ListViewScrollTracker.java

package com.mi.mishophome.util;

import android.support.v4.util.SparseArrayCompat;
import android.view.View;
import android.widget.AbsListView;

/**
 * Created by wuyexiong on 12/2/14.
 */
public class ListViewScrollTracker {
    private AbsListView mListView;
    private SparseArrayCompat<Integer> mPositions;
    private SparseArrayCompat<Integer> mListViewItemHeights = new SparseArrayCompat<>();
    private int mFirstVisiblePosition;
    private int mScrollY;

    public ListViewScrollTracker(final AbsListView listView) {
        mListView = listView;
    }

    /**
     * Call from an AbsListView.OnScrollListener to calculate the incremental offset (change in scroll offset
     * since the last calculation).
     *
     * @param firstVisiblePosition First visible item position in the list.
     * @param visibleItemCount     Number of visible items in the list.
     * @return The incremental offset, or 0 if it wasn't possible to calculate the offset.
     */
    public int calculateIncrementalOffset(final int firstVisiblePosition, final int visibleItemCount) {
        // Remember previous positions, if any
        SparseArrayCompat<Integer> previousPositions = mPositions;

        // Store new positions
        mPositions = new SparseArrayCompat<Integer>();
        for (int i = 0; i < visibleItemCount; i++) {
            mPositions.put(firstVisiblePosition + i, mListView.getChildAt(i).getTop());
        }

        if (previousPositions != null) {
            // Find position which exists in both mPositions and previousPositions, then return the difference
            // of the new and old Y values.
            for (int i = 0; i < previousPositions.size(); i++) {
                int position = previousPositions.keyAt(i);
                int previousTop = previousPositions.get(position);
                Integer newTop = mPositions.get(position);
                if (newTop != null) {
                    return newTop - previousTop;
                }
            }
        }

        return 0; // No view's position was in both previousPositions and mPositions
    }

    public int calculateScrollY(final int firstVisiblePosition, final int visibleItemCount) {
        mFirstVisiblePosition = firstVisiblePosition;
        if (visibleItemCount > 0) {
            View c = mListView.getChildAt(0); //this is the first visible row
            int scrollY = -c.getTop();
            mListViewItemHeights.put(firstVisiblePosition, c.getMeasuredHeight());
            for (int i = 0; i < firstVisiblePosition; ++i) {
                if (mListViewItemHeights.get(i) != null) // (this is a sanity check)
                    scrollY += mListViewItemHeights.get(i); //add all heights of the views that are gone
            }
            mScrollY = scrollY;
            return scrollY;
        }
        return 0;
    }

    public int getFirstVisiblePosition() {
        return mFirstVisiblePosition;
    }

    public int getScrollY() {
        return mScrollY;
    }

    public void clear() {
        mPositions = null;
    }
}