孟老板 Paging3 (一) 入门

Paging3 (一) 入门

前言:

官方分页工具, 确实香. 但数据源不开放, 无法随意增删改操作; 只能借助 Room; 但列表数据不一定都要用 Room吧;

如果偏查询的分页数据用 Paging3 ; 其他一概用老Adapter; 这倒也算个方案. [苦笑]

分页数据的内存中缓存。该功能可确保您的应用在处理分页数据时高效利用系统资源。
内置的请求重复信息删除功能，可确保您的应用高效利用网络带宽和系统资源。
可配置的 RecyclerView 适配器，会在用户滚动到已加载数据的末尾时自动请求数据。
对 Kotlin 协程和 Flow 以及 LiveData 和 RxJava 的一流支持。
内置对错误处理功能的支持，包括刷新和重试功能。

导包:

dependencies {

  val paging_version = "3.0.0"

  //唯一必导包

  implementation("androidx.paging:paging-runtime:$paging_version")

  // 测试用

  testImplementation("androidx.paging:paging-common:$paging_version")

  // optional - RxJava2 support

  implementation("androidx.paging:paging-rxjava2:$paging_version")

  // optional - RxJava3 support

  implementation("androidx.paging:paging-rxjava3:$paging_version")

  // 适配 Guava 库  - 高效java扩展库

  implementation("androidx.paging:paging-guava:$paging_version")

  // 适配 Jetpack Compose  - 代码构建View; 干掉 layout

  implementation("androidx.paging:paging-compose:1.0.0-alpha09")

}

1. 简单使用:

1.1 数据源 PagingSource

自定义数据源, 继承 PagingSource

它有两个泛型参数, 1. 页码key, 没有特殊需求的话一般就是 Int 类型; 2.集合实体类型

重写两个方法: 1.load() 加载数据的方法; 2.getRefreshKey 初始加载的页码; 暂且返回 1 或 null

LoadResult.Page 后面再讲;

class DynamicDataSource: PagingSource<Int, DynamicTwo>() {

    //模拟最大页码

    private var maxPage = 2

    //模拟数据

    private fun fetchItems(startPosition: Int, pageSize: Int): MutableList<DynamicTwo> {

        Log.d("ppppppppppppppppppppp", "startPosition=${startPosition};;;pageSize=${pageSize}")

        val list: MutableList<DynamicTwo> = ArrayList()

        for (i in startPosition until startPosition + pageSize) {

            val concert = DynamicTwo()

            concert.title = "我是标题${i}"

            concert.newsInfo = "我是内容${i}"

            concert.nickName = "小王${i}"

            list.add(concert)

        }

        return list

    }

    override fun getRefreshKey(state: PagingState<Int, DynamicTwo>): Int? = null

    override suspend fun load(params: LoadParams<Int>): LoadResult<Int, DynamicTwo> {

        val nextPageNumber = params.key ?: 1

        val size = params.loadSize

        Log.d("ppppppppppppppppppppp", "nextPageNumber=${nextPageNumber};;;size=${size}")

        val response = fetchItems((nextPageNumber-1) * size, size)

        return LoadResult.Page(

            data = response,

            prevKey = null, // Only paging forward.  只向后加载就给 null

            //nextKey 下一页页码;  尾页给 null;  否则当前页码加1

            nextKey = if(nextPageNumber >= maxPage) null else (nextPageNumber + 1)

        )

    }

}

1.2 ViewModel

代码比较简单. 内容我们一会再讲

class DynamicPagingModel(application: Application) : AndroidViewModel(application) {

    val flow = Pager(

        //配置

        PagingConfig(pageSize = 10, prefetchDistance = 2,initialLoadSize = 10)

    ) {

        //我们自定义的数据源

        DynamicDataSource()

    }.flow

        .cachedIn(viewModelScope)

}

1.3 前台使用:

初始化 Adapter 及 RecycleView

mViewModel?.flow?.collectLatest 绑定监听, 然后通过 submitData() 刷新列表;

mAdapter = SimplePagingAdapter(R.layout.item_dynamic_img_two, null)

mDataBind.rvRecycle.let {

    it.layoutManager = LinearLayoutManager(mActivity)

    it.adapter = mAdapter

}

//Activity 用 lifecycleScope

//Fragments 用 viewLifecycleOwner.lifecycleScope

viewLifecycleOwner.lifecycleScope.launchWhenCreated {

    mViewModel?.flow?.collectLatest {

        mAdapter.submitData(it)

    }

}

1.4 Adapter

必须继承 paging 的 PagingDataAdapter

DiffCallback() 或 handler NewViewHolder 不了解的可以看我的 ListAdapter 封装系列

open class SimplePagingAdapter(

    private val layout: Int,

    protected val handler: BaseHandler? = null

) :

    PagingDataAdapter<DynamicTwo, RecyclerView.ViewHolder>(DiffCallback()) {

    override fun onCreateViewHolder(parent: ViewGroup, viewType: Int): RecyclerView.ViewHolder {

        return NewViewHolder(

            DataBindingUtil.inflate(

                LayoutInflater.from(parent.context), layout, parent, false

            ), handler

        )

    }

    override fun onBindViewHolder(holder: RecyclerView.ViewHolder, position: Int) {

        if(holder is NewViewHolder){

            holder.bind(getItem(position))

        }

    }

}

over 简单的分页模拟数据已完成;

2. LoadResult

它是一个密封类; 它表示加载操作的结果;

2.1 LoadResult.Error

表示加载失败; 需提供 Throwable 对象.

public data class Error<Key : Any, Value : Any>(

    val throwable: Throwable

) : LoadResult<Key, Value>()

可用于:

异常时返回, HTTP, IO, 数据解析等异常;
服务器错误码响应
没有更多数据

2.1 LoadResult.Page

表示加载成功;

参数:

data 数据集合;

prevKey 前页页码 key; 　　//向下一页加载给null

nextKey 后页页码 key; 　　//向上一页加载给null

public data class Page<Key : Any, Value : Any> constructor(
    /**
     * Loaded data
     */
    val data: List<Value>,
    /**
     * [Key] for previous page if more data can be loaded in that direction, `null`
     * otherwise.
     */
    val prevKey: Key?,
    /**
     * [Key] for next page if more data can be loaded in that direction, `null` otherwise.
     */
    val nextKey: Key?,
    /**
     * Optional count of items before the loaded data.
     */
    @IntRange(from = COUNT_UNDEFINED.toLong())
    val itemsBefore: Int = COUNT_UNDEFINED,
    /**
     * Optional count of items after the loaded data.
     */
    @IntRange(from = COUNT_UNDEFINED.toLong())
    val itemsAfter: Int = COUNT_UNDEFINED
) : LoadResult<Key, Value>() {

3.PagingConfig

分页配置

参数:

pageSize: 每页容量

prefetchDistance: 当RecycleView 滑动到底部时, 会自动加载下一页. 如果能提前预加载, 可以省去部分等待加载的时间.

　　　　　　　　prefetchDistance 就是距离底部提前加载的距离. 默认 = pageSize; = 0 时将不会加载更多

enablePlaceholders: 允许使用占位符. 想了解的点这里

initialLoadSize: 初始加载数量, 默认 = pageSize * 3

maxSize: 似乎意义没有那么简单. 还没看源码,不清楚; 不能 < pageSize + prefetchDistance * 2

jumpThreshold: 某阈值! 好吧我摊牌了, 我不知道. [奸笑]

4.监听加载状态:

LoadState: 表示加载状态密封类;

LoadState.NotLoading: 加载完毕, 并且界面也已相应更新

LoadState.Error: 加载失败.

LoadState.Loading: 正在加载..

lifecycleScope.launch {

    mAdapter.loadStateFlow.collectLatest { loadStates ->

        when(loadStates.refresh){

            is LoadState.Loading -> {

                Log.d("pppppppppppppp", "加载中")

            }

            is LoadState.Error -> {

                Log.d("pppppppppppppp", "加载失败")

            }

            is LoadState.NotLoading -> {

                Log.d("pppppppppppppp", "完事了")

            }

            else -> {

                Log.d("pppppppppppppp", "这是啥啊")

            }

        }

    }

    //或者:

    mAdapter.addLoadStateListener { ... }

}

5. 状态适配器 LoadStateAdapter

用于直接在显示的分页数据列表中呈现加载状态。 例如: 尾部显示 正在加载, 加载失败, 没有更多等;

5.1 自定义 MyLoadStateAdapter 继承 LoadStateAdapter

重写 onCreateViewHolder, onBindViewHolder

retry: 如果加载失败, 想要重试, 则提供该高阶函数参数; 否则不需要它

class MyLoadStateAdapter(

    /**

     * 当下一页加载失败时, 继续尝试加载下一页;

     */

    private val retry: () -> Unit

) : LoadStateAdapter<LoadStateViewHolder>() {

    override fun onCreateViewHolder(

        parent: ViewGroup,

        loadState: LoadState

    ) = LoadStateViewHolder(parent, retry)

    override fun onBindViewHolder(

        holder: LoadStateViewHolder,

        loadState: LoadState

    ) = holder.bind(loadState)

}

5.2 自定义 LoadStateViewHolder

功能:

加载中显示 Loading;
加载失败显示错误信息. 包括 http, IO 异常, 后台给的错误 msg 等;
没有更多

class LoadStateViewHolder (

    parent: ViewGroup,

    retry: () -> Unit

) : RecyclerView.ViewHolder(

    LayoutInflater.from(parent.context)

        .inflate(R.layout.view_loading_more, parent, false)

) {

    private val binding = ViewLoadingMoreBinding.bind(itemView)

    init {

        //当点击重试按钮时, 调用 PagingDataAdapter 的 retry() 重新尝试加载

        binding.btnLoadingRetry.setOnClickListener {

            retry()

        }

    }

    fun bind(loadState: LoadState) {

        // 当加载失败时.

        if(loadState is LoadState.Error){

            // 将没有更多封装成 NoMoreException;  此时显示没有更多 View

            if(loadState.error is NoMoreException){

                hideNoMoreUi(false) //显示 没有更多 View

                hideErrUi(true)     //隐藏 失败 View

            }else{

                hideNoMoreUi(true)

                hideErrUi(false, loadState.error.message)   //显示失败 View时, 填充错误 msg

            }

        }else{

            hideNoMoreUi(true)

            hideErrUi(true)

        }

        //加载中..

        binding.pbLoadingBar.visibility = if(loadState is LoadState.Loading){

            View.VISIBLE

        }else{

            View.GONE

        }

    }

    /**

     * 隐藏没有更多View;

     */

    private fun hideNoMoreUi(hide: Boolean){

        if(hide){

            binding.tvLoadingHint.visibility = View.GONE

        }else{

            binding.tvLoadingHint.visibility = View.VISIBLE

        }

    }

    /**

     * 隐藏 加载失败View;

     */

    private fun hideErrUi(hide: Boolean, msg: String? = null){

        if(hide){

            binding.tvLoadingError.visibility = View.GONE

            binding.btnLoadingRetry.visibility = View.GONE

        }else{

            binding.tvLoadingError.text = msg

            binding.tvLoadingError.visibility = View.VISIBLE

            binding.btnLoadingRetry.visibility = View.VISIBLE

        }

    }

}

顺便补一下 NoMoreException; 用法? 在下面 PagingSource 喽.

class NoMoreException: RuntimeException()

5.3 layout view_loading_more.xml

包含: TextView: 没有更多; ProgressBar: 加载中; TextView: 错误信息; Button: 重试按钮

<layout>

<androidx.constraintlayout.widget.ConstraintLayout

    xmlns:android="http://schemas.android.com/apk/res/android"

    xmlns:app="http://schemas.android.com/apk/res-auto"

    android:paddingHorizontal="16dp"

    android:layout_width="match_parent"

    android:layout_height="54dp">

    <View

        android:layout_width="match_parent"

        android:layout_height="1px"

        android:background="#e5e5e5"

        app:layout_constraintTop_toTopOf="parent"/>

    <TextView

        android:id="@+id/tv_loading_hint"

        android:layout_width="wrap_content"

        android:layout_height="wrap_content"

        android:textSize="14sp"

        android:textColor="#798080"

        android:text="已经到底了"

        android:visibility="gone"

        app:layout_constraintStart_toStartOf="parent"

        app:layout_constraintEnd_toEndOf="parent"

        app:layout_constraintTop_toTopOf="parent"

        app:layout_constraintBottom_toBottomOf="parent"/>

    <ProgressBar

        android:id="@+id/pb_loading_bar"

        android:layout_width="32dp"

        android:layout_height="32dp"

        android:visibility="gone"

        android:indeterminateTint="#7671F8"

        app:layout_constraintStart_toStartOf="parent"

        app:layout_constraintEnd_toEndOf="parent"

        app:layout_constraintTop_toTopOf="parent"

        app:layout_constraintBottom_toBottomOf="parent"/>

    <TextView

        android:id="@+id/tv_loading_error"

        android:layout_width="0dp"

        android:layout_height="wrap_content"

        android:textColor="@color/shape_red"

        android:text="错误信息"

        android:layout_marginEnd="8dp"

        android:maxLines="2"

        android:ellipsize="end"

        android:visibility="gone"

        app:layout_constraintTop_toTopOf="parent"

        app:layout_constraintBottom_toBottomOf="parent"

        app:layout_constraintEnd_toStartOf="@id/btn_loading_retry"

        app:layout_constraintStart_toStartOf="parent"/>

    <Button

        android:id="@+id/btn_loading_retry"

        android:layout_width="60dp"

        android:layout_height="38dp"

        android:textColor="@color/white"

        android:text="重试"

        android:visibility="gone"

        android:background="@drawable/shape_blue_7671f8_r8"

        app:layout_constraintEnd_toEndOf="parent"

        app:layout_constraintTop_toTopOf="parent"

        app:layout_constraintBottom_toBottomOf="parent"/>

</androidx.constraintlayout.widget.ConstraintLayout>

</layout>

5.4 PagingSource 需要根据情况返回不同的 LoadResult

代码如下, 直接看注释就可以了;

class DynamicDataSource: PagingSource<Int, DynamicTwo>() {

    private var maxPage = 1

    override fun getRefreshKey(state: PagingState<Int, DynamicTwo>): Int? = null

    override suspend fun load(params: LoadParams<Int>): LoadResult<Int, DynamicTwo> {

        try {

            val nextPageNumber = params.key ?: 1

            //超过页码时,  返回没有更多状态 NoMoreException

            if(nextPageNumber > maxPage){

                return LoadResult.Error(NoMoreException())

            }

            //这是 Retrofit 网络请求

            val map = mapOf("page" to nextPageNumber, "pageSize" to params.loadSize)

            val param = ApiManager.INSTANCE.getJsonBody(map)

            val response = ApiManager.INSTANCE.mApi.getDynamicList(param)

            //后台 响应错误码时;  用 RuntimeException 返回错误信息

            if(response.code != 200){

                return LoadResult.Error(RuntimeException(response.msg))

            }

            //解析响应数据

            val jo = response.data

            val list = jo?.getAsJsonArray("newsList")?.toString()?.toBeanList<DynamicTwo>() ?: mutableListOf()

            maxPage = jo?.get("totalPage").toString().toInt()

            //返回正常数据

            return LoadResult.Page(

                data = list,

                prevKey = null, // Only paging forward. 只向后加载就给null

                // nextKey 下一页页码;  尾页给 null;  否则当前页码加1

                nextKey = nextPageNumber + 1

            )

        } catch (e: IOException) {

            // IOException for network failures.

            return LoadResult.Error(e)

        } catch (e: HttpException) {

            // HttpException for any non-2xx HTTP status codes.

            return LoadResult.Error(e)

        } catch (e: Exception) {

            // IOException for network failures.

            return LoadResult.Error(e)

        }

    }

}

代码中请求参数只给了 page 和 pageSize; 其他参数怎么给?

DynamicDataSource 的构造方法传入;
动态参数怎么办? 写回调, 从ViewModel 中组装请求数据
麻烦怎么办? 创建 BaseDataSource. 将相似代码封装. 请求参数通过高阶函数从ViewModel组装;

5.5 前台使用:

首先正常初始化 Adapter, RecycleView, 并调用 mViewModel?.flow?.collectLatest

其次 RecycleView 的 adaper 不要给主数据Adapter; 而是给 withLoadStateFooter() 返回的 ConcatAdapter

val stateAdapter = mAdapter.withLoadStateFooter(MyLoadStateAdapter(mAdapter::retry))

mDataBind.rvRecycle.let {

    it.layoutManager = LinearLayoutManager(mActivity)

    // ****  这里不要给 mAdapter(主数据 Adapter);  而是给 stateAdapter ***

    it.adapter = stateAdapter

}


PagingDataAdapter 的 withLoadStateFooter 方法会返回一个新的 ConcatAdapter 对象; 请将这个 ConcatAdapter 设置给 RecycleView

withLoadStateFooter 的参数 就是我们自定义的 MyLoadStateAdapter;  retry -> mAdapter.retry()

5.6 看一下 LoadStateAdapter 的源码;

可以发现, 这是个单条目 Adapter.

并且只有当 LoadState.Loading, LoadState.Error 时才会出现; 当然也可以重写 displayLoadStateAsItem(), 让它所有状态都出现;

当列表状态变化时, 会设置 loadState 参数; 动态增删改 Item;

abstract class LoadStateAdapter<VH : RecyclerView.ViewHolder> : RecyclerView.Adapter<VH>() {

    var loadState: LoadState = LoadState.NotLoading(endOfPaginationReached = false)

        set(loadState) {

            if (field != loadState) {

                val oldItem = displayLoadStateAsItem(field)

                val newItem = displayLoadStateAsItem(loadState)

                if (oldItem && !newItem) {

                    notifyItemRemoved(0)

                } else if (newItem && !oldItem) {

                    notifyItemInserted(0)

                } else if (oldItem && newItem) {

                    notifyItemChanged(0)

                }

                field = loadState

            }

        }

    final override fun onCreateViewHolder(parent: ViewGroup, viewType: Int): VH {

        return onCreateViewHolder(parent, loadState)

    }

    final override fun onBindViewHolder(holder: VH, position: Int) {

        onBindViewHolder(holder, loadState)

    }

    final override fun getItemViewType(position: Int): Int = getStateViewType(loadState)


　　//条目数量, final 不可重写;

    final override fun getItemCount(): Int = if (displayLoadStateAsItem(loadState)) 1 else 0

    abstract fun onCreateViewHolder(parent: ViewGroup, loadState: LoadState): VH

    abstract fun onBindViewHolder(holder: VH, loadState: LoadState)

    open fun getStateViewType(loadState: LoadState): Int = 0

　　
　　//只有当 Loading, Error 时, 才显示

    open fun displayLoadStateAsItem(loadState: LoadState): Boolean {

        return loadState is LoadState.Loading || loadState is LoadState.Error

    }

}

5.7 LoadStateAdapter 改建头尾

如果我们把它强行改造成 Header footer:

重写 displayLoadStateAsItem() 不管什么状态, 都返回true
loadState 不能重写, 所以 notifyItemChanged(0) 必被调用;
暴力一点, 直接重写 notifyItemChanged() 让它什么都不做? 好吧它也是 final, 不能重写
既然要调刷新, 那就调吧 [破涕为笑]; 那怎么办尽量少执行无用代码呗, 那就 onBindViewHolder() 啥也不干;
头尾由前端控制, Adapter 只需要把这个固定View显示就 ok 了
如果能阻止 notifyItemChanged(0) 那就更好了. 聪明的你有没有办法呢. [666]

最终 Adapter:

class EndViewAdapter(val v: View) : LoadStateAdapter<EndHolder>() {

    override fun onCreateViewHolder(

        parent: ViewGroup,

        loadState: LoadState

    ) = EndHolder(v)

    override fun onBindViewHolder(holder: EndHolder, loadState: LoadState){

        //啥也不干

    }

    override fun displayLoadStateAsItem(loadState: LoadState) = true

}

class EndHolder(itemView: View) : RecyclerView.ViewHolder(itemView)

好吧, 一运行, 崩了 [捂脸]; called attach on a child which is not detached

怎么办, 取消 RecycleView 的刷新闪烁动画:

(mDataBind.rvRecycle.itemAnimator as SimpleItemAnimator).supportsChangeAnimations = false;

整个 RecycleView 的条目刷新动画都没了, 这不是个事啊! 但博主已经没办法了 [捂脸]

没办法了怎么办? 不用 Header 了? 当然不是, 我们只是不用 LoadStateAdapter 做头尾了; 我们用 ConcatAdapter 做头尾;

就是在 withLoadState... 之后, 再自己组装 ConcatAdapter

6. MAP: 数据转换; 有的时候, 我们需要对响应数据进行预先处理;

例如: 根据条件,预先改变实体内容;

val flow: Flow<PagingData<DynamicTwo>> = Pager(

    PagingConfig(pageSize = 10, prefetchDistance = 2,initialLoadSize = 10)

) {

    DynamicDataSource()

}.flow

    .cachedIn(viewModelScope)

    .map {

        it.map { entity ->

            // 这里根据条件,  预先处理数据

            if(entity.isLike == 1){

                entity.nickName = "变变变, 我是百变小魔女"

            }else{

                entity.nickName = "呜哈哈哈"

            }

            entity

        }

    }

例如: 组合实体; 根据条件产生不同实体;

val flow: Flow<PagingData<GroupEntity>> = Pager(

    PagingConfig(pageSize = 10, prefetchDistance = 2,initialLoadSize = 10)

) {

    DynamicDataSource()

}.flow

    .cachedIn(viewModelScope)

    .map {

        it.map { entity ->

            // 这里根据条件,  预先处理数据

            if(entity.isLike == 1){

                GroupEntity.DynamicTwoItem(entity)

            }else{

                GroupEntity.DynamicItem(DynamicEntity())

            }

        }

    }

sealed class GroupEntity{

    class DynamicTwoItem (val entity: DynamicTwo): GroupEntity()

    class DynamicItem (val entity: DynamicEntity): GroupEntity()

}

又例如: 插入实体分隔符等

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孟老板 Paging3 (一) 入门

Paging3 (一) 入门

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