How to move the snap position from center to left of RecycleView using SnapHelper? - android-recyclerview

Question

I have an RecycleView that contains ImageViews and my question is how can i move the snap to be on the left side of the RecycleView instead of the center?
When i move the ImageViews they get snapped in the center and I can move them to the left or right inside that "snap window" by overriding the CalculateDistanceToFinalSnap method. I think I would now need to move that "snap window" to the left side of the RecycleView but I don't know how, or maybe there is another way, please help.
Here is a image of my problem, maybe it will help you to understand more clearly:
image

Answer 1

#Jessie Zhang -MSFT's solution works for me. The code was a little oddly formatted and I had some difficulty bringing it over. Here is the same solution (for a horizontal snap only) in Kotlin.
class StartSnapHelper: LinearSnapHelper() {
override fun calculateDistanceToFinalSnap(layoutManager: RecyclerView.LayoutManager, targetView: View): IntArray? {
return if (layoutManager.canScrollHorizontally()) {
val outer = mutableListOf<Int>()
outer.add(distanceToStart(targetView, getHorizontalHelper(layoutManager)))
outer.add(0)
outer.toIntArray()
} else {
super.calculateDistanceToFinalSnap(layoutManager, targetView)
}
}
override fun findSnapView(layoutManager: RecyclerView.LayoutManager?): View? {
return if (layoutManager is LinearLayoutManager) {
if (layoutManager.canScrollHorizontally()) {
getStartView(layoutManager, getHorizontalHelper(layoutManager))
} else {
super.findSnapView(layoutManager)
}
} else {
super.findSnapView(layoutManager)
}
}
private fun distanceToStart(targetView: View, helper: OrientationHelper): Int {
return helper.getDecoratedStart(targetView) - helper.startAfterPadding
}
private fun getStartView(layoutManager: RecyclerView.LayoutManager, orientationHelper: OrientationHelper): View? {
val firstChild = (layoutManager as LinearLayoutManager).findFirstVisibleItemPosition()
val isLastItem = (layoutManager.findLastCompletelyVisibleItemPosition() == layoutManager.itemCount - 1)
if (firstChild == RecyclerView.NO_POSITION || isLastItem) {
return null
}
val child = layoutManager.findViewByPosition(firstChild)
return if (orientationHelper.getDecoratedEnd(child) >= orientationHelper.getDecoratedMeasurement(child) / 2
&& orientationHelper.getDecoratedEnd(child) > 0) {
child;
} else {
if (layoutManager.findFirstCompletelyVisibleItemPosition() == layoutManager.itemCount -1) {
null
} else {
layoutManager.findViewByPosition(firstChild + 1)
}
}
}
private fun getHorizontalHelper(layoutManager: RecyclerView.LayoutManager): OrientationHelper {
return OrientationHelper.createHorizontalHelper(layoutManager)
}
}

Answer 2

I have achieved this function ,we juse need to create a class and extent class LinearSnapHelper and override method CalculateDistanceToFinalSnap and FindSnapView. You can check out the full demo here .
The main code is as follows:
public class StartSnapHelper: LinearSnapHelper
{
private OrientationHelper mVerticalHelper, mHorizontalHelper;
public StartSnapHelper()
{
}
public override void AttachToRecyclerView(RecyclerView recyclerView)
{
base.AttachToRecyclerView(recyclerView);
}
public override int[] CalculateDistanceToFinalSnap(RecyclerView.LayoutManager layoutManager, View targetView)
{
//return base.CalculateDistanceToFinalSnap(layoutManager, targetView);
int[] outer = new int[2];
if (layoutManager.CanScrollHorizontally())
{
outer[0] = distanceToStart(targetView, getHorizontalHelper(layoutManager));
} else {
outer[0] = 0;
}
if (layoutManager.CanScrollVertically()) {
outer[1] = distanceToStart(targetView, getVerticalHelper(layoutManager));
} else {
outer[1] = 0;
}
return outer;
}
private int distanceToStart(View targetView, OrientationHelper helper)
{
return helper.GetDecoratedStart(targetView) - helper.StartAfterPadding;
}
public override View FindSnapView(RecyclerView.LayoutManager layoutManager)
{
if (layoutManager is LinearLayoutManager) {
if (layoutManager.CanScrollHorizontally())
{
return getStartView(layoutManager, getHorizontalHelper(layoutManager));
}
else
{
return getStartView(layoutManager, getVerticalHelper(layoutManager));
}
}
return base.FindSnapView(layoutManager);
}
private View getStartView(RecyclerView.LayoutManager layoutManager,
OrientationHelper helper)
{
if (layoutManager is LinearLayoutManager) {
int firstChild = ((LinearLayoutManager)layoutManager).FindFirstVisibleItemPosition();
bool isLastItem = ((LinearLayoutManager)layoutManager)
.FindLastCompletelyVisibleItemPosition()
== layoutManager.ItemCount - 1;
if (firstChild == RecyclerView.NoPosition || isLastItem)
{
return null;
}
View child = layoutManager.FindViewByPosition(firstChild);
if (helper.GetDecoratedEnd(child) >= helper.GetDecoratedMeasurement(child) / 2
&& helper.GetDecoratedEnd(child) > 0)
{
return child;
}
else
{
if (((LinearLayoutManager)layoutManager).FindLastCompletelyVisibleItemPosition()
== layoutManager.ItemCount - 1)
{
return null;
}
else
{
return layoutManager.FindViewByPosition(firstChild + 1);
}
}
}
return base.FindSnapView(layoutManager);
}
private OrientationHelper getVerticalHelper(RecyclerView.LayoutManager layoutManager)
{
if (mVerticalHelper == null)
{
mVerticalHelper = OrientationHelper.CreateVerticalHelper(layoutManager);
}
return mVerticalHelper;
}
private OrientationHelper getHorizontalHelper(RecyclerView.LayoutManager layoutManager)
{
if (mHorizontalHelper == null)
{
mHorizontalHelper = OrientationHelper.CreateHorizontalHelper(layoutManager);
}
return mHorizontalHelper;
}
}
And use like this:
SnapHelper snapHelperStart = new StartSnapHelper();
snapHelperStart.AttachToRecyclerView(recyclerView);