Bottom padded ScrollView/RecyclerView weird behaviour

[moisoni97]

I notice that when a ScrollView has a bottom padding, the overscroll effect starts before the ScrollView reaches the end, resulting in a weird glitch where the view is overscrolled and then kinda snaps to reach the padding.

<ScrollView
            android:id="@+id/scrollView"
            android:layout_width="match_parent"
            android:layout_height="match_parent"
            android:clipToPadding="false"
            android:paddingBottom="140dp"
            android:scrollbars="none">

Also, this can be fixed if we trigger the overscroll effect only when the user has genuinely reached the end of the content, accounting for any padding.

@UiThread
public final class ScrollViewOverScrollHelper implements IOverScrollDecoratorAdapter {

    private final ScrollView mScrollView;
    private final View mContentView;

    public ScrollViewOverScrollHelper(@NonNull ScrollView scrollView) {
        mScrollView = scrollView;
        mContentView = scrollView.getChildAt(0);
    }

    @Override
    public View getView() {
        return mScrollView;
    }

    @Override
    public boolean isInAbsoluteStart() {
        return mScrollView.getScrollY() <= 0;
    }

    @Override
    public boolean isInAbsoluteEnd() {
        if (mContentView == null) {
            return true;
        }

        int scrollY = mScrollView.getScrollY();
        int contentHeight = mContentView.getHeight();
        int scrollViewHeight = mScrollView.getHeight();
        int paddingBottom = mScrollView.getPaddingBottom();

        return scrollY >= (contentHeight - scrollViewHeight + paddingBottom);
    }
}

ScrollView scrollView = findViewById(R.id.scroll_view);
new VerticalOverScrollBounceEffectDecorator(new PaddedScrollViewOverScrollDecorAdapter(scrollView));

And similarly for RecyclerView. Unlike the ScrollView, it does not have any weird behaviour if it has a big bottom padding, but doesn't overscroll at all from bottom to top (if there are not enough items to fill the screen).

@UiThread
public final class RecyclerViewOverScrollHelper implements IOverScrollDecoratorAdapter {

    private final RecyclerView mRecyclerView;
    private final LinearLayoutManager mLayoutManager;

    public RecyclerViewOverScrollHelper(@NonNull RecyclerView recyclerView) {
        mRecyclerView = recyclerView;
        mLayoutManager = (LinearLayoutManager) recyclerView.getLayoutManager();
    }

    @Override
    public View getView() {
        return mRecyclerView;
    }

    @Override
    public boolean isInAbsoluteStart() {
        if (mLayoutManager.getItemCount() == 0) return false;
        return mLayoutManager.findFirstCompletelyVisibleItemPosition() == 0;
    }

    @Override
    public boolean isInAbsoluteEnd() {
        int totalItemCount = mLayoutManager.getItemCount();
        if (totalItemCount == 0) return false;
        int lastVisibleItemPosition = mLayoutManager.findLastCompletelyVisibleItemPosition();

        return lastVisibleItemPosition == totalItemCount - 1;
    }
}

RecyclerView recyclerView = findViewById(R.id.recycler_view);
new VerticalOverScrollBounceEffectDecorator(new RecyclerViewOverScrollHelper(recyclerView));

I think this should be added in a future release.