AdapterView 和 RecyclerView 的连续滚动

android

RecyclerView

tutorial

概述

应用中一个常见的使用场景就是：当用户滚动浏览的项目时，会自动加载更多的项目（又叫做无限滚动）。它的原理是：当滚动到达底部之前，一旦当前剩余可见的项目达到了一个设定好的阈值，就会触发加载更多数据的请求。

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：

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;
}
// 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 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
}
}

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：

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

对于 RecyclerView 来说，我们也可以使用一个相似的方法：定义接口 EndlessRecyclerViewScrollListener；一个必须实现的方法 onLoadMore()。在 RecyclerView 中，LayoutManager 用于渲染列表元素并管理滚动，即提供与适配器相关的当前滚动位置的信息。基于上述理由，我们需要传入一个 LayoutManager 的实例，用于收集必须的信息，和用于确定加载更多数据的时机。

因此，RecyclerView 实现连续分页需要以下几个步骤：

把 EndlessRecyclerViewScrollListener.java 类拷贝到你的项目中
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
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：

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()`
}
}

复位连续滚动状态

当你准备执行新的搜索时，要确保清除列表上已经存在的数据，并马上通知适配器数据的变化。当然，还需要使用 resetState() 方法来重置 EndlessRecyclerViewScrollListener 的状态：

// 1. First, clear the array of data
listOfItems.clear();
// 2. Notify the adapter of the update
recyclerAdapterOfItems.notifyDataSetChanged(); // or notifyItemRangeRemoved
// 3. Reset endless scroll listener when performing a new search
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

如果在开发中遇到问题，请考虑下述的建议：

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：

// Delay before notifying the adapter since the scroll listeners
// can be called while RecyclerView data cannot be changed.
view.post(new Runnable() {
@Override
public void run() {
// Notify adapter with appropriate notify methods
adapter.notifyItemRangeInserted(curSize, allContacts.size() - 1);
}
});

在自定义的适配器中显示进度

想要在 ListView 的底部显示加载数据的进度，需要对适配器进行特殊处理。使用 getItemViewType(int position) 定义两种不同的视图类型，既正常行与最后一行的样子不同。