paging3实现RecyclerView二级列表

Paging3 是一个用于 Android 的分页库，它可以帮助你更加容易地实现分页功能。

要在 RecyclerView 中实现二级列表，你需要做以下几个步骤：

1. 创建一个数据类，用于保存二级列表的数据。

data class SubItem(val name: String, val description: String)
data class Item(val name: String, val subItems: List<SubItem>)

2. 创建一个 RecyclerViewAdapter 类，继承自 PagingDataAdapter。

class MyAdapter :
    PagingDataAdapter<Item, RecyclerView.ViewHolder>(ItemDiffCallback) {

    override fun onBindViewHolder(holder: RecyclerView.ViewHolder, position: Int) {
        val item = getItem(position)
        if (item != null) {
            when (holder) {
                is ItemViewHolder -> holder.bind(item)
                is SubItemViewHolder -> holder.bind(item.subItems[position - 1])
            }
        }
    }

    override fun onCreateViewHolder(parent: ViewGroup, viewType: Int): RecyclerView.ViewHolder {
        return when (viewType) {
            ITEM_VIEW_TYPE -> ItemViewHolder.create(parent)
            SUB_ITEM_VIEW_TYPE -> SubItemViewHolder.create(parent)
            else -> throw IllegalArgumentException("unknown view type $viewType")
        }
    }

    override fun getItemViewType(position: Int): Int {
        return if (position == 0) {
            ITEM_VIEW_TYPE
        } else {
            SUB_ITEM_VIEW_TYPE
        }
    }

    companion object {
        private const val ITEM_VIEW_TYPE = 0
        private const val SUB_ITEM_VIEW_TYPE = 1
    }
}

3. 创建 ViewHolder 类。

class ItemViewHolder private constructor(private val binding: ItemBinding) :
    RecyclerView.ViewHolder(binding.root) {

    fun bind(item: Item) {
        binding.name.text = item.name
    }

    companion object {
        fun create(parent: ViewGroup): ItemViewHolder {
            val layoutInflater = LayoutInflater.from(parent.context)
            val binding = ItemBinding.inflate(layoutInflater, parent, false)
            return ItemViewHolder(binding)
        }
    }
}

class SubItemViewHolder private constructor(private val binding: SubItemBinding) :
    RecyclerView.ViewHolder(binding.root) {

    fun bind(subItem: SubItem) {
        binding.name.text = subItem.name
        binding.description.text = subItem.description
    }

    companion object {
        fun create(parent: ViewGroup): SubItemViewHolder {
            val layoutInflater = LayoutInflater.from(parent.context)
            val binding = SubItemBinding.inflate(layoutInflater, parent, false)
            return SubItemViewHolder(binding)
        }
    }
}

4. 创建 DiffCallback。

object ItemDiffCallback : DiffUtil.ItemCallback<Item>() {
    override fun areItemsTheSame(oldItem: Item, newItem: Item): Boolean {
        return oldItem.name == newItem.name
    }

    override fun areContentsTheSame(oldItem: Item, newItem: Item): Boolean {
        return oldItem == newItem
    }
}

5. 在 Activity 中初始化 RecyclerView 和 Paging3。

class MyActivity : AppCompatActivity() {

    private val viewModel by viewModels<MyViewModel>()

    private val adapter = MyAdapter()

    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        val binding = ActivityMainBinding.inflate(layoutInflater)
        setContentView(binding.root)

        binding.recyclerView.adapter = adapter

        lifecycleScope.launch {
            viewModel.flow.collectLatest { pagingData ->
                adapter.submitData(pagingData)
            }
        }
    }
}

6. 在 ViewModel 中创建数据源。

class MyViewModel : ViewModel() {

    private val repository = MyRepository()

    val flow = Pager(PagingConfig(pageSize = 10)) {
        repository.getPager()
    }.flow
}

7. 在 Repository 中创建数据源。

class MyRepository {

    fun getPager(): PagingSource<Int, Item> {
        return object : PagingSource<Int, Item>() {
            override suspend fun load(params: LoadParams<Int>): LoadResult<Int, Item> {
                val page = params.key ?: 1
                val items = mutableListOf<Item>()
                // TODO: load data from API or database
                return LoadResult.Page(
                    data = items,
                    prevKey = if (page == 1) null else page - 1,
                    nextKey = if (items.isEmpty()) null else page + 1
                )
            }

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

这样就完成了 RecyclerView 的二级列表实现。通过 Paging3，你可以更加方便地实现分页功能，同时也可以更好地管理数据源。