引入库

implementation 'androidx.paging:paging-runtime-ktx:3.1.1'

paging 库，目前还是有点小bug ，后面说

数据模型定义

// 分页请求的数据响应，至少要返回一个总数量; page、pageIndex 都可本地定义，然后 可以计算出当前请求响应后，历史返回的总数量；最终计算出是否还有下一页
data class CustomerPageData(val totalCount: Int, val data: List<CustomerData>)

data class CustomerData(val id: String, val name: String)

分页 adapter

import android.graphics.Color
import android.view.LayoutInflater
import android.view.ViewGroup
import android.widget.TextView
import androidx.paging.PagingDataAdapter
import androidx.recyclerview.widget.DiffUtil
import com.stone.stoneviewskt.R
import com.stone.stoneviewskt.common.BaseViewHolder
import com.stone.stoneviewskt.data.CustomerData
import com.stone.stoneviewskt.util.logi

/**
 * desc:
 * author:  stone
 * email:   aa86799@163.com
 * blog :   https://stone.blog.csdn.net
 * time:    2023/6/10 11:20
 */
class PageListAdapter: PagingDataAdapter<CustomerData, BaseViewHolder>(object : DiffUtil.ItemCallback<CustomerData>() {
    /*
     * 如果 areItemsTheSame 返回 true，但是仍然会展示相同的数据，可能是因为 areContentsTheSame 返回 false
     */

    // 是否是同一条 item
    override fun areItemsTheSame(oldItem: CustomerData, newItem: CustomerData): Boolean {
        logi("areItemsTheSame") // 没有输出该日志  可能是官方的bug(不然实现这个回调有什么意义)
        return oldItem.id == newItem.id
    }

    // 是否内容相同
    override fun areContentsTheSame(oldItem: CustomerData, newItem: CustomerData): Boolean {
        logi("areContentsTheSame") // 没有输出该日志 可能是官方的bug(不然实现这个回调有什么意义)
        return oldItem.id == newItem.id
    }
}) {

    // item 点击事件
    private var mBlock: ((position: Int, data: CustomerData) -> Unit)? = null

    override fun onBindViewHolder(holder: BaseViewHolder, position: Int) {
        val data = getItem(position) ?: return
        holder.findView<TextView>(R.id.tv_name).text = data.name

        holder.itemView.setOnClickListener {
            mBlock?.invoke(position, data)
        }

        if (position % 2 == 0) {
            holder.itemView.setBackgroundColor(Color.parseColor("#c0ff00ff"))
        } else {
            holder.itemView.setBackgroundColor(Color.parseColor("#c0abc777"))
        }
    }

    override fun onCreateViewHolder(parent: ViewGroup, viewType: Int): BaseViewHolder {
        val itemView = LayoutInflater.from(parent.context).inflate(R.layout.item_page_list, parent, false)
        return BaseViewHolder(itemView)
    }

    fun setItemClick(block: (position: Int, data: CustomerData) -> Unit) {
        this.mBlock = block
    }

}

加载更多 adapter

import android.view.LayoutInflater
import android.view.View
import android.view.ViewGroup
import android.widget.ProgressBar
import android.widget.TextView
import androidx.paging.LoadState
import androidx.paging.LoadStateAdapter
import com.stone.stoneviewskt.R
import com.stone.stoneviewskt.common.BaseViewHolder
import com.stone.stoneviewskt.util.logi

/**
 * desc:    加载更多
 * author:  stone
 * email:   aa86799@163.com
 * time:    2023/6/8 16:11
 */
class LoadMoreAdapter(private val retryBlock: () -> Unit) : LoadStateAdapter<BaseViewHolder>() {
    override fun onBindViewHolder(holder: BaseViewHolder, loadState: LoadState) {
        when (loadState) {
            is LoadState.NotLoading -> { // 非加载中
                logi("LoadMoreAdapter---onBindViewHolder---LoadState.NotLoading")
                holder.itemView.findViewById<ProgressBar>(R.id.progressBar).visibility = View.GONE
                holder.itemView.findViewById<TextView>(R.id.tv_load).visibility = View.GONE

                holder.itemView.findViewById<TextView>(R.id.tv_all_message).visibility = View.VISIBLE

                holder.itemView.findViewById<TextView>(R.id.btn_retry).visibility = View.GONE
            }

            is LoadState.Loading -> { // 加载中
                holder.itemView.findViewById<ProgressBar>(R.id.progressBar).visibility = View.VISIBLE
                holder.itemView.findViewById<TextView>(R.id.tv_load).visibility = View.VISIBLE

                holder.itemView.findViewById<TextView>(R.id.tv_all_message).visibility = View.GONE

                holder.itemView.findViewById<TextView>(R.id.btn_retry).visibility = View.GONE
            }

            is LoadState.Error -> {
                holder.itemView.findViewById<ProgressBar>(R.id.progressBar).visibility = View.GONE
                holder.itemView.findViewById<TextView>(R.id.tv_load).visibility = View.GONE

                holder.itemView.findViewById<TextView>(R.id.tv_all_message).visibility = View.GONE

                val btnRetry = holder.itemView.findViewById<TextView>(R.id.btn_retry)
                btnRetry.visibility = View.VISIBLE
                btnRetry.text = "发生了错误：${loadState.error.message}，点击重试"
                btnRetry.setOnClickListener {
                    retryBlock()
                }
            }
        }
    }

    override fun onCreateViewHolder(parent: ViewGroup, loadState: LoadState): BaseViewHolder {
        val itemView = LayoutInflater.from(parent.context).inflate(R.layout.layout_load_more, parent, false)
        return BaseViewHolder(itemView)
    }

    // 是否作为 item 显示
    override fun displayLoadStateAsItem(loadState: LoadState): Boolean {
        logi("loadState -- $loadState")
        return loadState is LoadState.Loading
                || loadState is LoadState.Error
                || (loadState is LoadState.NotLoading && loadState.endOfPaginationReached)
        // loadState.endOfPaginationReached false 表示有更多的数据要加载
    }

}

空数据 adapter

import com.stone.stoneviewskt.R
import com.stone.stoneviewskt.adapter.BaseRvAdapter
import com.stone.stoneviewskt.common.BaseViewHolder

/**
 * desc:    空数据
 * author:  stone
 * email:   aa86799@163.com
 * time:    2023/6/9 10:45
 */
class EmptyAdapter: BaseRvAdapter<String>(R.layout.layout_empty) {

    override fun fillData(holder: BaseViewHolder, position: Int, data: String) {

    }
}

分页数据源

import androidx.paging.PagingSource
import androidx.paging.PagingState
import com.stone.stoneviewskt.BuildConfig
import com.stone.stoneviewskt.data.CustomerData
import com.stone.stoneviewskt.data.CustomerPageData
import com.stone.stoneviewskt.util.logi
import com.stone.stoneviewskt.util.showShort
import kotlinx.coroutines.delay
import kotlin.random.Random

/**
 * desc:    分页数据源
 * author:  stone
 * email:   aa86799@163.com
 * blog :   https://stone.blog.csdn.net
 * time:    2023/6/10 11:26
 */
class PageListPageSource : PagingSource<Int, CustomerData>() {

    private var flagA = false
    private var flagB = false
    companion object {
        private var mockEmpty = true // 静态字段 模拟空数据
    }

    override fun getRefreshKey(state: PagingState<Int, CustomerData>): Int? {
        return state.anchorPosition?.let { anchorPosition ->
            val anchorPage = state.closestPageToPosition(anchorPosition)
            anchorPage?.prevKey?.plus(1) ?: anchorPage?.nextKey?.minus(1)
        }
    }

    override suspend fun load(params: LoadParams<Int>): LoadResult<Int, CustomerData> {
        if (BuildConfig.DEBUG) { // 防止加载数据过快，loading 效果看不到
            delay(1000)
        }
        val currentPage = params.key ?: 1 // 当前页码
        val pageSize = params.loadSize // 每页多少条数据
        var bean: CustomerPageData? = null // 分页数据
        kotlin.runCatching { // 异常发生后，执行 onFailure {}
            // todo 真实开发时，移除模拟错误代码

            // 模拟 第2页 一定抛异常
            if (currentPage == 2 && !flagB) {
                flagA = true
            }
            if (flagA) {
                flagA = false
                flagB = true
                throw IllegalStateException("test-error")
            }


            if (currentPage % 2 == 0 && Random.nextInt() % 2 == 0) { // 当偶数页时，随机发生异常。  注意异常发生后，对请求的页码 -- currentPage是否有影响
                throw IllegalStateException("test-error")
            }
//            Api.createRetrofit(CustomerApiService::class.java).customerList(currentPage，pageSize) // 真实网络请求方式
            mockData(currentPage, pageSize)// todo 真实开发时，移除模拟数据代码
        }.onSuccess {
            bean = it
        }.onFailure {
            it.printStackTrace()
            if (it !is java.util.concurrent.CancellationException) {
                showShort("出错了" + it.message)
                return LoadResult.Error(it) // 如果返回 error，那后续 加载更多将不会触发
            }
        }
        val prevKey = if (currentPage > 1) currentPage - 1 else null // 当前页不是第一页的话，前一页为当前页减一
        val hasMore = currentPage * pageSize < (bean?.totalCount ?: 0)
        val nextKey = if (hasMore) currentPage + 1 else null // 当前页有数据的话，下一页就是，当前页加一，反之为空
        logi("currentPage:$currentPage    pageSize:$pageSize    prevKey:$prevKey     nextKey:$nextKey")
        return try {
            LoadResult.Page(
                data = bean?.data ?: arrayListOf(), // 数据
                prevKey = prevKey, // 为当前页的前一页
                nextKey = nextKey  // 为当前页的下一页
            )
        } catch (e: Exception) {
            e.printStackTrace()
            return LoadResult.Error(e)
        }
    }

    // 模拟分页数据
    private fun mockData(pageIndex: Int, pageSize: Int): CustomerPageData? {
        if (mockEmpty) {
            mockEmpty = false
            return null
        }
        logi("分页参数：[pageIndex-$pageIndex] [pageSize-$pageSize]")
        val list = arrayListOf<CustomerData>()
        val totalCount = 55
        val currentCount = if (totalCount > pageIndex * pageSize) pageSize else totalCount - (pageIndex - 1) * pageSize
         (1..currentCount).forEach {
             val currentPosition = (pageIndex - 1) * pageSize + it
             list.add(CustomerData("id-$currentPosition", "name-$currentPosition"))
        }
        return CustomerPageData(totalCount, list)
    }
}

ViewModel 提供加载数据源的方法

/**
 * desc:
 * author:  stone
 * email:   aa86799@163.com
 * blog :   https://stone.blog.csdn.net
 * time:    2023/6/10 12:12
 */
class PageListViewModel : ViewModel() {

    // todo 截止到 3.1.1版本，官方库有bug：超出 initialLoadSize 所在页码后， 再retry 没问题；反之retry，会数据重复
    // todo 为了规避 建议 真实开发时，initialLoadSize 和 pageSize 的值相同。 或看看后续版本是否有修复

    fun loadData(): Flow<PagingData<CustomerData>> {
        return Pager( //PagingConfig 定义一次从PagingSource加载的项数。
            config = PagingConfig(
                pageSize = 10,
                initialLoadSize = 15, // 初始取的 pageSize，通常要大于指定的 pageSize   但大于了，在retry时又会出现bug
                prefetchDistance = 1   // prefetchDistance条数据 时加载下一页
            ),
            //每页都会调用一次,请求数据
            pagingSourceFactory = { PageListPageSource() }
        ).flow.distinctUntilChangedBy { it }.cachedIn(viewModelScope) // 设置缓存
    }
}

结合以上实现的 Fragment

import android.os.Bundle
import androidx.lifecycle.ViewModelProvider
import androidx.lifecycle.lifecycleScope
import androidx.paging.LoadState
import androidx.recyclerview.widget.ConcatAdapter
import com.stone.stoneviewskt.R
import com.stone.stoneviewskt.common.BaseBindFragment
import com.stone.stoneviewskt.databinding.FragmentPageListBinding
import com.stone.stoneviewskt.util.logi
import com.stone.stoneviewskt.util.showShort
import kotlinx.coroutines.flow.collectLatest

/**
 * desc:    Paging3分页+ConcatAdapter+空数据视图+下拉刷新(SwipeRefreshLayout)+加载更多+错误重试
 * author:  stone
 * email:   aa86799@163.com
 * blog :   https://stone.blog.csdn.net
 * time:    2023/6/10 11:09
 */
class PageListFragment: BaseBindFragment<FragmentPageListBinding>(R.layout.fragment_page_list) {

    private val viewModel by lazy { ViewModelProvider(this)[PageListViewModel::class.java] }
    private lateinit var concatAdapter: ConcatAdapter // 可合并 adapter
    private val adapter by lazy { PageListAdapter() }
    private val loadMoreAdapter by lazy {
        LoadMoreAdapter { // 点击 重试按钮
            adapter.retry()
        }
    }
    private val emptyAdapter: EmptyAdapter by lazy { EmptyAdapter() }

    override fun onPreparedView(savedInstanceState: Bundle?) {
        super.onPreparedView(savedInstanceState)

        concatAdapter = adapter.withLoadStateFooter(loadMoreAdapter) // 将 loadMoreAdapter 添加为 footer 效果
        emptyAdapter.updateData(listOf("")) // 向 emptyAdapter 添加一项，当其显示时，只显示一个对应视图
        mBind.rvPage.adapter = concatAdapter // 这时 已 添加了 loadMoreAdapter

        // 设置加载状态监听
        adapter.addLoadStateListener {
            logi("监听到loadState -- $it")
            // 如果 状态是 未加载(加载完成也算) 或 发生错误
            if (it.refresh is LoadState.NotLoading) {
                mBind.swipeRefresh.isRefreshing = false
                if (adapter.itemCount == 0) { // 真实数据为空
                    if (concatAdapter.adapters.contains(loadMoreAdapter)) {
                        concatAdapter.removeAdapter(loadMoreAdapter) // 移除加载更多
                        logi("removeAdapter -- loadMoreAdapter")
                    }
                    if (!concatAdapter.adapters.contains(emptyAdapter)) {
                        concatAdapter.addAdapter(emptyAdapter) // 添加空数据视图   仅在未添加过时才添加
                    }
                } else {
                    if (concatAdapter.adapters.contains(emptyAdapter)) {
                        concatAdapter.removeAdapter(emptyAdapter)
                        concatAdapter.addAdapter(loadMoreAdapter) // 只添加一次 loadMore
                    }
                }

            } else if (it.refresh is LoadState.Error) {
                mBind.swipeRefresh.isRefreshing = false
            }
        }

        adapter.setItemClick { position, data ->
            showShort("点击查看 --- ${data.name}")
        }

        mBind.swipeRefresh.setOnRefreshListener {
            loadData()
        }

        mBind.swipeRefresh.isRefreshing = true
        loadData()
    }

    private fun loadData() {
        lifecycleScope.launchWhenResumed {
            viewModel.loadData().collectLatest { // 加载数据
                adapter.submitData(it) // 绑定分页数据源
            }
        }
    }
}

数据重复问题

截止到 3.1.1版本，官方库有bug：超出 initialLoadSize 所在页码后，再retry 没问题；反之retry，会数据重复。比如 pageSize=10，initialLoadSize=15，初始加载15个数据后，当前页码对应 pageSize=10 的情形下是 2；这时再请求第2页，发生了异常，调用 adapter#retry()后，会把第二页的10个条目加载进来并显示，这就发生了数据重复。而DiffUtil.ItemCallback中的两个函数，一直都没有被回调，没有任何日志打印出来… 这就很迷幻了…