/*
 * Copyright (c) 2010-2019 Belledonne Communications SARL.
 *
 * This file is part of linphone-android
 * (see https://www.linphone.org).
 *
 * This program is free software: you can redistribute it and/or modify
 * it under the terms of the GNU General Public License as published by
 * the Free Software Foundation, either version 3 of the License, or
 * (at your option) any later version.
 *
 * This program is distributed in the hope that it will be useful,
 * but WITHOUT ANY WARRANTY; without even the implied warranty of
 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
 * GNU General Public License for more details.
 *
 * You should have received a copy of the GNU General Public License
 * along with this program. If not, see <http://www.gnu.org/licenses/>.
 */
package org.linphone.chat;

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

abstract class ChatScrollListener extends RecyclerView.OnScrollListener {
    // The minimum amount of items to have below your current scroll position
    // before mLoading more.
    private static final int mVisibleThreshold = 5;
    // 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;

    private final LinearLayoutManager mLayoutManager;

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

    // 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;
        int totalItemCount = mLayoutManager.getItemCount();

        lastVisibleItemPosition = 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) {
            this.mPreviousTotalItemCount = totalItemCount;
            if (totalItemCount == 0) {
                this.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) {
            onLoadMore(totalItemCount);
            mLoading = true;
        }
    }

    // Defines the process for actually mLoading more data based on page
    protected abstract void onLoadMore(int totalItemsCount);
}