paging 的作用就是自动加载， 不需要监听列表的滚动状态
引入依赖包

dependencies {
      def paging_version = "2.1.1"

      implementation "androidx.paging:paging-runtime:$paging_version" // For Kotlin use paging-runtime-ktx

      // alternatively - without Android dependencies for testing
      testImplementation "androidx.paging:paging-common:$paging_version" // For Kotlin use paging-common-ktx

      // optional - RxJava support
      implementation "androidx.paging:paging-rxjava2:$paging_version" // For Kotlin use paging-rxjava2-ktx
    }

假设是从数据库中读取数据的场景

    @Query("SELECT * FROM STUDENT")
    DataSource.Factory<Integer, Student> queryAll();

数据适配器 使用 PagedListAdapter

public class PageAdapter extends PagedListAdapter<Student, PageAdapter.ViewHolder> {

    public PageAdapter() {
        super(new DiffUtil.ItemCallback<Student>() {
            @Override
            public boolean areItemsTheSame(@NonNull Student oldItem, @NonNull Student newItem) {
                return oldItem.getId() == newItem.getId();
            }
            @Override
            public boolean areContentsTheSame(@NonNull Student oldItem, @NonNull Student newItem) {
                return oldItem.equals(newItem);
            }
        });
    }

    @NonNull
    @Override
    public ViewHolder onCreateViewHolder(@NonNull ViewGroup parent, int viewType) {
        View view = LayoutInflater.from(parent.getContext()).inflate(R.layout.adapter_paging, parent, false);
        return new ViewHolder(view);
    }
    @Override
    public void onBindViewHolder(@NonNull ViewHolder holder, int position) {
        Student item = getItem(position);
        // 这里 item  可能会为空
        if (item != null) {
            holder.number.setText(String.valueOf(item.getStudent_number()));
        }

    }
    static class ViewHolder extends RecyclerView.ViewHolder {
        TextView number;
        ViewHolder(@NonNull View itemView) {
            super(itemView);
            number = itemView.findViewById(R.id.number);
        }
    }

}

在Controller 层 中

private LiveData<PagedList<Student>> allStudents;

recyclerView.setLayoutManager(new LinearLayoutManager(activity));
adapter = new PageAdapter();

recyclerView.setAdapter(adapter);
studentDao = StudentDatabase.getInstance(activity).getStudentDao();
      
 // 两个参数 是数据来源 和 分页的个数
allStudents = new LivePagedListBuilder<>(studentDao.queryAll(), 20)
                .build();
allStudents.observe(getViewLifecycleOwner(), new Observer<PagedList<Student>>() {
        @Override
         public void onChanged(PagedList<Student> students) {
                // 刷新
                adapter.submitList(students);
          }
       });

用在网络请求的情景

涉及到的类 DataSource , DataSource.Factory， PagedListAdapter

/**
 *   分页加载的 DataSource ;  -->PageKeyedDataSource 
 *   DataSource 有三个子类 ，根据不同的api 实现不同的子类
 */
class ArticleDataSource : PageKeyedDataSource<Int, ArticleData>() {
    private val repository = ArticleRepository()
    /**
     *  初始加载
     */
    override fun loadInitial(
        params: LoadInitialParams<Int>,
        callback: LoadInitialCallback<Int, ArticleData>
    ) {

        repository.getArticle(0).execute(object : BaseObserver<Article>() {
            override fun success(t: Article) {
                var datas = t.datas
                Log.e("=====", "$datas")
                callback.onResult(datas, null, 1)
            }

            override fun fail(msg: String) {
                Log.e("=====", msg)
            }
        })

    }

    /**
     * 加载下一页
     */
    override fun loadAfter(params: LoadParams<Int>, callback: LoadCallback<Int, ArticleData>) {
        repository.getArticle(params.key).execute(object : BaseObserver<Article>() {
            override fun success(t: Article) {
                var datas = t.datas
                callback.onResult(datas, (params.key + 1))
                Log.e("=====", "$datas")
            }

            override fun fail(msg: String) {
                Log.e("=====", msg)
            }
        })
    }

    override fun loadBefore(params: LoadParams<Int>, callback: LoadCallback<Int, ArticleData>) {
    }
}

DataSource.Factory

class ArticleDataSourceFactory : DataSource.Factory<Int, ArticleData>() {
    override fun create(): DataSource<Int, ArticleData> {
        return ArticleDataSource()
    }
}

在ViewModel 中只需要 实例化 实例化 DataSource.Factory 转 LiveData

class RecyclerViewModel : ViewModel() {
    val articleLiveData = ArticleDataSourceFactory().toLiveData(1)
    /**
      *  如果外部需要调用 下拉刷新，重头加载的
      *  在这里把 datasource 置空就会重新生成一个新的 DataSource()
      *  里面的 方法就会重新执行。
      */
     fun resetQuery() {
         articleLiveData.value?.dataSource?.invalidate()
     }
}

Activity 里头

  mViewModel?.articleLiveData?.observe(viewLifecycleOwner, Observer {
            adapter.submitList(it)
        })

在把 Adapter 继承 PagedListAdapter 就可以自动加载更多了 .