EndlessScrollListener.java

package org.gdg.frisbee.android.utils;

import android.widget.AbsListView;

public abstract class EndlessScrollListener implements AbsListView.OnScrollListener {
    // The minimum number of items to have below your current scroll position
    // before loading more.
    private int mVisibleThreshold = 5;
    // The current offset index of data you have loaded
    private int mCurrentPage = 0;
    // The total number of items in the dataset after the last load
    private int mPreviousTotalItemCount = 0;
    // True if we are still waiting for the last set of data to load.
    private boolean mLoading = true;
    // Sets the starting page index
    private int mStartingPageIndex = 0;

    public EndlessScrollListener() {
    }

    public EndlessScrollListener(int visibleThreshold) {
        mVisibleThreshold = visibleThreshold;
    }

    public EndlessScrollListener(int visibleThreshold, int startPage) {
        mVisibleThreshold = visibleThreshold;
        mStartingPageIndex = startPage;
        mCurrentPage = startPage;
    }

    // This happens many times a second during a scroll, so be wary of the code you place here.
    // We are given a few useful parameters to help us work out if we need to load some more data,
    // but first we check if we are waiting for the previous load to finish.
    @Override
    public void onScroll(AbsListView view, int firstVisibleItem, int visibleItemCount, int totalItemCount) {
        // If the total item count is zero and the previous isn't, assume the
        // list is invalidated and should be reset back to initial state
        if (totalItemCount < mPreviousTotalItemCount) {
            mCurrentPage = mStartingPageIndex;
            mPreviousTotalItemCount = totalItemCount;
            if (totalItemCount == 0) {
                mLoading = true;
            }
        }
        // If it's still loading, we check to see if the dataset count has
        // changed, if so we conclude it has finished loading and update the current page
        // number and total item count.
        if (mLoading && (totalItemCount > mPreviousTotalItemCount)) {
            mLoading = false;
            mPreviousTotalItemCount = totalItemCount;
            mCurrentPage++;
        }

        // If it isn't currently loading, we check to see if we have breached
        // the visibleThreshold and need to reload more data.
        // If we do need to reload some more data, we execute onLoadMore to fetch the data.
        if (!mLoading && (totalItemCount - visibleItemCount) <= (firstVisibleItem + mVisibleThreshold)) {
            mLoading = onLoadMore(mCurrentPage + 1);
        }
    }

    // Defines the process for actually loading more data based on page
    // Returns true if more data is being loaded; returns false if there is no more data to load.
    public abstract boolean onLoadMore(int page);

    @Override
    public void onScrollStateChanged(AbsListView view, int scrollState) {
        // Don't take any action on changed
    }
}