BaseEndlessScrollListener.java

package com.meg7.soas.ui.adapter;

import android.widget.AbsListView;

// Based on https://github.com/codepath/android_guides/wiki/Endless-Scrolling-with-AdapterViews
public abstract class BaseEndlessScrollListener implements AbsListView.OnScrollListener {

    /**
     * The minimum amount of items to have below your current scroll position
     * before loading more.
     */
    private int visibleThreshold = 5;
    /**
     * The current offset index of data you have loaded
     */
    private int currentPage = 0;
    /**
     * The total number of items in the dataset after the last load
     */
    private int previousTotalItemCount = 0;
    /**
     * True if we are still waiting for the last set of data to load.
     */
    private boolean loading = true;
    /**
     * Sets the starting page index
     */
    private int startingPageIndex = 0;

    public BaseEndlessScrollListener() {
    }

    public BaseEndlessScrollListener(int visibleThreshold) {
        this.visibleThreshold = visibleThreshold;
    }

    public BaseEndlessScrollListener(int visibleThreshold, int startPage) {
        this.visibleThreshold = visibleThreshold;
        this.startingPageIndex = startPage;
        this.currentPage = 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 < previousTotalItemCount) {
            this.currentPage = this.startingPageIndex;
            this.previousTotalItemCount = totalItemCount;
            if (totalItemCount == 0) {
                this.loading = 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 (loading && (totalItemCount > previousTotalItemCount)) {
            loading = false;
            previousTotalItemCount = totalItemCount;
            currentPage++;
        }

        // 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 (!loading && (totalItemCount - visibleItemCount) <= (firstVisibleItem + visibleThreshold)) {
            onLoadMore(currentPage + 1, totalItemCount);
            loading = true;
        }
    }

    /**
     * Defines the process for actually loading more data based on page
     */
    public abstract void onLoadMore(int page, int totalItemsCount);

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