EndlessRecyclerScrollListener.java

package br.com.catbag.gifreduxsample.ui;

import android.support.v7.widget.GridLayoutManager;
import android.support.v7.widget.LinearLayoutManager;
import android.support.v7.widget.RecyclerView;
import android.support.v7.widget.StaggeredGridLayoutManager;

/**
 at https://gist.github.com/nesquena/d09dc68ff07e845cc622
 Credits from this gist to https://gist.github.com/nesquena and https://gist.github.com/rogerhu
 **/

public abstract class EndlessRecyclerScrollListener extends RecyclerView.OnScrollListener {
    // The minimum amount of items to have below your current scroll position
    // before mLoading 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;

    private RecyclerView.LayoutManager mLayoutManager;

    public EndlessRecyclerScrollListener(LinearLayoutManager layoutManager) {
        mLayoutManager = layoutManager;
    }

    public EndlessRecyclerScrollListener(GridLayoutManager layoutManager) {
        mLayoutManager = layoutManager;
        mVisibleThreshold = mVisibleThreshold * layoutManager.getSpanCount();
    }

    public EndlessRecyclerScrollListener(StaggeredGridLayoutManager layoutManager) {
        mLayoutManager = layoutManager;
        mVisibleThreshold = mVisibleThreshold * layoutManager.getSpanCount();
    }

    public int getLastVisibleItem(int[] lastVisibleItemPositions) {
        int maxSize = 0;
        for (int i = 0; i < lastVisibleItemPositions.length; i++) {
            if (i == 0) {
                maxSize = lastVisibleItemPositions[i];
            } else if (lastVisibleItemPositions[i] > maxSize) {
                maxSize = lastVisibleItemPositions[i];
            }
        }
        return maxSize;
    }

    // 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 onScrolled(RecyclerView view, int dx, int dy) {
        int lastVisibleItemPosition = 0;
        int totalItemCount = mLayoutManager.getItemCount();

        if (mLayoutManager instanceof StaggeredGridLayoutManager) {
            int[] lastVisibleItemPositions = ((StaggeredGridLayoutManager) mLayoutManager)
                    .findLastVisibleItemPositions(null);
            // get maximum element within the list
            lastVisibleItemPosition = getLastVisibleItem(lastVisibleItemPositions);
        } else if (mLayoutManager instanceof GridLayoutManager) {
            lastVisibleItemPosition = ((GridLayoutManager) mLayoutManager)
                    .findLastVisibleItemPosition();
        } else if (mLayoutManager instanceof LinearLayoutManager) {
            lastVisibleItemPosition = ((LinearLayoutManager) mLayoutManager)
                    .findLastVisibleItemPosition();
        } 

        // 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 mLoading, we check to see if the dataset count has
        // changed, if so we conclude it has finished mLoading and update the current page
        // number and total item count.
        if (mLoading && (totalItemCount > mPreviousTotalItemCount)) {
            mLoading = false;
            mPreviousTotalItemCount = totalItemCount;
        }

        // If it isn’t currently mLoading, we check to see if we have breached
        // the mVisibleThreshold and need to reload more data.
        // If we do need to reload some more data, we execute onLoadMore to fetch the data.
        // threshold should reflect how many total columns there are too
        if (!mLoading && (lastVisibleItemPosition + mVisibleThreshold) > totalItemCount) {
            mCurrentPage++;
            onLoadMore(mCurrentPage, totalItemCount, view);
            mLoading = true;
        }
    }
    
    // Call this method whenever performing new searches
    public void resetState() {
        mCurrentPage = mStartingPageIndex;
        mPreviousTotalItemCount = 0;
        mLoading = true;
    }

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

}