Develop Reference

Implementing auto fling at an interval on a Recycler View in android

I have a recycler view with the following attributes in the xml file.
NOTE : I AM DISPLAYING ONLY ONE ITEM OF AT A TIME ON THE SCREEN FOR THIS RECYCLER VIEW.
<MyCustomRecyclerView
android:id="#+id/my_rv"
android:layout_width="match_parent"
android:layout_height="wrap_content"
android:clipToPadding="false"
android:nestedScrollingEnabled="false"
android:orientation="horizontal"
android:overScrollMode="never"
android:paddingHorizontal="4dp"
app:layoutManager="androidx.recyclerview.widget.LinearLayoutManager"/>
And I am using a PagerSnapHelper to move to the position left or right based on the center of the view on the screen.
val snapHelper = PagerSnapHelper()
snapHelper.attachToRecyclerView(this)
It's working fine for a manual scroll action performed.
Now, I want to add an auto scroll as well after a certain interval of time (say 2.5 seconds). I have created a handler and posted a runnable on it with a delay of 2.5 seconds. I am trying to call fling(velocityX, velocityY) of the RecyclerView with a good enough value of velocityX
val scrollHandler = Handler()
val SCROLL_INTERVAL:Long = 2500 //scroll period in ms
val runnable = object : Runnable {
override fun run() {
//velocityX = 7500
fling(7500, 0)
scrollHandler.postDelayed(this, SCROLL_INTERVAL.toLong())
}
}
But the PagerSnaperHelper::findTargetSnapPosition() not returning correct target position because the View actually has not changed on the screen as in case of a manual scroll. It is returning the position of the element which is already visible on the screen.
#Override
public int findTargetSnapPosition(RecyclerView.LayoutManager layoutManager, int velocityX,
int velocityY) {
final int itemCount = layoutManager.getItemCount();
if (itemCount == 0) {
return RecyclerView.NO_POSITION;
}
final OrientationHelper orientationHelper = getOrientationHelper(layoutManager);
if (orientationHelper == null) {
return RecyclerView.NO_POSITION;
}
// A child that is exactly in the center is eligible for both before and after
View closestChildBeforeCenter = null;
int distanceBefore = Integer.MIN_VALUE;
View closestChildAfterCenter = null;
int distanceAfter = Integer.MAX_VALUE;
// Find the first view before the center, and the first view after the center
final int childCount = layoutManager.getChildCount();
for (int i = 0; i < childCount; i++) {
final View child = layoutManager.getChildAt(i);
if (child == null) {
continue;
}
final int distance = distanceToCenter(layoutManager, child, orientationHelper);
if (distance <= 0 && distance > distanceBefore) {
// Child is before the center and closer then the previous best
distanceBefore = distance;
closestChildBeforeCenter = child;
}
if (distance >= 0 && distance < distanceAfter) {
// Child is after the center and closer then the previous best
distanceAfter = distance;
closestChildAfterCenter = child;
}
}
// Return the position of the first child from the center, in the direction of the fling
final boolean forwardDirection = isForwardFling(layoutManager, velocityX, velocityY);
if (forwardDirection && closestChildAfterCenter != null) {
return layoutManager.getPosition(closestChildAfterCenter);
} else if (!forwardDirection && closestChildBeforeCenter != null) {
return layoutManager.getPosition(closestChildBeforeCenter);
}
// There is no child in the direction of the fling. Either it doesn't exist (start/end of
// the list), or it is not yet attached (very rare case when children are larger then the
// viewport). Extrapolate from the child that is visible to get the position of the view to
// snap to.
View visibleView = forwardDirection ? closestChildBeforeCenter : closestChildAfterCenter;
if (visibleView == null) {
return RecyclerView.NO_POSITION;
}
int visiblePosition = layoutManager.getPosition(visibleView);
int snapToPosition = visiblePosition
+ (isReverseLayout(layoutManager) == forwardDirection ? -1 : +1);
if (snapToPosition < 0 || snapToPosition >= itemCount) {
return RecyclerView.NO_POSITION;
}
return snapToPosition;
}
I would like to know how can I achieve the desired result?

I got a workaround to solve this. Before calling fling(), I called scrollBy(x,y) to scroll the items as if it would have happened during a manual scroll.
val runnable = object : Runnable {
override fun run() {
scrollBy(400,0)
//velocityX = 7500
fling(7500, 0)
scrollHandler.postDelayed(this, SCROLL_INTERVAL.toLong())
}
}

How to scroll only one item by one in recyclerview?

I have a horizontal list which it's using a recycler view and a PagerSnapHelper to simulate a view pager.
I want to swipe scroll one element at the time, but the problem is that i can scroll to the last element with one swipe.
So my problem is that I want to stop the swipe after one element swiped so if someone has any advice will be appreciated.

You could try adding a logic by checking the current position as mentioned in below post:
SnapHelper Item Position
public void onScrollStateChanged(RecyclerView recyclerView, int newState) {
super.onScrollStateChanged(recyclerView, newState);
if(newState == RecyclerView.SCROLL_STATE_IDLE) {
View centerView = snapHelper.findSnapView(mLayoutManager);
int pos = mLayoutManager.getPosition(centerView);
Log.e("Snapped Item Position:",""+pos);
}
}
or Something like this:
LinearSnapHelper snapHelper = new LinearSnapHelper() {
#Override
public int findTargetSnapPosition(RecyclerView.LayoutManager layoutManager, int velocityX, int velocityY) {
View centerView = findSnapView(layoutManager);
if (centerView == null)
return RecyclerView.NO_POSITION;
int position = layoutManager.getPosition(centerView);
int targetPosition = -1;
if (layoutManager.canScrollHorizontally()) {
if (velocityX < 0) {
targetPosition = position - 1;
} else {
targetPosition = position + 1;
}
}
if (layoutManager.canScrollVertically()) {
if (velocityY < 0) {
targetPosition = position - 1;
} else {
targetPosition = position + 1;
}
}
final int firstItem = 0;
final int lastItem = layoutManager.getItemCount() - 1;
targetPosition = Math.min(lastItem, Math.max(targetPosition, firstItem));
return targetPosition;
}
};
snapHelper.attachToRecyclerView(recyclerView);
Ref: https://ogeek.cn/qa/?qa=441913/&show=441914

How to center the Clicked position in the Recyclerview

I want to center the clicked position in the Recyclerview. I am able to scroll the Recyclerview to certain position but i want to middle that position in the screen.
I used this method to scroll to that position.
videoRecyclerview.scrollToPosition(position);

If you are using a RecyclerView and LinearLayoutManager this will work:
private void scrollToCenter(View v) {
int itemToScroll = mRecyclerView.getChildPosition(v);
int centerOfScreen = mRecyclerView.getWidth() / 2 - v.getWidth() / 2;
mLayoutManager.scrollToPositionWithOffset(itemToScroll, centerOfScreen);
}

if you use linearlayoutManager, you can use this code,
linearLayoutManager.scrollToPositionWithOffset(2, 20);
(linearLayoutManager.void scrollToPositionWithOffset (int position,
int offset))
Setting the offset to 0 should align with the top

first move scroll to your item, but whenever recyclerView scrolls it just brings the item in visible region, it is never sure that the item is in center or not, so we find the center item and then check if we are on next to center to item or behind it, here is working logic
recyclerView.smoothScrollToPosition(index);
int firstVisibleItemPosition = rvLayoutManager.findFirstVisibleItemPosition();
int lastVisibleItemPosition = rvLayoutManager.findLastVisibleItemPosition();
int centerPosition = (firstVisibleItemPosition + lastVisibleItemPosition) / 2;
if (index > centerPosition) {
recyclerView.smoothScrollToPosition(index + 1);
} else if (index < centerPosition) {
recyclerView.smoothScrollToPosition(index - 1);
}

If you need smooth scroll to centre for LieanerLayoutManager both horizontal & vertical
Copy the entire code and simply call** scrollToCenter:
public void scrollToCenter(LinearLayoutManager layoutManager, RecyclerView recyclerList, int clickPosition) {
RecyclerView.SmoothScroller smoothScroller = createSnapScroller(recyclerList, layoutManager);
if (smoothScroller != null) {
smoothScroller.setTargetPosition(clickPosition);
layoutManager.startSmoothScroll(smoothScroller);
}
}
// This number controls the speed of smooth scroll
private static final float MILLISECONDS_PER_INCH = 70f;
private final static int DIMENSION = 2;
private final static int HORIZONTAL = 0;
private final static int VERTICAL = 1;
#Nullable
private LinearSmoothScroller createSnapScroller(RecyclerView mRecyclerView, RecyclerView.LayoutManager layoutManager) {
if (!(layoutManager instanceof RecyclerView.SmoothScroller.ScrollVectorProvider)) {
return null;
}
return new LinearSmoothScroller(mRecyclerView.getContext()) {
#Override
protected void onTargetFound(View targetView, RecyclerView.State state, Action action) {
int[] snapDistances = calculateDistanceToFinalSnap(layoutManager, targetView);
final int dx = snapDistances[HORIZONTAL];
final int dy = snapDistances[VERTICAL];
final int time = calculateTimeForDeceleration(Math.max(Math.abs(dx), Math.abs(dy)));
if (time > 0) {
action.update(dx, dy, time, mDecelerateInterpolator);
}
}
#Override
protected float calculateSpeedPerPixel(DisplayMetrics displayMetrics) {
return MILLISECONDS_PER_INCH / displayMetrics.densityDpi;
}
};
}
private int[] calculateDistanceToFinalSnap(#NonNull RecyclerView.LayoutManager layoutManager, #NonNull View targetView) {
int[] out = new int[DIMENSION];
if (layoutManager.canScrollHorizontally()) {
out[HORIZONTAL] = distanceToCenter(layoutManager, targetView,
OrientationHelper.createHorizontalHelper(layoutManager));
}
if (layoutManager.canScrollVertically()) {
out[VERTICAL] = distanceToCenter(layoutManager, targetView,
OrientationHelper.createHorizontalHelper(layoutManager));
}
return out;
}
private int distanceToCenter(#NonNull RecyclerView.LayoutManager layoutManager,
#NonNull View targetView, OrientationHelper helper) {
final int childCenter = helper.getDecoratedStart(targetView)
+ (helper.getDecoratedMeasurement(targetView) / 2);
final int containerCenter;
if (layoutManager.getClipToPadding()) {
containerCenter = helper.getStartAfterPadding() + helper.getTotalSpace() / 2;
} else {
containerCenter = helper.getEnd() / 2;
}
return childCenter - containerCenter;
}

it's worked for me with this code :
layoutManager.scrollToPositionWithOffset(pos - 1,0);

//on the click callback
view.OnClickListener { callback?.onItemClicked(it)}
// code in activity or your fragment
override fun onItemClicked(view: View) {
val position = recyclerView.getChildLayoutPosition(view)
recyclerView.smoothScrollToPosition(position)
}

simplest hack ever, this was good enough for me:
videoRecyclerview.scrollToPosition(position+2);
if position+2 is within the arraylist.

Header in listview like instagram disappears

I have a problem in listview. If I scroll list, all is good. But if I click on checkbox, header disappears. I checked, notifyOfChange() not started. I think this is due to the drawing of view. Who knows how to make the header is drawn in the last instance.
I use the following code:
#Override
public void onScroll(AbsListView view, int firstVisibleItem, int visibleItemCount, int totalItemCount) {
//the listview has only few children (of course according to the height of each child) who are visible
for(int i=0; i < list.getChildCount(); i++)
{
View child = list.getChildAt(i);
MuscleViewHolder holder = (MuscleViewHolder) child.getTag();
//if the view is the first item at the top we will do some processing
if(i == 0)
{
boolean isAtBottom = child.getHeight() <= holder.headerLayout.getBottom();
int offset = holder.previousTop - child.getTop();
if(!(isAtBottom && offset > 0))
{
holder.previousTop = child.getTop();
holder.headerLayout.offsetTopAndBottom(offset);
holder.headerLayout.invalidate();
}
} //if the view is not the first item it "may" need some correction because of view re-use
else if (holder.headerLayout.getTop() != 0)
{
int offset = -1 * holder.headerLayout.getTop();
holder.headerLayout.offsetTopAndBottom(offset);
holder.previousTop = 0;
holder.headerLayout.invalidate();
}
}
}

The problem is solved. I replace the code to the following
if(!(isAtBottom && offset > 0)) {
holder.setPreviousTop(child.getTop());
if(itNotScroll){
holder.getHeaderLayout().offsetTopAndBottom(-holder.getPreviousTop());
itNotScroll = false;
} else {
holder.getHeaderLayout().offsetTopAndBottom(offset);
}
holder.getHeaderLayout().invalidate();
}
And yet, I did so, so that you can open only one element in expaned listview

How to set the point, after which the ViewPager swipes to another View?

If more than 50% of the follower of View B is visible,
on release the ViewPager will animate to the View B instead of goind back to View A.
How can I change this point of switching from 50% to x%?

You can do this by overriding the onScrollChanged() function, here's an example:
#Override
public void onScrollChanged(ViewPager vp, int x, int y, int oldx, int oldy) {
// We take the last son in the ViewPager
View view = (View) vp.getChildAt(vp.getChildCount() - 1);
if (diff < view.getBottom()/2) {
// do the animation
}
}
Let me know if that worked or not (in order for me to know if I should remove the Answer from here).
Thanks

You can build your own customized ViewPager and override the proper method with your own float-value. something like :
public CustomViewpager extends ViewPager {
#Override
private int determineTargetPage(int currentPage, float pageOffset, int velocity, int deltaX) {
int targetPage;
if (Math.abs(deltaX) > mFlingDistance && Math.abs(velocity) > mMinimumVelocity) {
targetPage = velocity > 0 ? currentPage : currentPage + 1;
} else {
//change your values here for whatever you need for your purposes
final float truncator = currentPage >= mCurItem ? 0.4f : 0.6f;
targetPage = (int) (currentPage + pageOffset + truncator);
}
if (mItems.size() > 0) {
final ItemInfo firstItem = mItems.get(0);
final ItemInfo lastItem = mItems.get(mItems.size() - 1);
// Only let the user target pages we have items for
targetPage = Math.max(firstItem.position, Math.min(targetPage, lastItem.position));
}
return targetPage;
}
}