package com.codepath.customadapter;

import android.widget.AbsListView;

/**
 * Created by Administrator on 2016/7/11.
 */
public abstract class EndlessScrollListener implements AbsListView.OnScrollListener {
 //在你滑动项下最少为多少时开始加载数据
 private int visibleThreshold = 5;
 //已经加载数据的当前页码
 private int currentPage = 0;
 //上一次加载数据后数据库的数据量
 private int previousTotalItemCount = 0;
 //我们是否在等待最后一组数据的加载
 private boolean loading = true;
 //设置开始页的下标
 private int startingPageIndex = 0;
 public EndlessScrollListener() {

 }
 public EndlessScrollListener(int visibleThreshold) {
  this.visibleThreshold = visibleThreshold;
 }
 public EndlessScrollListener(int visibleThreshold, int startingPageIndex) {
  this.visibleThreshold = visibleThreshold;
  this.startingPageIndex = startingPageIndex;
 }
 //这个方法可能会在滑动调用很多次，所以在设计时要保持谨慎
 //我们需要一些有用的参数来帮助我们，当我们需要加载更多数据的时候
 //但是我们首先要检查是否我们在等待先前的加载结束

//onScroll()当列表或网格视图被滚动后将会调用，参数一：报告状态的视图参数二：第一个可以看见的项的下标，参数三：可见项的数量参数四：listAdapter中所有的项数
 @Override
 public void onScroll(AbsListView view, int firstVisibleItem, int visibleItemCount, int totalItemCount) {

  //如果总项数为0，而且先前没有项，那么这个列表是无效的应该被设定为初始状态
  if (totalItemCount < previousTotalItemCount) {
   this.currentPage = this.startingPageIndex;
   this.previousTotalItemCount = totalItemCount;
   if (totalItemCount == 0) {this.loading = true;}
  }
  //如果仍在加载中我们可以检查一下数据集合是否改变了，如果改变的话那就是已经完成了loading需要更新当前
  //页数和数据总量
  if (loading && (totalItemCount > previousTotalItemCount)) {
   loading = false;
   previousTotalItemCount = totalItemCount;
   currentPage++;
  }
  //如果当前没有加载，我们需要检查当前是否达到了阈值，如果是的话我们需要
  //加载更多的数据，执行onLoadMore
  if (!loading && (firstVisibleItem + visibleItemCount + visibleThreshold) >= totalItemCount) {
   loading = onLoadMore(currentPage + 1, totalItemCount);
  }
 }
 //定义实际加载数据的过程，如果数据加载完成返回false，如果正在加载返回true；
 public abstract boolean onLoadMore(int page, int totalItemCount);

 @Override
 public void onScrollStateChanged(AbsListView view, int scrollState) {
  //不采取动作
 }
}

public class MainActivity extends Activity {
 @Override
 protected void onCreate(Bundle savedInstance) {
  //向平常一样
   ListView lvItems = (ListView) findViewById(R.id.lvItens);
   //将监听器绑定到上面
    lvItems.setOnScrollListener(new EndlessScrollListener() {
   @Override
   public boolean onLoadMore(int page, int totalItemsCount) {
    // 当新数据需要绑定到列表上的时候触发
    // 加载数据需要的代码Add whatever code is needed to append new items to your AdapterView
    customLoadMoreDataFromApi(page); 
    // or customLoadMoreDataFromApi(totalItemsCount); 
    return true; //数据加载中为true，不然为false； ONLY if more data is actually being loaded; false otherwise.
   }
  });
 }
 //加载更多的数据
 public void customLoadMoreDataFromApi(int offset) {
  //这个方法通常会发起一些网络请求，然后向适配器添加更多的数据
  //将偏移量数据作为参数附在请求里来获得一个数据的分页
  //解析API返回的值并且获得新的对象构建适配器
 }
}

 protected void onCreate(Bundle savedInstanceState) {
  // Configure the RecyclerView获得RecylerView的实例
  RecyclerView rvItems = (RecyclerView) findViewById(R.id.rvContacts);
  LinearLayoutManager linearLayoutManager = new LinearLayoutManager(this);
  recyclerView.setLayoutManager(linearLayoutManager);
  // Add the scroll listener
  rvItems.addOnScrollListener(new EndlessRecyclerViewScrollListener(linearLayoutManager) {
   @Override
   public void onLoadMore(int page, int totalItemsCount) {
    // Triggered only when new data needs to be appended to the list
    // Add whatever code is needed to append new items to the bottom of the list
    customLoadMoreDataFromApi(page); 
   }
  });
 }

 // Append more data into the adapter
 // This method probably sends out a network request and appends new data items to your adapter. 
 public void customLoadMoreDataFromApi(int page) {
  // Send an API request to retrieve appropriate data using the offset value as a parameter.
  // --> Deserialize API response and then construct new objects to append to the adapter
  // --> Notify the adapter of the changes
 }
}

public abstract class EndlessRecyclerViewScrollListener extends RecyclerView.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;

 RecyclerView.LayoutManager mLayoutManager;

 public EndlessRecyclerViewScrollListener(LinearLayoutManager layoutManager) {
  this.mLayoutManager = layoutManager;
 }

 public EndlessRecyclerViewScrollListener(GridLayoutManager layoutManager) {
  this.mLayoutManager = layoutManager;
  visibleThreshold = visibleThreshold * layoutManager.getSpanCount();
 }

 public EndlessRecyclerViewScrollListener(StaggeredGridLayoutManager layoutManager) {
  this.mLayoutManager = layoutManager;
  visibleThreshold = visibleThreshold * 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 LinearLayoutManager) {
   lastVisibleItemPosition = ((LinearLayoutManager) mLayoutManager).findLastVisibleItemPosition();
  } else if (mLayoutManager instanceof GridLayoutManager) {
   lastVisibleItemPosition = ((GridLayoutManager) 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 < 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;
  }

  // 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.
  // threshold should reflect how many total columns there are too
  if (!loading && (lastVisibleItemPosition + visibleThreshold) > totalItemCount) {
   currentPage++;
   onLoadMore(currentPage, totalItemCount);
   loading = true;
  }
 }

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

}