package com.codepath.customadapter;

import android.widget.AbsListView;

/**
 * Created by Administrator on 2016/7/11.
 */
public abstract class EndlessScrollListener implements AbsListView.OnScrollListener {
 //在你滑動項下最少為多少時開始加載數(shù)據(jù)
 private int visibleThreshold = 5;
 //已經(jīng)加載數(shù)據(jù)的當前頁碼
 private int currentPage = 0;
 //上一次加載數(shù)據(jù)后數(shù)據(jù)庫的數(shù)據(jù)量
 private int previousTotalItemCount = 0;
 //我們是否在等待最后一組數(shù)據(jù)的加載
 private boolean loading = true;
 //設置開始頁的下標
 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;
 }
 //這個方法可能會在滑動調(diào)用很多次，所以在設計時要保持謹慎
 //我們需要一些有用的參數(shù)來幫助我們，當我們需要加載更多數(shù)據(jù)的時候
 //但是我們首先要檢查是否我們在等待先前的加載結(jié)束

//onScroll()當列表或網(wǎng)格視圖被滾動后將會調(diào)用，參數(shù)一：報告狀態(tài)的視圖參數(shù)二：第一個可以看見的項的下標，參數(shù)三：可見項的數(shù)量參數(shù)四：listAdapter中所有的項數(shù)
 @Override
 public void onScroll(AbsListView view, int firstVisibleItem, int visibleItemCount, int totalItemCount) {

 //如果總項數(shù)為0，而且先前沒有項，那么這個列表是無效的應該被設定為初始狀態(tài)
 if (totalItemCount < previousTotalItemCount) {
  this.currentPage = this.startingPageIndex;
  this.previousTotalItemCount = totalItemCount;
  if (totalItemCount == 0) {this.loading = true;}
 }
 //如果仍在加載中我們可以檢查一下數(shù)據(jù)集合是否改變了，如果改變的話那就是已經(jīng)完成了loading需要更新當前
 //頁數(shù)和數(shù)據(jù)總量
 if (loading && (totalItemCount > previousTotalItemCount)) {
  loading = false;
  previousTotalItemCount = totalItemCount;
  currentPage++;
 }
 //如果當前沒有加載，我們需要檢查當前是否達到了閾值，如果是的話我們需要
 //加載更多的數(shù)據(jù)，執(zhí)行onLoadMore
 if (!loading && (firstVisibleItem + visibleItemCount + visibleThreshold) >= totalItemCount) {
  loading = onLoadMore(currentPage + 1, totalItemCount);
 }
 }
 //定義實際加載數(shù)據(jù)的過程，如果數(shù)據(jù)加載完成返回false，如果正在加載返回true；
 public abstract boolean onLoadMore(int page, int totalItemCount);

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

public class MainActivity extends Activity {
 @Override
 protected void onCreate(Bundle savedInstance) {
 //向平常一樣
  ListView lvItems = (ListView) findViewById(R.id.lvItens);
  //將監(jiān)聽器綁定到上面
  lvItems.setOnScrollListener(new EndlessScrollListener() {
  @Override
  public boolean onLoadMore(int page, int totalItemsCount) {
  // 當新數(shù)據(jù)需要綁定到列表上的時候觸發(fā)
  // 加載數(shù)據(jù)需要的代碼Add whatever code is needed to append new items to your AdapterView
  customLoadMoreDataFromApi(page); 
  // or customLoadMoreDataFromApi(totalItemsCount); 
  return true; //數(shù)據(jù)加載中為true，不然為false； ONLY if more data is actually being loaded; false otherwise.
  }
 });
 }
 //加載更多的數(shù)據(jù)
 public void customLoadMoreDataFromApi(int offset) {
 //這個方法通常會發(fā)起一些網(wǎng)絡請求，然后向適配器添加更多的數(shù)據(jù)
 //將偏移量數(shù)據(jù)作為參數(shù)附在請求里來獲得一個數(shù)據(jù)的分頁
 //解析API返回的值并且獲得新的對象構(gòu)建適配器
 }
}

 protected void onCreate(Bundle savedInstanceState) {
 // Configure the RecyclerView獲得RecylerView的實例
 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);

}