AdapterView 和 RecyclerView 的连续滚动

android

RecyclerView

tutorial

• • 概述
  • ListView 和 GridView 的实现方式
  • RecyclerView 的实现方式
    • 复位连续滚动状态
  • 故障排查
  • 在自定义的适配器中显示进度

概述

本文列举了 ListView、GridView 和 RecyclerView 的实现方法。它们的实现都是类似的，除了 RecyclerView 还需要传入 LayoutManager，这是因为它需要给无限滚动提供一些必要的信息。

无论哪个控件，实现无限滚动所需要的信息无非就包括这么几点：检测列表中剩余的可见元素，在到达最后一个元素之前开始获取数据的阈值。这个阈值可以用来决定什么时候开始加载更多。

示例图 1

要实现连续滚动的一个重点就是：一定要在用户到达列表的末尾前就获取数据。因此，添加一个阈值来让列表在预期的时候就加载数据。

示例图片 2

ListView 和 GridView 的实现方式

每个 AdapterView （例如 ListView 或 GridView）都支持 onScrollListener 事件的绑定，只要用户滑动列表，就会触发该事件。使用该体系，我们就可以定义一个基础类：EndlessScrollListener，它继承自 OnScrollListener，可以适用于大多数情况：

1. import android.widget.AbsListView;

   ---

2. ---

3. public abstract class EndlessScrollListener implements AbsListView.OnScrollListener { 
   

   ---

4. // The minimum number of items to have below your current scroll position 
   

   ---

5. // before loading more. 
   

   ---

6. private int visibleThreshold = 5; 
   

   ---

7. // The current offset index of data you have loaded 
   

   ---

8. private int currentPage = 0; 
   

   ---

9. // The total number of items in the dataset after the last load 
   

   ---

10. private int previousTotalItemCount = 0; 
    

    ---

11. // True if we are still waiting for the last set of data to load. 
    

    ---

12. private boolean loading = true; 
    

    ---

13. // Sets the starting page index 
    

    ---

14. private int startingPageIndex = 0; 
    

    ---

15. ---

16. public EndlessScrollListener() { 
    

    ---

17. } 
    

    ---

18. ---

19. public EndlessScrollListener(int visibleThreshold) { 
    

    ---

20. this.visibleThreshold = visibleThreshold; 
    

    ---

21. } 
    

    ---

22. ---

23. public EndlessScrollListener(int visibleThreshold, int startPage) { 
    

    ---

24. this.visibleThreshold = visibleThreshold; 
    

    ---

25. this.startingPageIndex = startPage; 
    

    ---

26. this.currentPage = startPage; 
    

    ---

27. } 
    

    ---

28. ---

29. // This happens many times a second during a scroll, so be wary of the code you place here. 
    

    ---

30. // We are given a few useful parameters to help us work out if we need to load some more data, 
    

    ---

31. // but first we check if we are waiting for the previous load to finish. 
    

    ---

32. @Override 
    

    ---

33. public void onScroll(AbsListView view, int firstVisibleItem, int visibleItemCount, int totalItemCount)  
    

    ---

34. { 
    

    ---

35. // If the total item count is zero and the previous isn‘t, assume the 
    

    ---

36. // list is invalidated and should be reset back to initial state 
    

    ---

37. if (totalItemCount < previousTotalItemCount) { 
    

    ---

38. this.currentPage = this.startingPageIndex; 
    

    ---

39. this.previousTotalItemCount = totalItemCount; 
    

    ---

40. if (totalItemCount == 0) { this.loading = true; }  
    

    ---

41. } 
    

    ---

42. // If it‘s still loading, we check to see if the dataset count has 
    

    ---

43. // changed, if so we conclude it has finished loading and update the current page 
    

    ---

44. // number and total item count. 
    

    ---

45. if (loading && (totalItemCount > previousTotalItemCount)) { 
    

    ---

46. loading = false; 
    

    ---

47. previousTotalItemCount = totalItemCount; 
    

    ---

48. currentPage++; 
    

    ---

49. } 
    

    ---

50. ---

51. // If it isn‘t currently loading, we check to see if we have breached 
    

    ---

52. // the visibleThreshold and need to reload more data. 
    

    ---

53. // If we do need to reload some more data, we execute onLoadMore to fetch the data. 
    

    ---

54. if (!loading && (firstVisibleItem + visibleItemCount + visibleThreshold) >= totalItemCount ) { 
    

    ---

55. loading = onLoadMore(currentPage + 1, totalItemCount); 
    

    ---

56. } 
    

    ---

57. } 
    

    ---

58. ---

59. // Defines the process for actually loading more data based on page 
    

    ---

60. // Returns true if more data is being loaded; returns false if there is no more data to load. 
    

    ---

61. public abstract boolean onLoadMore(int page, int totalItemsCount); 
    

    ---

62. ---

63. @Override 
    

    ---

64. public void onScrollStateChanged(AbsListView view, int scrollState) { 
    

    ---

65. // Don‘t take any action on changed 
    

    ---

66. } 
    

    ---

67. } 
    

    ---

注意：这是一个抽象类，要使用它，必须实现该类中的抽象方法：onLoadMore，用于检索新的数据。在 activity 中，可以用一个匿名内部类来实现这个抽象类，并把它绑定到适配器上。例如：

1. public class MainActivity extends Activity {

   ---

2. @Override 
   

   ---

3. protected void onCreate(Bundle savedInstanceState) { 
   

   ---

4. // ... the usual  
   

   ---

5. ListView lvItems = (ListView) findViewById(R.id.lvItems); 
   

   ---

6. // Attach the listener to the AdapterView onCreate 
   

   ---

7. lvItems.setOnScrollListener(new EndlessScrollListener() { 
   

   ---

8. @Override 
   

   ---

9. public boolean onLoadMore(int page, int totalItemsCount) { 
   

   ---

10. // Triggered only when new data needs to be appended to the list 
    

    ---

11. // Add whatever code is needed to append new items to your AdapterView 
    

    ---

12. loadNextDataFromApi(page);  
    

    ---

13. // or loadNextDataFromApi(totalItemsCount);  
    

    ---

14. return true; // ONLY if more data is actually being loaded; false otherwise. 
    

    ---

15. } 
    

    ---

16. }); 
    

    ---

17. } 
    

    ---

18. ---

19. ---

20. // Append the next page of data into the adapter 
    

    ---

21. // This method probably sends out a network request and appends new data items to your adapter.  
    

    ---

22. public void loadNextDataFromApi(int offset) { 
    

    ---

23. // Send an API request to retrieve appropriate paginated data  
    

    ---

24. // --> Send the request including an offset value (i.e `page`) as a query parameter. 
    

    ---

25. // --> Deserialize and construct new model objects from the API response 
    

    ---

26. // --> Append the new data objects to the existing set of items inside the array of items 
    

    ---

27. // --> Notify the adapter of the new items made with `notifyDataSetChanged()` 
    

    ---

28. } 
    

    ---

29. } 
    

    ---

现在，当你滚动列表时，每当剩余元素到达阈值时，列表就会自动加载下一页的数据。该方法对于 GridView 来说，一样的有效。

RecyclerView 的实现方式

因此，RecyclerView 实现连续分页需要以下几个步骤：

1. 把 EndlessRecyclerViewScrollListener.java 类拷贝到你的项目中
2. 在 RecyclerView 上调用 addOnScrollListener() 方法来启用连续分页。给该方法传入 EndlessRecyclerViewScrollListener 的实例，当新页需要加载时，实现 onLoadMore() 方法
3. 在 onLoadMore() 方法中，加载更多数据，并把它们填充到列表中

代码示例如下：

1. public class MainActivity extends Activity {

   ---

2. // Store a member variable for the listener 
   

   ---

3. private EndlessRecyclerViewScrollListener scrollListener; 
   

   ---

4. ---

5. @Override 
   

   ---

6. protected void onCreate(Bundle savedInstanceState) { 
   

   ---

7. // Configure the RecyclerView 
   

   ---

8. RecyclerView rvItems = (RecyclerView) findViewById(R.id.rvContacts); 
   

   ---

9. LinearLayoutManager linearLayoutManager = new LinearLayoutManager(this); 
   

   ---

10. rvItems.setLayoutManager(linearLayoutManager); 
    

    ---

11. // Retain an instance so that you can call `resetState()` for fresh searches 
    

    ---

12. scrollListener = new EndlessRecyclerViewScrollListener(linearLayoutManager) { 
    

    ---

13. @Override 
    

    ---

14. public void onLoadMore(int page, int totalItemsCount, RecyclerView view) { 
    

    ---

15. // Triggered only when new data needs to be appended to the list 
    

    ---

16. // Add whatever code is needed to append new items to the bottom of the list 
    

    ---

17. loadNextDataFromApi(page); 
    

    ---

18. } 
    

    ---

19. }; 
    

    ---

20. // Adds the scroll listener to RecyclerView 
    

    ---

21. rvItems.addOnScrollListener(scrollListener); 
    

    ---

22. } 
    

    ---

23. ---

24. // Append the next page of data into the adapter 
    

    ---

25. // This method probably sends out a network request and appends new data items to your adapter.  
    

    ---

26. public void loadNextDataFromApi(int offset) { 
    

    ---

27. // Send an API request to retrieve appropriate paginated data  
    

    ---

28. // --> Send the request including an offset value (i.e `page`) as a query parameter. 
    

    ---

29. // --> Deserialize and construct new model objects from the API response 
    

    ---

30. // --> Append the new data objects to the existing set of items inside the array of items 
    

    ---

31. // --> Notify the adapter of the new items made with `notifyItemRangeInserted()` 
    

    ---

32. } 
    

    ---

33. } 
    

    ---

复位连续滚动状态

1. // 1. First, clear the array of data

   ---

2. listOfItems.clear(); 
   

   ---

3. // 2. Notify the adapter of the update 
   

   ---

4. recyclerAdapterOfItems.notifyDataSetChanged(); // or notifyItemRangeRemoved 
   

   ---

5. // 3. Reset endless scroll listener when performing a new search 
   

   ---

6. scrollListener.resetState(); 
   

   ---

完整的连续滚动代码可以参考：code sample for usage，this code sample。

故障排查

• 对于 ListView 来说，请一定在 Activity 的 onCreate() 方法 或 Fragment 的 onCreateView() 方法中，给它设置 setOnScrollListener() 监听。否则，你可能会遇到一些想不到的问题
• 要使分页系统可以可靠地、持续地工作，在给列表添加新的数据之前，你应该确保清除适配器的数据。对 RecyclerView 来说，当需要通知适配器数据有更新时，强烈建议使用精度更细的通知方法。
• 要触发分页，始终记得 loadNextDataFromApi 方法调用时，需要把新数据添加到已经存在的数据源。按句话说，只有首次加载时才清除数据，以后的每次分页都是把新增的数据添加到原有的数据集中。
• 如果你遇到了下述的错误：Cannot call this method in a scroll callback. Scroll callbacks might be run during a measure & layout pass where you cannot change the RecyclerView data，那你应该按照 Stack Overflow 中的解决办法对代码进行改造：

1. // Delay before notifying the adapter since the scroll listeners

   ---

2. // can be called while RecyclerView data cannot be changed. 
   

   ---

3. view.post(new Runnable() { 
   

   ---

4. @Override 
   

   ---

5. public void run() { 
   

   ---

6. // Notify adapter with appropriate notify methods 
   

   ---

7. adapter.notifyItemRangeInserted(curSize, allContacts.size() - 1); 
   

   ---

8. } 
   

   ---

9. }); 
   

   ---

在自定义的适配器中显示进度