SmoothScrollToPosition Scrolls Wrong Amount From Partially Visible Items

Question

SmoothScrollToPosition Scrolls Wrong Amount From Partially Visible Items

Nathan Sokalski 4,111

I have a SmoothScroller that inherits from LinearSmoothScroller, and I override CalculateDtToFit as follows:

public override int CalculateDtToFit(int viewStart, int viewEnd, int boxStart, int boxEnd, int snapPreference)
{
    int height = viewEnd - viewStart;
    int result;
    if (((MainActivity)Xamarin.Essentials.Platform.CurrentActivity).rvPlayerNamesInput.GetAdapter().ItemCount == 1)
    { result = base.CalculateDtToFit(viewStart, viewEnd, boxStart, boxEnd, LinearSmoothScroller.SnapToEnd); }
    else if (this.TargetPosition == 0)
    { result = base.CalculateDtToFit(viewStart, viewEnd + height, boxStart, boxEnd, LinearSmoothScroller.SnapToEnd); }
    else if (this.TargetPosition == ((MainActivity)Xamarin.Essentials.Platform.CurrentActivity).rvPlayerNamesInput.GetAdapter().ItemCount - 1)
    { result = base.CalculateDtToFit(viewStart - height, viewEnd, boxStart, boxEnd, LinearSmoothScroller.SnapToEnd); }
    else
    { result = base.CalculateDtToFit(viewStart - height, viewEnd + height, boxStart, boxEnd, LinearSmoothScroller.SnapToEnd); }
    return result;
}

When I call SmoothScrollToPosition(x) to scroll to a RecyclerView item that is completely visible, everything works exactly as expected. However, for items that are only partially visible (not completely offscreen, just not 100% within view), it does not. What it looks like it is doing for the partially visible items is using base.CalculateDtToFit with the original values & SnapToAny rather than what I have in the code above. However, I have confirmed that the code above is being called & returning the desired values (for both completely & partially visible items), yet it only scrolls the returned value for completely visible items (partially visible items do scroll, but not by the amount returned here). This override is always called & returns the desired value, so why does it not always use the returned value?

JarvanZhang 23,971

Hi, njsokalski. Could you please share more details about the featur you want to achieve? If you want to scroll to a position and place each corresponding item at the top or end, change the value of the snapPreference property.

Here is the sample code, you could refer to it.

public class CustomSmoothScroller : LinearSmoothScroller
{
    public CustomSmoothScroller(Context context) : base(context)
    {
    }

    public override int CalculateDtToFit(int viewStart, int viewEnd, int boxStart, int boxEnd, int snapPreference)
    {
        return base.CalculateDtToFit(viewStart, viewEnd, boxStart, boxEnd, -1);
    }
}

public class MyLinearLayoutManager : LinearLayoutManager
{
    private MainActivity mainActivity;

    public MyLinearLayoutManager(Context context) : base(context)
    {
        this.mainActivity = (MainActivity)context;
    }

    public override void SmoothScrollToPosition(RecyclerView recyclerView, RecyclerView.State state, int position)
    {
        CustomSmoothScroller linearSmoothScroller = new CustomSmoothScroller(mainActivity);
        linearSmoothScroller.TargetPosition = position;
        StartSmoothScroll(linearSmoothScroller);
    }
}

Nathan Sokalski 4,111 Reputation points

2021-11-30T19:00:10.917+00:00

Each item in my RecyclerView is a GridLayout which contains an EditText & Button. When the user selects the EditText, I want to scroll so that the previous, selected & next item are visible & aligned to the end of the RecyclerView. This is why my CalculateDtToFit detects whether the selected item is the only, first, last, or other item so that I can adjust viewStart & viewEnd, as well as use SnapToEnd. If, for example, my RecyclerView currently displays the following:

Item4 (Partially Visible)
Item5 (Completely Visible)
Item6 (Completely Visible)
Item7 (Partially Visible)

If I click Item4 (which is currently partially visible), it scrolls upward to result in the following:

Item4 (Completely Visible)
Item5 (Completely Visible)
Item6 (Completely Visible)
Item7 (Partially Visible)

But what I want & expect is the following:

Item2 (Partially Visible)
Item3 (Completely Visible)
Item4 (Completely Visible)
Item5 (Completely Visible)

Notice that in the result, it scrolled the selected item (Item4) into view on the edge it was coming from (SnapToAny instead SnapToEnd) and only enough to display itself (instead of Item3, itself & Item5). If I had instead selected Item5 (which is currently completely visible), it would result in the following:

Item3 (Partially Visible)
Item4 (Completely Visible)
Item5 (Completely Visible)
Item6 (Completely Visible)

This is what I want (the previous, selected, and next items snapped to the end). The only difference between these scenarios is whether the selected item is partially or completely visible.

JarvanZhang 23,971

I want to scroll so that the previous, selected & next item are visible & aligned to the end of the RecyclerView.

For this, try to scroll to the next position instead.

if (position == dataList.Count - 1)
{
    recyclerView.SmoothScrollToPosition(position);
}
else
{
    recyclerView.SmoothScrollToPosition(position + 1);
}

Set the snapPreference to SnapToEnd.

public class CustomSmoothScroller : LinearSmoothScroller
{
    public CustomSmoothScroller(Context context) : base(context)
    {
    }

    public override int CalculateDtToFit(int viewStart, int viewEnd, int boxStart, int boxEnd, int snapPreference)
    {
        return base.CalculateDtToFit(viewStart, viewEnd, boxStart, boxEnd + 100, LinearSmoothScroller.SnapToEnd);
    }
}