LinearLayoutManager with fixed computeVerticalScrollOffset()

AdvancedLinearLayoutManager.java

import android.annotation.SuppressLint;
import android.content.Context;
import android.util.AttributeSet;
import android.view.View;

import java.util.HashMap;
import java.util.Map;

import androidx.recyclerview.widget.LinearLayoutManager;
import androidx.recyclerview.widget.RecyclerView;

/**
 * @author heruoxin @ CatchingNow Inc.
 * @since 2019-08-04
 * 
 * Based on:
 * https://medium.com/@rituel521/improving-accuracy-of-computeverticalscrolloffset-for-linearlayoutmanager-38699a9d03b
 * Fixed an issue that ignored paddingTop
 */
public class AdvancedLinearLayoutManager extends LinearLayoutManager {

    public AdvancedLinearLayoutManager(Context context) {
        super(context);
    }

    public AdvancedLinearLayoutManager(Context context, int orientation, boolean reverseLayout) {
        super(context, orientation, reverseLayout);
    }

    public AdvancedLinearLayoutManager(Context context, AttributeSet attrs, int defStyleAttr, int defStyleRes) {
        super(context, attrs, defStyleAttr, defStyleRes);
    }


    // map of child adapter position to its height.
    @SuppressLint("UseSparseArrays")
    private Map<Integer, Integer> childSizesMap = new HashMap<>();

    @Override
    public void onLayoutCompleted(RecyclerView.State state) {
        super.onLayoutCompleted(state);
        for (int i = 0; i < getChildCount(); i++) {
            View child = getChildAt(i);
            if (child != null) childSizesMap.put(getPosition(child), getDecoratedMeasuredHeight(child));
        }
    }

    @Override
    public int computeVerticalScrollOffset(RecyclerView.State state) {
        if (getChildCount() == 0) return 0;
        int firstChildPosition = findFirstVisibleItemPosition();
        View firstChild = findViewByPosition(firstChildPosition);
        if (firstChild == null) return super.computeVerticalScrollOffset(state);
        int scrolledY = - getDecoratedTop(firstChild);
        for (int i = 0; i < firstChildPosition; i++) {
            Integer size = childSizesMap.get(i);
            scrolledY += size == null ? 0 : size;
        }
        return scrolledY + getPaddingTop();
    }

}