Context context, ArrayList<PhotoBag> itemList) {
this.photoBag = itemList;
this.context = context;
}
#Override
public PickerAdapterHolder onCreateViewHolder(ViewGroup parent, int viewType) {
View layoutView = LayoutInflater.from(parent.getContext()).inflate(R.layout.facebook_picker_item, null);
return new PickerAdapterHolder(layoutView);
}
#Override
public void onBindViewHolder(final PickerAdapterHolder holder, final int position) {
if(photoBag.get(position).isSelected()){
int border = Math.round(TypedValue.applyDimension(TypedValue.COMPLEX_UNIT_DIP, 5, context.getResources().getDisplayMetrics()));
Bitmap photo = photoBag.get(position).getPhoto();
photo = Bitmap.createScaledBitmap(photo,photo.getWidth() - (border*2), photo.getHeight() - (border*2), false);
photo = addWhiteBorder(photo,border);
holder.photo.setImageBitmap(photo);
}else {
holder.photo.setImageBitmap(photoBag.get(position).getPhoto());
}
}
#Override
public int getItemCount() {
return this.photoBag.size();
}
private Bitmap addWhiteBorder(Bitmap bmp, int borderSize) {
Bitmap bmpWithBorder = Bitmap.createBitmap(bmp.getWidth() + borderSize * 2, bmp.getHeight() + borderSize * 2, bmp.getConfig());
Canvas canvas = new Canvas(bmpWithBorder);
canvas.drawColor(Color.WHITE);
canvas.drawBitmap(bmp, borderSize, borderSize, null);
return bmpWithBorder;
}

remove those 2 lines from onItemClick
picker.setAdapter(adapter);
picker.scrollToPosition(position);
every time you setAdapter it resets position, and now you don't need to set a new position again.
this should work. If it doesn't, check this answer of mine (and their comments) about providing ID How to remain at a scroll position in RecyclerView after adding items at its first index and call notifydatasetchange

How to keep ViewHolder's width after view was recycled

I'm using RecyclerView as horizontal list to show my images.
If I scroll to the fifth picture, the first two or three are recycled and ViewHolder loses its width. If I scroll back to the first image, the images are loaded again and that leads to jumps while scrolling.
Here is R.layout.fragment_details_view_img_item
<ImageView
xmlns:android="http://schemas.android.com/apk/res/android"
android:id="#+id/details_view_img_item"
android:background="#color/red"
android:adjustViewBounds="true"
android:layout_width="wrap_content"
android:layout_height="wrap_content" >
</ImageView>
My ViewHolder and Adapter:
private class ViewHolder extends RecyclerView.ViewHolder {
ImageView img;
public ViewHolder(ImageView imgV){
super(imgV);
img = imgV;
}
}
private class ImageListAdapter extends RecyclerView.Adapter<ViewHolder> {
[...]
#Override
public ViewHolder onCreateViewHolder(ViewGroup parent, int position) {
View v = LayoutInflater.from(getActivity()).inflate(R.layout.fragment_details_view_img_item, parent, false);
v.setOnClickListener(listener);
logDebug("onCreateViewHolder");
return new ViewHolder((ImageView) v);
}
#Override
public void onBindViewHolder(ViewHolder viewHolder, int i) {
logDebug("onBindViewHolder");
ImageItem item = data.get(i);
if (item != null) {
ImageView imgView = viewHolder.img;
imgView.setTag(item);
String imgurl = ImageUtil.imgUrlForAvailableHeightInPX(item, parentHeight);
ImageLoader.instance().loadASYNC(imgurl, imgView);
}
}
#Override
public void onViewRecycled(ViewHolder holder) {
super.onViewRecycled(holder);
logDebug("onViewRecycled: " + holder.img.getTag());
}
}
So how can I keep ViewHolder's width?

Before I start to approach this problem, one thing needs to be clear. The ViewHolder doesn't have any width, the ImageView it "holds" does have width, and that's what you're trying to control.
Now, considering this, your issue is (after making certain assumptions from looking at your code) that you need to know the width of a certain image when it is at a certain given height and while maintaining aspect ratio - before it arrives from the server.
This is a tricky one.
One option would be to preload all your images. This, however, is very costly with memory and could lead to memory crashes.
A better option would be to load all the images' details, without actually downloading the images' pixels. Then, you'll need to remember the aspect ratio of all the images in some cache, and set the dimension of the image you're loading prior to actually downloading the image contents.
To download an image's dimensions without downloading the image itself, you should use something like this:
public float getImageAspectRatio(InputStream inputStreamFromServer)
{
BitmapFactory.Options decodeOptions = new BitmapFactory.Options();
decodeOptions.inJustDecodeBounds = true;
BitmapFactory.decodeStream(inputStreamFromServer, null, decodeOptions);
final float imageDesiredWidth = decodeOptions.outWidth;
final float imageDesiredHeight = decodeOptions.outHeight;
return imageDesiredWidth / imageDesiredHeight;
}
Once you have this method, you'll need to preload all your images using this function:
private float[] mAspectRatios;
public void decodeAllAspectRatios(List<String> imageUrls)
{
mAspectRatios = new float[imageUrls.size()];
InputStream inputStream;
int index = 0;
for (String url : imageUrls)
{
// Get the input stream from the image url using whatever method you use.
mAspectRatios[index] = getImageAspectRatio(inputStream);
index++;
}
}
Important: Your RecyclerView should not begin working until this method finished working.
Once you have preloaded all your aspect ratios, we go back to your ViewHolder:
#Override
public void onBindViewHolder(ViewHolder viewHolder, int i) {
logDebug("onBindViewHolder");
ImageItem item = data.get(i);
if (item != null) {
ImageView imgView = viewHolder.img;
// set the layout params of the image, making it fit the correct size prior to loading the bitmap.
imgView.setLayoutParams(new LayoutParams(parentHeight * mAspectRatios[i], parentHeight));
imgView.setTag(item);
String imgurl = ImageUtil.imgUrlForAvailableHeightInPX(item, parentHeight);
ImageLoader.instance().loadASYNC(imgurl, imgView);
}
}
So long as you want your RecyclerView to display varying width images depending on their aspect ratio, I believe this is your best option.

GridView android 75x75 exact screen size of square

I want to make a GridView that contains 75dp width and 75dp height imageView. And also i want this square fill the screen size, with no space in between. Right now i am just using 500 count. So its randomly printing 500 square images. Its not filling up the space. Please check the image below for tablet version.
Here is my Adapter:
public class ImageAdapter extends BaseAdapter {
private Context mContext;
public ImageAdapter(Context c) {
mContext = c;
}
public int getCount() {
return 500;
}
public Object getItem(int position) {
return null;
}
public long getItemId(int position) {
return 0;
}
// create a new ImageView for each item referenced by the Adapter
public View getView(int position, View convertView, ViewGroup parent) {
ImageView imageView;
if (convertView == null) { // if it's not recycled, initialize some
// attributes
imageView = new ImageView(mContext);
imageView.setLayoutParams(new GridView.LayoutParams(75, 75));
imageView.setScaleType(ImageView.ScaleType.CENTER_CROP);
imageView.setPadding(0, 0, 0, 0);
} else {
imageView = (ImageView) convertView;
}
imageView.setImageResource(R.drawable.sample_0);
return imageView;
}
}

In getCount you could divide the area of the screen by the area of the squares you're filling it with.
public int getCount() {
Display display = getWindowManager().getDefaultDisplay();
Point size = new Point();
display.getSize(size);
int area = size.x * size.y;
// TODO: you may want to pass in the square dimensions or make them constants
return area / (75 * 75);
}
Of course, this isn't right, since it uses the display and not the parent ViewGroup. Instead of trying to do this all in the adapter, have the constructor for this take in the dimensions of the parent and do the math there, saving an instance variable with the count:
private int numBoxes = 0;
private static final SQUARE_SIZE = 75;
public ImageAdapter(Context c, ViewGroup parent) {
mContext = c;
numBoxes = (parent.getWidth() * parent.getHeight()) / (SQUARE_SIZE * SQUARE_SIZE);
}
public int getCount() {
return numBoxes;
}

Expanding on the answer by Nick White, you need to make sure the parent layout has been laid out before you can get the height and width. If the parent width and/or height are "wrap_content" or "match_parent", the getHeight() and getWidth() will return zero in the adapter's constructor, if the adapter is being created in the Activity onCreate().
This answer gives a way to listen for onLayout events and handle sizing. I did try to do something similar, but I found the results in my case were less than perfect, as the grid would draw then redraw after I applied the size changes. So far the best results I've had with grid sizing is to use qualified resource values to size the image views inside the grid. The resource size value is then set according to the screen density or size by creating various values in values-xxx resource folders.
In my case, I was altering the size of the cell images to make a certain number fit the screen by applying something like:
Integer size = getResources().getInteger(R.int.image_size));
imageView.setLayoutParams(new GridView.LayoutParams(size, size));
It should be possible to do something similar to calculate the number of columns for the screen size.

Scroll a ListView by pixels in Android

I want to scroll the a ListView in Android by number of pixels. For example I want to scroll the list 10 pixels down (so that the first item on the list has its top 10 pixel rows hidden).
I thought the obviously visible scrollBy or scrollTo methods on ListView would do the job, but they don't, instead they scroll the whole list wrongly (In fact, the getScrollY always return zero even though I have scrolled the list using my finger.)
What I'm doing is I'm capturing Trackball events and I want to scroll the listview smoothly according to the motion of the trackball.

The supported way to scroll a ListView widget is:
mListView.smoothScrollToPosition(position);
http://developer.android.com/reference/android/widget/AbsListView.html#smoothScrollToPosition(int)
However since you mentioned specifically that you would like to offset the view vertically, you must call:
mListView.setSelectionFromTop(position, yOffset);
http://developer.android.com/reference/android/widget/ListView.html#setSelectionFromTop(int,%20int)
Note that you can also use smoothScrollByOffset(yOffset). However it is only supported on API >= 11
http://developer.android.com/reference/android/widget/ListView.html#smoothScrollByOffset(int)

If you look at the source for the scrollListBy() method added in api 19 you will see that you can use the package scoped trackMotionScroll method.
public class FutureListView {
private final ListView mView;
public FutureListView(ListView view) {
mView = view;
}
/**
* Scrolls the list items within the view by a specified number of pixels.
*
* #param y the amount of pixels to scroll by vertically
*/
public void scrollListBy(int y) {
if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.KITKAT) {
mView.scrollListBy(y);
} else {
// scrollListBy just calls trackMotionScroll
trackMotionScroll(-y, -y);
}
}
private void trackMotionScroll(int deltaY, int incrementalDeltaY) {
try {
Method method = AbsListView.class.getDeclaredMethod("trackMotionScroll", int.class, int.class);
method.setAccessible(true);
method.invoke(mView, deltaY, incrementalDeltaY);
} catch (Exception ex) {
throw new RuntimeException(ex);
};
}
}

Here is some code from my ListView subclass. It can easily be adapted so it can be used in Activity code.
getListItemsHeight() returns the total pixel height of the list, and fills an array with vertical pixel offsets of each item. While this information is valid, getListScrollY() returns the current vertical pixel scroll position, and scrollListToY() scrolls the list to pixel position.
If the size or the content of the list changes, getListItemsHeight() has to be called again.
private int m_nItemCount;
private int[] m_nItemOffY;
private int getListItemsHeight()
{
ListAdapter adapter = getAdapter();
m_nItemCount = adapter.getCount();
int height = 0;
int i;
m_nItemOffY = new int[m_nItemCount];
for(i = 0; i< m_nItemCount; ++i){
View view = adapter.getView(i, null, this);
view.measure(MeasureSpec.makeMeasureSpec(0, MeasureSpec.UNSPECIFIED),
MeasureSpec.makeMeasureSpec(0, MeasureSpec.UNSPECIFIED));
m_nItemOffY[i] = height;
height += view.getMeasuredHeight();
}
return height;
}
private int getListScrollY()
{
int pos, nScrollY, nItemY;
View view;
pos = getFirstVisiblePosition();
view = getChildAt(0);
nItemY = view.getTop();
nScrollY = m_nItemOffY[pos] - nItemY;
return nScrollY;
}
private void scrollListToY(int nScrollY)
{
int i, off;
for(i = 0; i < m_nItemCount; ++i){
off = m_nItemOffY[i] - nScrollY;
if(off >= 0){
setSelectionFromTop(i, off);
break;
}
}
}

For now, ListViewCompat is probably a better solution.
android.support.v4.widget.ListViewCompat.scrollListBy(#NonNull ListView listView, int y)

if you want to move by pixels then u can use this
public void scrollBy(ListView l, int px){
l.setSelectionFromTop(l.getFirstVisiblePosition(),l.getChildAt(0).getTop() - px);
}
this works for even ones with massive headers