将 Paging 3 alpha 更新为稳定导致索引问题 Android答案

【问题标题】：Updating Paging 3 alpha to stable cause indexing issue Android将 Paging 3 alpha 更新为稳定导致索引问题 Android
【发布时间】：2021-12-13 17:19:49
【问题描述】：

嘿，我正在使用 Paging 3 库和 ViewPager 2。它在其中加载无限数据。

implementation "androidx.paging:paging-runtime-ktx:3.0.0-alpha07"

DataSource.kt

package com.example.viewpagerexample

import java.util.*

class DataSource(
    private val size: Int = 5,
    private val currentDate: Date,
    private val limitDate: Date?
) {

    fun returnData(pageNumber: Int): List<Date> {

        val dateList = mutableListOf<Date>()
        val startDateForPage = startDate(pageNumber)
        val tempCalendar = Calendar.getInstance()

        tempCalendar.time = startDateForPage
        val lastDateForPage = endDate(startDateForPage)

        while (tempCalendar.time < lastDateForPage) {
            if (limitDate == null ||
                tempCalendar.time.before(limitDate) ||
                tempCalendar.time == limitDate
            ) {
                dateList.add(tempCalendar.time)
                tempCalendar.add(Calendar.DATE, 1)
            } else {
                break
            }
        }
        return dateList
    }

    private fun startDate(pageNumber: Int): Date {
        if (pageNumber == 0) {
            return currentDate
        } else {
            Calendar.getInstance().let {
                it.time = currentDate
                it.add(Calendar.DATE, pageNumber * size)
                return it.time
            }
        }
    }

    private fun endDate(firstDateForPage: Date): Date {
        Calendar.getInstance().let {
            it.time = firstDateForPage
            it.add(Calendar.DATE, size)
            return it.time
        }
    }
}

ViewPagerPagingSource.kt

class ViewPagerPagingSource(
    private val dataSource: DataSource
) : PagingSource<Int, Date>() {

    override suspend fun load(params: LoadParams<Int>): LoadResult<Int, Date> {
        val position = params.key ?: 0

        return try {
            val data = dataSource.returnData(position)
            LoadResult.Page(
                data = data,
                prevKey = if (data.isEmpty()) null else position - 1,
                nextKey = if (data.isEmpty()) null else position + 1,
                itemsBefore = LoadResult.Page.COUNT_UNDEFINED,
                itemsAfter = LoadResult.Page.COUNT_UNDEFINED
            )
        } catch (exception: IOException) {
            LoadResult.Error(exception)
        }
    }

}

所有代码工作正常。当应用程序启动时，它会加载当前日期页面。

现在当我将库更新到这个版本时

implementation "androidx.paging:paging-runtime-ktx:3.0.1"

我猜它跳转到 -1 页面，看起来像这样

我不明白为什么这会导致问题，并且我编辑了 ViewPagerPagingSource 以实现另一种方法

override fun getRefreshKey(state: PagingState<Int, Date>): Int? {
        return state.anchorPosition
}

我不明白更新库后导致问题的原因。我正在添加我的 github 存储库Example。请有人建议我代码中出了什么问题？

更新

我尝试学习分页概念并更新我的代码。另外，请按照@dlam 的建议进行更改。

如果我将日期前几天作为当前日期传递，则再次跳转 1 页。 Github

2021-11-21 21:20:40.820 5460-5460/com.example.viewpagerexample E/Page -1: [06/11/2021, 07/11/2021, 08/11/2021, 09/11/2021, 10/11/2021]
2021-11-21 21:20:40.821 5460-5460/com.example.viewpagerexample E/Page 0: [11/11/2021, 12/11/2021, 13/11/2021, 14/11/2021, 15/11/2021]
2021-11-21 21:20:40.821 5460-5460/com.example.viewpagerexample E/Page 1: [16/11/2021, 17/11/2021, 18/11/2021, 19/11/2021, 20/11/2021]

更新 2

@MuhannadFakhouri 回答后我尝试了这个逻辑

package com.example.viewpagerexample

import java.util.*

class DataSource(
    private val size: Int = 5,
    private val currentDate: Date,
    private val limitDate: Date? = null
) {

    fun returnData(pageNumber: Int): Result {
        val dateList = mutableListOf<Date>()

        val startDateForPage = startDate(pageNumber)
        val tempCalendar = Calendar.getInstance()

        tempCalendar.time = startDateForPage
        val lastDateForPage = endDate(startDateForPage)

        var index = size
        while (tempCalendar.time <= lastDateForPage && index-- > 0) {
            dateList.add(tempCalendar.time)
            tempCalendar.add(Calendar.DATE, 1)
        }
        val limitCalendar = Calendar.getInstance().apply { limitDate }
        return Result(
            result = dateList,
            pageNumber - 1,
            if (dateList.lastOrNull()?.let { Calendar.getInstance().apply { time = it } }
                    ?.let {
                        it.get(Calendar.YEAR) == limitCalendar.get(Calendar.YEAR) &&
                                it.get(Calendar.DAY_OF_YEAR) == limitCalendar.get(Calendar.DAY_OF_YEAR)
                    } == true)
                null
            else
                pageNumber + 1
        )
    }

    private fun startDate(pageNumber: Int): Date {
        Calendar.getInstance().let {
            it.time = currentDate
            it.add(Calendar.DATE, pageNumber * size)
            return it.time
        }
    }

    private fun endDate(firstDateForPage: Date): Date? {
        Calendar.getInstance().let {
            it.time = firstDateForPage
            it.add(Calendar.DATE, size)

            limitDate?.let { limit ->
                return if (it.time > limit) limit else it.time
            } ?: run {
                return it.time
            }
        }
    }

    data class Result(
        val result: MutableList<Date>,
        val prevKey: Int?,
        val nextKey: Int?
    )
}

日志

2022-01-08 23:39:12.601 7886-7886/com.example.viewpagerexample E/Page -1: [24/12/2021, 25/12/2021, 26/12/2021, 27/12/2021, 28/12/2021]
2022-01-08 23:39:12.603 7886-7886/com.example.viewpagerexample E/Page 0: [29/12/2021, 30/12/2021, 31/12/2021, 01/01/2022, 02/01/2022]
2022-01-08 23:39:12.604 7886-7886/com.example.viewpagerexample E/Page 1: [03/01/2022, 04/01/2022, 05/01/2022, 06/01/2022, 07/01/2022]

截图

它需要显示 29/12/2021 日期，但它显示了 03/01/2022

新的详细解释

Youtube Link with detail explanination

首先它打开只有一个按钮的屏幕。当我点击它时，它将打开 viewpager 屏幕。每当我点击它打开不同的日期屏幕。我的主要观点是，如果我作为 当前日期 传递，它将在 当前日期 作为查看寻呼机主屏幕打开。如果我通过任何日期，例如 29/12/2021，它将打开这个日期。新附加的视频显示了它的行为方式。它将显示代码、日志和模拟器。

第一次当我点击它打开 04/01/22 这是索引 -1 你可以检查视频

2022-01-09 00:01:03.108 9637-9637/com.example.viewpagerexample E/Page -1: [04/01/2022, 05/01/2022, 06/01/2022, 07/01/2022, 08/01/2022]
2022-01-09 00:01:03.109 9637-9637/com.example.viewpagerexample E/Page 0: [09/01/2022]
2022-01-09 00:01:03.109 9637-9637/com.example.viewpagerexample E/Page 1: []

第二次还是一样

2022-01-09 00:01:06.488 9637-9637/com.example.viewpagerexample E/Page -1: [04/01/2022, 05/01/2022, 06/01/2022, 07/01/2022, 08/01/2022]
2022-01-09 00:01:06.488 9637-9637/com.example.viewpagerexample E/Page 0: [09/01/2022]
2022-01-09 00:01:06.488 9637-9637/com.example.viewpagerexample E/Page 1: []

第三次打开 09/01/22 正确。

2022-01-09 00:01:09.181 9637-9637/com.example.viewpagerexample E/Page -1: [04/01/2022, 05/01/2022, 06/01/2022, 07/01/2022, 08/01/2022]
2022-01-09 00:01:09.181 9637-9637/com.example.viewpagerexample E/Page 0: [09/01/2022]
2022-01-09 00:01:09.181 9637-9637/com.example.viewpagerexample E/Page 1: []

我的问题是为什么这会导致这种问题有时打开正确的 index 有时会打开错误的 index。

谢谢

日志

2022-01-09 14:15:25.501 21246-21246/com.example.viewpagerexample D/Debugging paging: load: null java.lang.Throwable
        at com.example.viewpagerexample.ViewPagerPagingSource.load(ViewPagerPagingSource.kt:16)
        at androidx.paging.PageFetcherSnapshot.doInitialLoad(PageFetcherSnapshot.kt:283)
        at androidx.paging.PageFetcherSnapshot.access$doInitialLoad(PageFetcherSnapshot.kt:54)
        at androidx.paging.PageFetcherSnapshot$pageEventFlow$1.invokeSuspend(PageFetcherSnapshot.kt:163)
        at androidx.paging.PageFetcherSnapshot$pageEventFlow$1.invoke(Unknown Source:8)
        at androidx.paging.PageFetcherSnapshot$pageEventFlow$1.invoke(Unknown Source:4)
        at androidx.paging.CancelableChannelFlowKt$cancelableChannelFlow$1.invokeSuspend(CancelableChannelFlow.kt:30)
        at androidx.paging.CancelableChannelFlowKt$cancelableChannelFlow$1.invoke(Unknown Source:8)
        at androidx.paging.CancelableChannelFlowKt$cancelableChannelFlow$1.invoke(Unknown Source:4)
        at androidx.paging.SimpleChannelFlowKt$simpleChannelFlow$1$1$producer$1$1.invokeSuspend(SimpleChannelFlow.kt:57)
        at androidx.paging.SimpleChannelFlowKt$simpleChannelFlow$1$1$producer$1$1.invoke(Unknown Source:8)
        at androidx.paging.SimpleChannelFlowKt$simpleChannelFlow$1$1$producer$1$1.invoke(Unknown Source:4)
        at kotlinx.coroutines.intrinsics.UndispatchedKt.startUndispatchedOrReturn(Undispatched.kt:89)
        at kotlinx.coroutines.CoroutineScopeKt.coroutineScope(CoroutineScope.kt:264)
        at androidx.paging.SimpleChannelFlowKt$simpleChannelFlow$1$1$producer$1.invokeSuspend(SimpleChannelFlow.kt:52)
        at kotlin.coroutines.jvm.internal.BaseContinuationImpl.resumeWith(ContinuationImpl.kt:33)
        at kotlinx.coroutines.DispatchedTask.run(DispatchedTask.kt:106)
        at kotlinx.coroutines.EventLoop.processUnconfinedEvent(EventLoop.common.kt:69)
        at kotlinx.coroutines.internal.DispatchedContinuationKt.resumeCancellableWith(DispatchedContinuation.kt:375)
        at kotlinx.coroutines.internal.DispatchedContinuationKt.resumeCancellableWith$default(DispatchedContinuation.kt:278)
        at kotlinx.coroutines.DispatchedCoroutine.afterResume(Builders.common.kt:256)
        at kotlinx.coroutines.AbstractCoroutine.resumeWith(AbstractCoroutine.kt:102)
        at kotlin.coroutines.jvm.internal.BaseContinuationImpl.resumeWith(ContinuationImpl.kt:46)
        at kotlinx.coroutines.DispatchedTask.run(DispatchedTask.kt:106)
        at android.os.Handler.handleCallback(Handler.java:883)
        at android.os.Handler.dispatchMessage(Handler.java:100)
        at android.os.Looper.loop(Looper.java:214)
        at android.app.ActivityThread.main(ActivityThread.java:7697)
        at java.lang.reflect.Method.invoke(Native Method)
        at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:516)
        at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:950)
2022-01-09 14:15:25.512 21246-21246/com.example.viewpagerexample D/Debugging paging: load: -1 java.lang.Throwable
        at com.example.viewpagerexample.ViewPagerPagingSource.load(ViewPagerPagingSource.kt:16)
        at androidx.paging.PageFetcherSnapshot.doLoad(PageFetcherSnapshot.kt:406)
        at androidx.paging.PageFetcherSnapshot.access$doLoad(PageFetcherSnapshot.kt:54)
        at androidx.paging.PageFetcherSnapshot$collectAsGenerationalViewportHints$$inlined$collect$1.emit(Collect.kt:135)
        at kotlinx.coroutines.flow.FlowKt__ChannelsKt.emitAllImpl$FlowKt__ChannelsKt(Channels.kt:62)
        at kotlinx.coroutines.flow.FlowKt__ChannelsKt.access$emitAllImpl$FlowKt__ChannelsKt(Channels.kt:1)
        at kotlinx.coroutines.flow.FlowKt__ChannelsKt$emitAllImpl$1.invokeSuspend(Unknown Source:14)
        at kotlin.coroutines.jvm.internal.BaseContinuationImpl.resumeWith(ContinuationImpl.kt:33)
        at kotlinx.coroutines.DispatchedTask.run(DispatchedTask.kt:106)
        at kotlinx.coroutines.EventLoop.processUnconfinedEvent(EventLoop.common.kt:69)
        at kotlinx.coroutines.DispatchedTaskKt.resumeUnconfined(DispatchedTask.kt:244)
        at kotlinx.coroutines.DispatchedTaskKt.dispatch(DispatchedTask.kt:161)
        at kotlinx.coroutines.CancellableContinuationImpl.dispatchResume(CancellableContinuationImpl.kt:397)
        at kotlinx.coroutines.CancellableContinuationImpl.resumeImpl(CancellableContinuationImpl.kt:431)
        at kotlinx.coroutines.CancellableContinuationImpl.resumeImpl$default(CancellableContinuationImpl.kt:420)
        at kotlinx.coroutines.CancellableContinuationImpl.resumeWith(CancellableContinuationImpl.kt:328)
        at kotlinx.coroutines.flow.SharedFlowImpl.tryEmit(SharedFlow.kt:368)
        at androidx.paging.HintHandler$HintFlow.setValue(HintHandler.kt:133)
        at androidx.paging.HintHandler$processHint$1.invoke(HintHandler.kt:85)
        at androidx.paging.HintHandler$processHint$1.invoke(HintHandler.kt:79)
        at androidx.paging.HintHandler$State.modify(HintHandler.kt:119)
        at androidx.paging.HintHandler.processHint(HintHandler.kt:79)
        at androidx.paging.PageFetcherSnapshot.accessHint(PageFetcherSnapshot.kt:197)
        at androidx.paging.PageFetcher$PagerUiReceiver.accessHint(PageFetcher.kt:215)
        at androidx.paging.PagingDataDiffer$collectFrom$2$1$1.invokeSuspend(PagingDataDiffer.kt:175)
        at kotlin.coroutines.jvm.internal.BaseContinuationImpl.resumeWith(ContinuationImpl.kt:33)
        at kotlinx.coroutines.DispatchedTask.run(DispatchedTask.kt:106)
        at android.os.Handler.handleCallback(Handler.java:883)
        at android.os.Handler.dispatchMessage(Handler.java:100)
        at android.os.Looper.loop(Looper.java:214)
        at android.app.ActivityThread.main(ActivityThread.java:7697)
        at java.lang.reflect.Method.invoke(Native Method)
        at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:516)
        at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:950)
2022-01-09 14:15:25.519 21246-21246/com.example.viewpagerexample D/Debugging paging: load: -2 java.lang.Throwable
        at com.example.viewpagerexample.ViewPagerPagingSource.load(ViewPagerPagingSource.kt:16)
        at androidx.paging.PageFetcherSnapshot.doLoad(PageFetcherSnapshot.kt:406)
        at androidx.paging.PageFetcherSnapshot.access$doLoad(PageFetcherSnapshot.kt:54)
        at androidx.paging.PageFetcherSnapshot$collectAsGenerationalViewportHints$$inlined$collect$1.emit(Collect.kt:135)
        at kotlinx.coroutines.flow.FlowKt__ChannelsKt.emitAllImpl$FlowKt__ChannelsKt(Channels.kt:62)
        at kotlinx.coroutines.flow.FlowKt__ChannelsKt.access$emitAllImpl$FlowKt__ChannelsKt(Channels.kt:1)
        at kotlinx.coroutines.flow.FlowKt__ChannelsKt$emitAllImpl$1.invokeSuspend(Unknown Source:14)
        at kotlin.coroutines.jvm.internal.BaseContinuationImpl.resumeWith(ContinuationImpl.kt:33)
        at kotlinx.coroutines.DispatchedTask.run(DispatchedTask.kt:106)
        at kotlinx.coroutines.EventLoop.processUnconfinedEvent(EventLoop.common.kt:69)
        at kotlinx.coroutines.DispatchedTaskKt.resumeUnconfined(DispatchedTask.kt:244)
        at kotlinx.coroutines.DispatchedTaskKt.dispatch(DispatchedTask.kt:161)
        at kotlinx.coroutines.CancellableContinuationImpl.dispatchResume(CancellableContinuationImpl.kt:397)
        at kotlinx.coroutines.CancellableContinuationImpl.resumeImpl(CancellableContinuationImpl.kt:431)
        at kotlinx.coroutines.CancellableContinuationImpl.resumeImpl$default(CancellableContinuationImpl.kt:420)
        at kotlinx.coroutines.CancellableContinuationImpl.resumeWith(CancellableContinuationImpl.kt:328)
        at kotlinx.coroutines.flow.SharedFlowImpl.tryEmit(SharedFlow.kt:368)
        at androidx.paging.HintHandler$HintFlow.setValue(HintHandler.kt:133)
        at androidx.paging.HintHandler$processHint$1.invoke(HintHandler.kt:85)
        at androidx.paging.HintHandler$processHint$1.invoke(HintHandler.kt:79)
        at androidx.paging.HintHandler$State.modify(HintHandler.kt:119)
        at androidx.paging.HintHandler.processHint(HintHandler.kt:79)
        at androidx.paging.PageFetcherSnapshot.accessHint(PageFetcherSnapshot.kt:197)
        at androidx.paging.PageFetcher$PagerUiReceiver.accessHint(PageFetcher.kt:215)
        at androidx.paging.PagingDataDiffer.get(PagingDataDiffer.kt:271)
        at androidx.paging.AsyncPagingDataDiffer.getItem(AsyncPagingDataDiffer.kt:214)
        at androidx.paging.PagingDataAdapter.getItem(PagingDataAdapter.kt:231)
        at com.example.viewpagerexample.ViewPagerAdapter.onBindViewHolder(ViewPagerAdapter.kt:11)
        at com.example.viewpagerexample.ViewPagerAdapter.onBindViewHolder(ViewPagerAdapter.kt:8)
        at androidx.recyclerview.widget.RecyclerView$Adapter.onBindViewHolder(RecyclerView.java:7254)
        at androidx.recyclerview.widget.RecyclerView$Adapter.bindViewHolder(RecyclerView.java:7337)
        at androidx.recyclerview.widget.RecyclerView$Recycler.tryBindViewHolderByDeadline(RecyclerView.java:6194)
        at androidx.recyclerview.widget.RecyclerView$Recycler.tryGetViewHolderForPositionByDeadline(RecyclerView.java:6460)
        at androidx.recyclerview.widget.RecyclerView$Recycler.getViewForPosition(RecyclerView.java:6300)
        at androidx.recyclerview.widget.RecyclerView$Recycler.getViewForPosition(RecyclerView.java:6296)
        at androidx.recyclerview.widget.LinearLayoutManager$LayoutState.next(LinearLayoutManager.java:2330)
        at androidx.recyclerview.widget.LinearLayoutManager.layoutChunk(LinearLayoutManager.java:1631)
        at androidx.recyclerview.widget.LinearLayoutManager.fill(LinearLayoutManager.java:1591)
        at androidx.recyclerview.widget.LinearLayoutManager.onLayoutChildren(LinearLayoutManager.java:668)
        at androidx.recyclerview.widget.RecyclerView.dispatchLayoutStep2(RecyclerView.java:4309)
        at androidx.recyclerview.widget.RecyclerView.dispatchLayout(RecyclerView.java:4012)
        at androidx.recyclerview.widget.RecyclerView.onLayout(RecyclerView.java:4578)
        at android.view.View.layout(View.java:22085)
        at android.view.ViewGroup.layout(ViewGroup.java:6290)
        at androidx.viewpager2.widget.ViewPager2.onLayout(ViewPager2.java:527)
        at android.view.View.layout(View.java:22085)
        at android.view.ViewGroup.layout(ViewGroup.java:6290)
        at androidx.constraintlayout.widget.ConstraintLayout.onLayout(ConstraintLayout.java:1873

【问题讨论】：

我不太了解 Paging 以查看您是否做错了什么，请在 Google 问题跟踪器上尝试 creating an issue。现在您可以使用适合您的版本。

标签： android kotlin android-paging android-paging-library android-paging-3

【解决方案1】：

我注意到你在这里提交了一个错误：https://issuetracker.google.com/204328119，但我也想更新这个答案，以供其他未来在 SO 上阅读此问题的人使用。

核心问题是在 ViewPager 准备好开始绑定这些项目之前，您正在 Paging 上开始收集。你应该使用lifecycleScope.launchWhenCreated 而不是lifecycleScope.launc 来解决这个问题：

lifecycleScope.launchWhenCreated {
  viewModel.dataList.collectLatest {
    adapter.submitData(it)
  }
}

我注意到的另一个问题（也可以解决此问题）是您启用了占位符，但没有在LoadResult.Page 中传递itemsBefore 或itemsAfter。启用占位符并具有静态计数也会为您的视图提供要绑定的列表大小，但是由于您传递了COUNT_UNDEFINED，因此 Paging 无法使用 null 占位符正确填充列表，因为它不知道要添加多少。

【讨论】：

我可以在itemBefore 或itemAfrer 中传递什么值？
您只需在LoadResult.Page 的构造函数中传递该页面前后的Int 项目数。例如，如果您有 [null, null, 3, null]，您将在 Page 中为 itemsBefore 传递 2 和为 itemsAfter 传递 1，其 data 仅包含 3。
我尝试学习分页逻辑并尝试更新我的逻辑，但仍然导致同样的问题Github。
在我的ViewPagerViewModel 中，我创建了两个函数currentDate，其中我在当前日期前几天通过。它再次导致从索引 1 开始的相同问题。但如果我通过当前日期，它在limitDate 中工作正常。你知道为什么会导致这个问题吗？

【解决方案2】：

我没有深入研究这一点以及这与库的版本有何关系，但我注意到您的数据源逻辑中存在故障您正在执行以下检查

                if (lastDateForPage == limitDate)
                    null
                else
                    pageNumber + 1

在这个循环之后

        while (tempCalendar.time <= lastDateForPage && index-- > 0) {
            dateList.add(tempCalendar.time)
            tempCalendar.add(Calendar.DATE, 1)
        }

循环实际上可能会在没有达到限制的情况下中断，这意味着您的数据源不能保证总是达到 limitDate。

更改条件lastDateForPage == limitDate 以检查以下内容，应该可以解决问题

            val limitCalendar = Calendar.getInstance().apply { limitDate }

            if (dateList.lastOrNull()?.let { Calendar.getInstance().apply { time = it } }
                    ?.let {
                        it.get(Calendar.YEAR) == limitCalendar.get(Calendar.YEAR) &&
                                it.get(Calendar.DAY_OF_YEAR) == limitCalendar.get(Calendar.DAY_OF_YEAR)
                    } == true)
                null
            else
                pageNumber + 1

【讨论】：

我试过你的代码它没有工作它给出错误的日期:(
那我需要你描述一下你想要实现的用例，这样我才能理解你面临的问题
我需要在当前页面浏览器中显示为日期。如果我将日期传递为29/11/2021，则需要以该日期开始屏幕。但现在它采用了错误的索引。
我在详细信息部分添加了新的解释，带有 YouTube 链接，请查看。如果有任何疑问，请告诉我。谢谢
我无法重现该问题，无论如何让我们假设您的数据源没有问题，将此日志语句 Log.d("Debugging paging", "load: ${params.key} ${Throwable().stackTraceToString()}") 添加到您的 ViewPagerPagingSource.load 并发布其输出