有人知道如何在 android 中实现延迟加载 recyclerView 吗？我是 android 的新手，我不知道如何解决这个问题。如果有人提供帮助，我将不胜感激。

最佳答案

你可以使用 EndlessRecyclerView

概览 一个常见的应用程序功能是在用户滚动项目时自动加载更多项目(也称为无限滚动)。这是通过在用户超过剩余项目的阈值后触发更多数据请求来完成的。

此处记录了 ListView、GridView 和 RecyclerView(ListView 的后继者)的方法。两者代码相似，只是需要传入RecyclerView中的LayoutManager，提供实现无限滚动的必要信息。

在这两种情况下，实现滚动所需的信息包括确定列表中的最后一个可见项目和某种类型的阈值以在到达最后一个项目之前开始获取更多数据。此数据可用于决定何时从外部源加载更多数据: To provide the appearance of endless scrolling, it's important to fetch data before the user gets to the end of the list. Adding a threshold value therefore helps anticipate the need to append more data.

使用 ListView 或 GridView 实现
每个 AdapterView(例如 ListView 和 GridView)都支持绑定(bind)到 OnScrollListener 事件，只要用户滚动浏览集合就会触发这些事件。使用这个系统，我们可以定义一个基本的 EndlessScrollListener，它通过创建我们自己的扩展 OnScrollListener 的类来支持大多数用例:

import android.widget.AbsListView; 

public abstract class EndlessScrollListener implements AbsListView.OnScrollListener {
    // The minimum number 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;

    public EndlessScrollListener() {
    }

    public EndlessScrollListener(int visibleThreshold) {
        this.visibleThreshold = visibleThreshold;
    }

    public EndlessScrollListener(int visibleThreshold, int startPage) {
        this.visibleThreshold = visibleThreshold;
        this.startingPageIndex = startPage;
        this.currentPage = startPage;
    }


    @Override
    public void onScroll(AbsListView view, int firstVisibleItem, int visibleItemCount, int totalItemCount) 
        {
        // 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;
            currentPage++;
        }

        // 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.
        if (!loading && (firstVisibleItem + visibleItemCount + visibleThreshold) >= totalItemCount ) {
            loading = onLoadMore(currentPage + 1, totalItemCount);
        }
    }

    // Defines the process for actually loading more data based on page
    // Returns true if more data is being loaded; returns false if there is no more data to load.
    public abstract boolean onLoadMore(int page, int totalItemsCount);

    @Override
    public void onScrollStateChanged(AbsListView view, int scrollState) {
        // Don't take any action on changed
    }
}

请注意，这是一个抽象类，为了使用它，您必须扩展这个基类并定义 onLoadMore 方法来实际检索新数据。我们现在可以在任何扩展 EndlessScrollListener 的 Activity 中定义一个匿名类并将其绑定(bind)到 AdapterView。例如:

public class MainActivity extends Activity {
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        // ... the usual 
        ListView lvItems = (ListView) findViewById(R.id.lvItems);
        // Attach the listener to the AdapterView onCreate
        lvItems.setOnScrollListener(new EndlessScrollListener() {
          @Override
          public boolean 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 your AdapterView
              loadNextDataFromApi(page); 
              // or loadNextDataFromApi(totalItemsCount); 
              return true; // ONLY if more data is actually being loaded; false otherwise.
          }
        });
    }


   // Append the next page of data into the adapter
   // This method probably sends out a network request and appends new data items to your adapter. 
   public void loadNextDataFromApi(int offset) {
      // Send an API request to retrieve appropriate paginated data 
      //  --> Send the request including an offset value (i.e `page`) as a query parameter.
      //  --> Deserialize and construct new model objects from the API response
      //  --> Append the new data objects to the existing set of items inside the array of items
      //  --> Notify the adapter of the new items made with `notifyDataSetChanged()`
   }
}

现在当您滚动时，项目将自动填充，因为一旦用户超过 visibleThreshold 就会触发 onLoadMore 方法。这种方法同样适用于 GridView，监听器可以访问页面和 totalItemsCount，以支持分页和基于偏移量的抓取。

使用 RecyclerView 实现 我们可以通过定义一个需要实现 onLoadMore() 方法的接口(interface) EndlessRecyclerViewScrollListener 来对 RecyclerView 使用类似的方法。 LayoutManager 负责在 RecyclerView 中呈现项目应放置的位置并管理滚动，提供有关当前滚动位置相对于适配器的信息。出于这个原因，我们需要传递一个 LayoutManager 的实例来收集必要的信息以确定何时加载更多数据。 为 RecyclerView 实现无限分页需要以下步骤:
为 RecyclerView 实现无限分页需要以下步骤:
1. 将 EndlessRecyclerViewScrollListener.java 复制到您的应用程序中。
2. 在 RecyclerView 上调用 addOnScrollListener(...) 以启用无限分页。传入一个 EndlessRecyclerViewScrollListener 实例并实现 onLoadMore，只要需要加载新页面以填充列表，它就会触发。
3. 在上述 onLoadMore 方法中，通过发送网络请求或从其他来源加载额外的项目到适配器中。

要开始处理步骤 2 和 3 的滚动事件，我们需要在 Activity 或 Fragment 中使用 addOnScrollListener() 方法，并使用布局管理器传入 EndlessRecyclerViewScrollListener 的实例，如下所示:

public class MainActivity extends Activity {
    // Store a member variable for the listener
    private EndlessRecyclerViewScrollListener scrollListener;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
       // Configure the RecyclerView
       RecyclerView rvItems = (RecyclerView) findViewById(R.id.rvContacts);
       LinearLayoutManager linearLayoutManager = new LinearLayoutManager(this);
       rvItems.setLayoutManager(linearLayoutManager);
       // Retain an instance so that you can call `resetState()` for fresh searches
       scrollListener = new EndlessRecyclerViewScrollListener(linearLayoutManager) {
           @Override
           public void onLoadMore(int page, int totalItemsCount, RecyclerView view) {
               // 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
               loadNextDataFromApi(page);
           }
      };
      // Adds the scroll listener to RecyclerView
      rvItems.addOnScrollListener(scrollListener);
  }

  // Append the next page of data into the adapter
  // This method probably sends out a network request and appends new data items to your adapter. 
  public void loadNextDataFromApi(int offset) {
      // Send an API request to retrieve appropriate paginated data 
      //  --> Send the request including an offset value (i.e `page`) as a query parameter.
      //  --> Deserialize and construct new model objects from the API response
      //  --> Append the new data objects to the existing set of items inside the array of items
      //  --> Notify the adapter of the new items made with `notifyItemRangeInserted()`
  }
}

https://github.com/codepath/android_guides/wiki/Endless-Scrolling-with-AdapterViews-and-RecyclerView

关于android - 在 android 中延迟加载 recyclerView，我们在Stack Overflow上找到一个类似的问题： https://stackoverflow.com/questions/35901697/