前言

在现代移动应用中，大数据列表的展示是一种常见的需求。然而，处理大量数据的网络加载过程可能会带来一些挑战，比如长时间的等待、空数据或错误信息的展示。为了提供更好的用户体验，我们需要在数据加载过程中显示合适的加载状态，使用户能够清楚地知道当前数据的加载状态，并在必要时提供相应的反馈和交互。

Paging 3 是 Google 推出的用于处理大数据列表的分页库，它为我们提供了一种优雅的方式来处理数据加载和分页。而在 Paging 3 中，处理网络数据加载状态是非常重要的一部分。用户可能会在滚动列表时不断加载新的数据，或者在加载失败时需要进行错误重试。在这些情况下，合理的加载状态展示对于提供良好的用户体验至关重要。

在本文中，我们将深入探讨在使用 Paging 3 进行网络数据加载时，如何处理加载状态，以及如何使用 Paging 3 提供的工具和方法来实现更好的用户体验。我们将学习如何使用 LoadState 和 CombinedLoadStates 类来获取加载状态信息，并在 RecyclerView 中添加加载状态项。同时，还会介绍如何自定义加载状态视图，以满足应用的样式和需求，并实现点击加载失败视图时进行重试的功能。

Paging 3 加载状态

在使用 Paging 3 进行网络数据加载时，了解和理解加载状态是非常重要的。Paging 3 提供了一些类和接口来表示数据加载状态，帮助我们在界面上展示合适的加载提示和交互。主要涉及的概念有两个：

• 加载状态类型LoadState

• 加载类型LoadType

1. 加载状态的类型LoadState：

Paging3中用LoadState来表示加载状态， LoadState是一个枚举类，表示加载状态的不同阶段。它包含以下几个状态：

2. 加载类型 LoadType

3. CombinedLoadStates 类

CombinedLoadStates 是一个包含了所有加载类型的类，即它包含三个属性：分别对应三种加载类型refresh、prepend、append ，它将数据加载的各个类型进行了组合。通过 CombinedLoadStates，我们可以同时获得每个加载类型对应的详细信息，包括加载状态以及相关的异常信息。

在 RecyclerView 中，我们通常需要根据 CombinedLoadStates 的内容来决定是否展示加载状态项或错误视图。例如，当列表正在加载中时，我们可以显示加载中的动画或提示信息；当数据加载成功时，我们可以隐藏加载状态项；当加载失败时，我们可以展示加载失败的提示并提供重试功能，比如：

private val loadStateListener = object : Function1<CombinedLoadStates, Unit> {
    override fun invoke(combinedLoadStates: CombinedLoadStates) {
        when (val state = combinedLoadStates.refresh) {
            is LoadState.Loading -> {
                println("refresh Loading")
                // Show loading animation or loading indicator
            }
            is LoadState.Error -> {
                // Show error message or handle error state
                val errorMessage = state.error.message ?: "Unknown error"
                // Display the error message to the user
            }
            is LoadState.NotLoading -> {
                println("refresh NotLoading")
                // Hide loading animation or loading indicator
            }
        }
        when (val state = combinedLoadStates.append) {
            is LoadState.Loading -> {
                println("append Loading")
                // Show loading animation or loading indicator
            }
            is LoadState.Error -> {
                // Show error message or handle error state
                val errorMessage = state.error.message ?: "Unknown error"
                // Display the error message to the user
            }
            is LoadState.NotLoading -> {
                println("append NotLoading")
                // Hide loading animation or loading indicator
            }
        }
    }
}
private val loadStateListener = object : Function1<CombinedLoadStates, Unit> {
    override fun invoke(combinedLoadStates: CombinedLoadStates) {
        when (val state = combinedLoadStates.refresh) {
            is LoadState.Loading -> {
                println("refresh Loading")
                // Show loading animation or loading indicator
            }
            is LoadState.Error -> {
                // Show error message or handle error state
                val errorMessage = state.error.message ?: "Unknown error"
                // Display the error message to the user
            }
            is LoadState.NotLoading -> {
                println("refresh NotLoading")

                // Hide loading animation or loading indicator
            }
        }


        when (val state = combinedLoadStates.append) {
            is LoadState.Loading -> {
                println("append Loading")
                // Show loading animation or loading indicator
            }
            is LoadState.Error -> {
                // Show error message or handle error state
                val errorMessage = state.error.message ?: "Unknown error"
                // Display the error message to the user
            }
            is LoadState.NotLoading -> {
                println("append NotLoading")

                // Hide loading animation or loading indicator
            }
        }

    }

}
private val loadStateListener = object : Function1<CombinedLoadStates, Unit> {
    override fun invoke(combinedLoadStates: CombinedLoadStates) {
        when (val state = combinedLoadStates.refresh) {
            is LoadState.Loading -> {
                println("refresh Loading")
                // Show loading animation or loading indicator
            }
            is LoadState.Error -> {
                // Show error message or handle error state
                val errorMessage = state.error.message ?: "Unknown error"
                // Display the error message to the user
            }
            is LoadState.NotLoading -> {
                println("refresh NotLoading")

                // Hide loading animation or loading indicator
            }
        }


        when (val state = combinedLoadStates.append) {
            is LoadState.Loading -> {
                println("append Loading")
                // Show loading animation or loading indicator
            }
            is LoadState.Error -> {
                // Show error message or handle error state
                val errorMessage = state.error.message ?: "Unknown error"
                // Display the error message to the user
            }
            is LoadState.NotLoading -> {
                println("append NotLoading")

                // Hide loading animation or loading indicator
            }
        }

    }

}

在PagingSoure中发送数据加载状态

首先我们根据上文，可以知道，数据加载状态分为：LoadState.Loading/LoadState.NotLoading/LoadState.Error

而对于PagingSource来讲，它的返回值为LoadResult，LoadResult 有三种可选值：

一般情况下，我们不需要考虑LoadResult.Invalid 的状态，本文也暂且不做此种状态的讨论。

通常，当数据请求正常时，PagingSource返回LoadResult.Page ，对应数据加载状态LoadState.NotLoading，当请求出错时，返回LoadResult.Error ，而当框架判断加载状态为LoadState.Loading 才会调用PagingSource.load方法，因此三种数据加载状态和PagingSource

对应了起来：

class MyPagingSource(private val apiService: ApiService) : PagingSource<Int, ListItem>() {
    override fun getRefreshKey(state: PagingState<Int, ListItem>): Int? {
        return null
    }
    override suspend fun load(params: LoadParams<Int>): LoadResult<Int, ListItem> {
        try {
            // 获取请求的页数
            val pageNumber = params.key ?: 1
            var listItems = mutableListOf<ListItem>()
            // 加载数据
            apiService.getItems(pageNumber, params.loadSize)
                .onSuccess {
                    listItems.addAll(it)
                }.onFailure {
                    // 加载出错： 返回加载错误的 LoadResult.Error 对象
                    return LoadResult.Error(it)
                }
            val prevKey = if (pageNumber > 1) pageNumber - 1 else null
            val nextKey = if (listItems.isNotEmpty()) pageNumber + 1 else null
            // 本次加载结束： 构建 LoadResult.Page 对象并返回
            return LoadResult.Page(
                data = listItems,
                prevKey = prevKey,
                nextKey = nextKey
            )
        } catch (e: Exception) {
            // 加载出错： 返回加载错误的 LoadResult.Error 对象
            return LoadResult.Error(e)
        }
    }
}
class MyPagingSource(private val apiService: ApiService) : PagingSource<Int, ListItem>() {
    override fun getRefreshKey(state: PagingState<Int, ListItem>): Int? {
        return null
    }




    override suspend fun load(params: LoadParams<Int>): LoadResult<Int, ListItem> {
        try {
            // 获取请求的页数
            val pageNumber = params.key ?: 1



            var listItems = mutableListOf<ListItem>()
            // 加载数据
            apiService.getItems(pageNumber, params.loadSize)
                .onSuccess {
                    listItems.addAll(it)
                }.onFailure {
                    // 加载出错： 返回加载错误的 LoadResult.Error 对象
                    return LoadResult.Error(it)
                }


            val prevKey = if (pageNumber > 1) pageNumber - 1 else null
            val nextKey = if (listItems.isNotEmpty()) pageNumber + 1 else null


            // 本次加载结束： 构建 LoadResult.Page 对象并返回
            return LoadResult.Page(
                data = listItems,
                prevKey = prevKey,
                nextKey = nextKey
            )
        } catch (e: Exception) {
            // 加载出错： 返回加载错误的 LoadResult.Error 对象
            return LoadResult.Error(e)
        }

    }

}


class MyPagingSource(private val apiService: ApiService) : PagingSource<Int, ListItem>() {
    override fun getRefreshKey(state: PagingState<Int, ListItem>): Int? {
        return null
    }




    override suspend fun load(params: LoadParams<Int>): LoadResult<Int, ListItem> {
        try {
            // 获取请求的页数
            val pageNumber = params.key ?: 1



            var listItems = mutableListOf<ListItem>()
            // 加载数据
            apiService.getItems(pageNumber, params.loadSize)
                .onSuccess {
                    listItems.addAll(it)
                }.onFailure {
                    // 加载出错： 返回加载错误的 LoadResult.Error 对象
                    return LoadResult.Error(it)
                }


            val prevKey = if (pageNumber > 1) pageNumber - 1 else null
            val nextKey = if (listItems.isNotEmpty()) pageNumber + 1 else null


            // 本次加载结束： 构建 LoadResult.Page 对象并返回
            return LoadResult.Page(
                data = listItems,
                prevKey = prevKey,
                nextKey = nextKey
            )
        } catch (e: Exception) {
            // 加载出错： 返回加载错误的 LoadResult.Error 对象
            return LoadResult.Error(e)
        }

    }

}

网络加载状态处理示例

在以下的示例中，我们将解决两个问题：

• 初始化数据加载成功或失败的处理

• 加载下一页时成功或失败的处理

初始化数据加载状态处理

1. 定义不同加载状态下的UI

加载状态可以分为：

1. 正在加载数据

2. 加载失败

3. 加载成功

4. 加载的内容为空

• 正在加载数据的UI： loading圈 + 正在加载中的文案

<?xml version="1.0" encoding="utf-8"?>
<androidx.constraintlayout.widget.ConstraintLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:app="http://schemas.android.com/apk/res-auto"
    android:layout_width="match_parent"
    android:layout_height="match_parent">
    <ProgressBar
        android:id="@+id/progressBar"
        style="@style/ProgressBarStyle"
        android:layout_width="100dp"
        android:layout_height="100dp"
        android:layout_gravity="center"
        app:layout_constraintBottom_toTopOf="@+id/textView"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintHorizontal_bias="0.5"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toTopOf="parent"
        app:layout_constraintVertical_chainStyle="packed" />
    <TextView
        android:id="@+id/textView"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_marginTop="8dp"
        android:text="正在加载中"
        android:textColor="@color/color_797980"
        android:textSize="32sp"
        app:layout_constraintBottom_toBottomOf="parent"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintHorizontal_bias="0.5"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toBottomOf="@+id/progressBar" />
</androidx.constraintlayout.widget.ConstraintLayout>
<?xml version="1.0" encoding="utf-8"?>




<androidx.constraintlayout.widget.ConstraintLayout xmlns:android="http://schemas.android.com/apk/res/android"



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


    android:layout_width="match_parent"

    android:layout_height="match_parent">





    <ProgressBar
        android:id="@+id/progressBar"
        style="@style/ProgressBarStyle"
        android:layout_width="100dp"
        android:layout_height="100dp"
        android:layout_gravity="center"
        app:layout_constraintBottom_toTopOf="@+id/textView"
        app:layout_constraintEnd_toEndOf="parent"

        app:layout_constraintHorizontal_bias="0.5"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toTopOf="parent"
        app:layout_constraintVertical_chainStyle="packed" />


    <TextView
        android:id="@+id/textView"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_marginTop="8dp"
        android:text="正在加载中"
        android:textColor="@color/color_797980"

        android:textSize="32sp"
        app:layout_constraintBottom_toBottomOf="parent"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintHorizontal_bias="0.5"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toBottomOf="@+id/progressBar" />

</androidx.constraintlayout.widget.ConstraintLayout>
<?xml version="1.0" encoding="utf-8"?>




<androidx.constraintlayout.widget.ConstraintLayout xmlns:android="http://schemas.android.com/apk/res/android"



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


    android:layout_width="match_parent"

    android:layout_height="match_parent">





    <ProgressBar
        android:id="@+id/progressBar"
        style="@style/ProgressBarStyle"
        android:layout_width="100dp"
        android:layout_height="100dp"
        android:layout_gravity="center"
        app:layout_constraintBottom_toTopOf="@+id/textView"
        app:layout_constraintEnd_toEndOf="parent"

        app:layout_constraintHorizontal_bias="0.5"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toTopOf="parent"
        app:layout_constraintVertical_chainStyle="packed" />


    <TextView
        android:id="@+id/textView"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_marginTop="8dp"
        android:text="正在加载中"
        android:textColor="@color/color_797980"

        android:textSize="32sp"
        app:layout_constraintBottom_toBottomOf="parent"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintHorizontal_bias="0.5"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toBottomOf="@+id/progressBar" />

</androidx.constraintlayout.widget.ConstraintLayout>

• 加载失败的UI： 加载异常的提示 + 重新加载的按钮

<?xml version="1.0" encoding="utf-8"?>
<androidx.constraintlayout.widget.ConstraintLayout xmlns:android="http://schemas.android.com/apk/res/android"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:background="@color/F2EAE7E7"
    xmlns:app="http://schemas.android.com/apk/res-auto">
    <ImageView
        android:id="@+id/ivEmpty"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:src="@drawable/ic_net_error"
        app:layout_constraintBottom_toTopOf="@id/emptyText"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toTopOf="parent"
        app:layout_constraintVertical_chainStyle="packed" />
    <TextView
        android:id="@+id/emptyText"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="@string/load_error_retry"
        android:textSize="24sp"
        app:layout_constraintBottom_toBottomOf="@id/btnRetry"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toBottomOf="@id/ivEmpty" />
    <Button
        android:id="@+id/btnRetry"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:background="@color/A09D9D"
        android:layout_marginTop="18dp"
        android:paddingTop="10dp"
        android:paddingBottom="10dp"
        android:paddingStart="40dp"
        android:paddingEnd="40dp"
        android:text="@string/load_retry"
        android:textSize="24sp"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintTop_toBottomOf="@id/emptyText"
        app:layout_constraintBottom_toBottomOf="parent" />
</androidx.constraintlayout.widget.ConstraintLayout>
<?xml version="1.0" encoding="utf-8"?>




<androidx.constraintlayout.widget.ConstraintLayout xmlns:android="http://schemas.android.com/apk/res/android"



    android:layout_width="match_parent"

    android:layout_height="match_parent"
    android:background="@color/F2EAE7E7"
    xmlns:app="http://schemas.android.com/apk/res-auto">



    <ImageView
        android:id="@+id/ivEmpty"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:src="@drawable/ic_net_error"
        app:layout_constraintBottom_toTopOf="@id/emptyText"
        app:layout_constraintEnd_toEndOf="parent"

        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toTopOf="parent"
        app:layout_constraintVertical_chainStyle="packed" />

    <TextView
        android:id="@+id/emptyText"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="@string/load_error_retry"
        android:textSize="24sp"
        app:layout_constraintBottom_toBottomOf="@id/btnRetry"
        app:layout_constraintEnd_toEndOf="parent"

        app:layout_constraintStart_toStartOf="parent"

        app:layout_constraintTop_toBottomOf="@id/ivEmpty" />





    <Button
        android:id="@+id/btnRetry"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:background="@color/A09D9D"
        android:layout_marginTop="18dp"
        android:paddingTop="10dp"
        android:paddingBottom="10dp"
        android:paddingStart="40dp"
        android:paddingEnd="40dp"
        android:text="@string/load_retry"
        android:textSize="24sp"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintTop_toBottomOf="@id/emptyText"
        app:layout_constraintBottom_toBottomOf="parent" />



</androidx.constraintlayout.widget.ConstraintLayout>
<?xml version="1.0" encoding="utf-8"?>




<androidx.constraintlayout.widget.ConstraintLayout xmlns:android="http://schemas.android.com/apk/res/android"



    android:layout_width="match_parent"

    android:layout_height="match_parent"
    android:background="@color/F2EAE7E7"
    xmlns:app="http://schemas.android.com/apk/res-auto">



    <ImageView
        android:id="@+id/ivEmpty"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:src="@drawable/ic_net_error"
        app:layout_constraintBottom_toTopOf="@id/emptyText"
        app:layout_constraintEnd_toEndOf="parent"

        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toTopOf="parent"
        app:layout_constraintVertical_chainStyle="packed" />

    <TextView
        android:id="@+id/emptyText"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="@string/load_error_retry"
        android:textSize="24sp"
        app:layout_constraintBottom_toBottomOf="@id/btnRetry"
        app:layout_constraintEnd_toEndOf="parent"

        app:layout_constraintStart_toStartOf="parent"

        app:layout_constraintTop_toBottomOf="@id/ivEmpty" />





    <Button
        android:id="@+id/btnRetry"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:background="@color/A09D9D"
        android:layout_marginTop="18dp"
        android:paddingTop="10dp"
        android:paddingBottom="10dp"
        android:paddingStart="40dp"
        android:paddingEnd="40dp"
        android:text="@string/load_retry"
        android:textSize="24sp"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintTop_toBottomOf="@id/emptyText"
        app:layout_constraintBottom_toBottomOf="parent" />



</androidx.constraintlayout.widget.ConstraintLayout>

• 加载内容为空的UI： 暂无内容 + 图标

<?xml version="1.0" encoding="utf-8"?>
<androidx.constraintlayout.widget.ConstraintLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:app="http://schemas.android.com/apk/res-auto"
    android:layout_width="match_parent"
    android:layout_height="match_parent">
    <ImageView
        android:id="@+id/ivEmpty"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:src="@drawable/ic_empty"
        app:layout_constraintBottom_toTopOf="@id/emptyText"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toTopOf="parent"
        app:layout_constraintVertical_chainStyle="packed" />
    <TextView
        android:id="@+id/emptyText"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_marginTop="20dp"
        android:text="暂无内容"
        android:textSize="28sp"
        app:layout_constraintBottom_toBottomOf="parent"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toBottomOf="@id/ivEmpty" />
</androidx.constraintlayout.widget.ConstraintLayout>
<?xml version="1.0" encoding="utf-8"?>




<androidx.constraintlayout.widget.ConstraintLayout xmlns:android="http://schemas.android.com/apk/res/android"



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


    android:layout_width="match_parent"

    android:layout_height="match_parent">





    <ImageView
        android:id="@+id/ivEmpty"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:src="@drawable/ic_empty"
        app:layout_constraintBottom_toTopOf="@id/emptyText"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toTopOf="parent"
        app:layout_constraintVertical_chainStyle="packed" />




    <TextView
        android:id="@+id/emptyText"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_marginTop="20dp"
        android:text="暂无内容"
        android:textSize="28sp"
        app:layout_constraintBottom_toBottomOf="parent"
        app:layout_constraintEnd_toEndOf="parent"

        app:layout_constraintStart_toStartOf="parent"

        app:layout_constraintTop_toBottomOf="@id/ivEmpty" />





</androidx.constraintlayout.widget.ConstraintLayout>
<?xml version="1.0" encoding="utf-8"?>




<androidx.constraintlayout.widget.ConstraintLayout xmlns:android="http://schemas.android.com/apk/res/android"



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


    android:layout_width="match_parent"

    android:layout_height="match_parent">





    <ImageView
        android:id="@+id/ivEmpty"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:src="@drawable/ic_empty"
        app:layout_constraintBottom_toTopOf="@id/emptyText"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toTopOf="parent"
        app:layout_constraintVertical_chainStyle="packed" />




    <TextView
        android:id="@+id/emptyText"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_marginTop="20dp"
        android:text="暂无内容"
        android:textSize="28sp"
        app:layout_constraintBottom_toBottomOf="parent"
        app:layout_constraintEnd_toEndOf="parent"

        app:layout_constraintStart_toStartOf="parent"

        app:layout_constraintTop_toBottomOf="@id/ivEmpty" />





</androidx.constraintlayout.widget.ConstraintLayout>

在定义完各个不同加载状态的UI后，我们就需要将这几个UI 放到一个全局的加载状态XML中：

<?xml version="1.0" encoding="utf-8"?>
<FrameLayout xmlns:android="http://schemas.android.com/apk/res/android"
    android:layout_width="match_parent"
    android:layout_height="match_parent">
    <ViewStub
        android:layout_width="match_parent"
        android:layout_height="match_parent"
        android:id="@+id/loadingView"
        android:layout="@layout/layout_progress_dialog" />
    <ViewStub
        android:layout_width="match_parent"
        android:layout_height="match_parent"
        android:id="@+id/emptyView"
        android:layout="@layout/layout_empty" />
    <ViewStub
        android:layout_width="match_parent"
        android:layout_height="match_parent"
        android:id="@+id/errorView"
        android:layout="@layout/layout_error" />
</FrameLayout>
<?xml version="1.0" encoding="utf-8"?>




<FrameLayout xmlns:android="http://schemas.android.com/apk/res/android"
    android:layout_width="match_parent"

    android:layout_height="match_parent">

    <ViewStub
        android:layout_width="match_parent"
        android:layout_height="match_parent"
        android:id="@+id/loadingView"
        android:layout="@layout/layout_progress_dialog" />



    <ViewStub
        android:layout_width="match_parent"
        android:layout_height="match_parent"
        android:id="@+id/emptyView"
        android:layout="@layout/layout_empty" />




    <ViewStub
        android:layout_width="match_parent"
        android:layout_height="match_parent"
        android:id="@+id/errorView"
        android:layout="@layout/layout_error" />

</FrameLayout>
<?xml version="1.0" encoding="utf-8"?>




<FrameLayout xmlns:android="http://schemas.android.com/apk/res/android"
    android:layout_width="match_parent"

    android:layout_height="match_parent">

    <ViewStub
        android:layout_width="match_parent"
        android:layout_height="match_parent"
        android:id="@+id/loadingView"
        android:layout="@layout/layout_progress_dialog" />



    <ViewStub
        android:layout_width="match_parent"
        android:layout_height="match_parent"
        android:id="@+id/emptyView"
        android:layout="@layout/layout_empty" />




    <ViewStub
        android:layout_width="match_parent"
        android:layout_height="match_parent"
        android:id="@+id/errorView"
        android:layout="@layout/layout_error" />

</FrameLayout>

这里我们用ViewStub来做延迟加载，防止不必要的资源浪费，比如加载数据成功并且数据量大于0，因此加载错误的XML和加载内容为空内容XML就没有必要加载出来。

2. 自定义View控制不同状态下UI的切换

1. 首先我们定义多个页面状态：

sealed class PageState {
    object Loading : PageState()
    object Empty : PageState()
    class Error(val e: Throwable) : PageState()
    object NotLoading : PageState()
}
sealed class PageState {
    object Loading : PageState()
    object Empty : PageState()
    class Error(val e: Throwable) : PageState()
    object NotLoading : PageState()
}
sealed class PageState {
    object Loading : PageState()
    object Empty : PageState()
    class Error(val e: Throwable) : PageState()
    object NotLoading : PageState()
}

根据上一节，我们定义四种不同的状态

2. 根据页面状态控制不同状态UI的显隐：

自定义类PageStateHost，继承自FrameLayout

class PageStateHost(
    private val context: Context,
    attributeSet: AttributeSet? = null,
    private val refreshAction: () -> Unit = {}
): FrameLayout(context, attributeSet) {
    private val binding by lazy {
        LayoutPageStateBinding.inflate(LayoutInflater.from(context), this, false)
    }
    private var errorView: View? = null
    fun init(viewGroup: ViewGroup) {
        (parent as? ViewGroup)?.removeView(binding.root)
        viewGroup.addView(
            binding.root,
            ViewGroup.LayoutParams.MATCH_PARENT,
            ViewGroup.LayoutParams.MATCH_PARENT
        )
    }
    fun setState(state: PageState) {
        when (state) {
            is PageState.Loading -> {
                binding.loadingView.visibility = View.VISIBLE
                binding.errorView.visibility = View.GONE
                binding.emptyView.visibility = View.GONE
            }
            is PageState.Error -> {
                binding.loadingView.visibility = View.GONE
                errorView = errorView ?: binding.errorView.inflate()
                errorView?.visibility = VISIBLE
                binding.emptyView.visibility = View.GONE
                val retryBtn = errorView?.findViewById<Button>(R.id.btnRetry)
                retryBtn?.setOnClickListener {
                    refreshAction()
                }
            }
            is PageState.Empty -> {
                binding.loadingView.visibility = View.GONE
                binding.errorView.visibility = View.GONE
                binding.emptyView.visibility = View.VISIBLE
            }
            else -> {
                binding.loadingView.visibility = View.GONE
                binding.errorView.visibility = View.GONE
                binding.emptyView.visibility = View.GONE
            }
        }
    }
}
class PageStateHost(
    private val context: Context,
    attributeSet: AttributeSet? = null,
    private val refreshAction: () -> Unit = {}
): FrameLayout(context, attributeSet) {




    private val binding by lazy {
        LayoutPageStateBinding.inflate(LayoutInflater.from(context), this, false)
    }

    private var errorView: View? = null

    fun init(viewGroup: ViewGroup) {
        (parent as? ViewGroup)?.removeView(binding.root)
        viewGroup.addView(
            binding.root,
            ViewGroup.LayoutParams.MATCH_PARENT,
            ViewGroup.LayoutParams.MATCH_PARENT
        )
    }


    fun setState(state: PageState) {
        when (state) {
            is PageState.Loading -> {
                binding.loadingView.visibility = View.VISIBLE
                binding.errorView.visibility = View.GONE
                binding.emptyView.visibility = View.GONE
            }




            is PageState.Error -> {
                binding.loadingView.visibility = View.GONE
                errorView = errorView ?: binding.errorView.inflate()
                errorView?.visibility = VISIBLE
                binding.emptyView.visibility = View.GONE
                val retryBtn = errorView?.findViewById<Button>(R.id.btnRetry)
                retryBtn?.setOnClickListener {
                    refreshAction()
                }
            }
            is PageState.Empty -> {
                binding.loadingView.visibility = View.GONE
                binding.errorView.visibility = View.GONE
                binding.emptyView.visibility = View.VISIBLE


            }


            else -> {
                binding.loadingView.visibility = View.GONE
                binding.errorView.visibility = View.GONE
                binding.emptyView.visibility = View.GONE
            }

        }
    }
}
class PageStateHost(
    private val context: Context,
    attributeSet: AttributeSet? = null,
    private val refreshAction: () -> Unit = {}
): FrameLayout(context, attributeSet) {




    private val binding by lazy {
        LayoutPageStateBinding.inflate(LayoutInflater.from(context), this, false)
    }

    private var errorView: View? = null

    fun init(viewGroup: ViewGroup) {
        (parent as? ViewGroup)?.removeView(binding.root)
        viewGroup.addView(
            binding.root,
            ViewGroup.LayoutParams.MATCH_PARENT,
            ViewGroup.LayoutParams.MATCH_PARENT
        )
    }


    fun setState(state: PageState) {
        when (state) {
            is PageState.Loading -> {
                binding.loadingView.visibility = View.VISIBLE
                binding.errorView.visibility = View.GONE
                binding.emptyView.visibility = View.GONE
            }




            is PageState.Error -> {
                binding.loadingView.visibility = View.GONE
                errorView = errorView ?: binding.errorView.inflate()
                errorView?.visibility = VISIBLE
                binding.emptyView.visibility = View.GONE
                val retryBtn = errorView?.findViewById<Button>(R.id.btnRetry)
                retryBtn?.setOnClickListener {
                    refreshAction()
                }
            }
            is PageState.Empty -> {
                binding.loadingView.visibility = View.GONE
                binding.errorView.visibility = View.GONE
                binding.emptyView.visibility = View.VISIBLE


            }


            else -> {
                binding.loadingView.visibility = View.GONE
                binding.errorView.visibility = View.GONE
                binding.emptyView.visibility = View.GONE
            }

        }
    }
}

在此处，我们用ViewStub来控制不同状态UI的显隐，避免不必要的资源浪费。

为了该FrameLayout使用的方便，我们定义了init方法，将需要展示状态的ViewGroup传入。另外为了能够让用户在加载异常时重新发起数据加载请求，构造函数中提供lambda表达式，让用户自定义重新加载的操作。

3. 页面中使用自定义View实现页面加载状态展示

在BaseActivity中定义初始化PageStateHost的逻辑

abstract class BaseActivity: AppCompatActivity() {
    private var pageStateHost: PageStateHost? = null
    fun initPageState(viewGroup: ViewGroup) {
        if (this is PagingPageState) {
            pageStateHost = PageStateHost(view.context, refreshAction = refreshDataAction).apply {
                init(viewGroup)
                lifecycleScope.launch {
                    pageState.collectLatest {
                        setState(it)
                    }
                }
            }
        }
    }
    interface PagingPageState {
        val pageState: MutableStateFlow<PageState>
        val refreshDataAction: () -> Unit
    }
}
abstract class BaseActivity: AppCompatActivity() {


    private var pageStateHost: PageStateHost? = null

    fun initPageState(viewGroup: ViewGroup) {
        if (this is PagingPageState) {
            pageStateHost = PageStateHost(view.context, refreshAction = refreshDataAction).apply {
                init(viewGroup)
                lifecycleScope.launch {
                    pageState.collectLatest {
                        setState(it)
                    }
                }
            }
        }
    }





    interface PagingPageState {
        val pageState: MutableStateFlow<PageState>
        val refreshDataAction: () -> Unit
    }
}
abstract class BaseActivity: AppCompatActivity() {


    private var pageStateHost: PageStateHost? = null

    fun initPageState(viewGroup: ViewGroup) {
        if (this is PagingPageState) {
            pageStateHost = PageStateHost(view.context, refreshAction = refreshDataAction).apply {
                init(viewGroup)
                lifecycleScope.launch {
                    pageState.collectLatest {
                        setState(it)
                    }
                }
            }
        }
    }





    interface PagingPageState {
        val pageState: MutableStateFlow<PageState>
        val refreshDataAction: () -> Unit
    }
}

在这里，我们首先定义一个接口PagingPageState，该接口中使用Flow来记录当前页面的状态，并定义无参函数作为重新加载的逻辑。

接着，我们定义initPageState方法用户初始化PageStateHost，在该方法中，我们首先调用PageStateHost.init方法，然后调用Flow的collectLatest方法用于收集最新页面状态，从而调用setState展示对应状态的UI

3. 在MainActivity中实现PagingPageState，完成页面状态类初始化，并发送页面加载状态

class MainActivity : BaseActivity(), BaseActivity.PagingPageState {
    private val loadStateListener = object : Function1<CombinedLoadStates, Unit> {
        override fun invoke(combinedLoadStates: CombinedLoadStates) {
            when (val state = combinedLoadStates.refresh) {
                is LoadState.Loading -> {
              
                    lifecycleScope.launch {
                        pageState.emit(PageState.Loading)
                    }
                }
                is LoadState.Error -> {
                    // Show error message or handle error state
                    val errorMessage = state.error.message ?: "Unknown error"
                    // Display the error message to the user
                    lifecycleScope.launch {
                        pageState.emit(PageState.Error(Throwable(errorMessage)))
                    }
                }
                is LoadState.NotLoading -> {
                    // Hide loading animation or loading indicator
                    lifecycleScope.launch {
                        if (adapter.itemCount == 0) {
                            pageState.emit(PageState.Empty)
                        } else {
                            pageState.emit(PageState.NotLoading)
                        }
                    }
                }
            }
        }
    }
    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        setContentView(binding.root)
        initPageState(binding.root)
        initView()
        addDataObserve()
    }
    override val pageState: MutableStateFlow<PageState> = MutableStateFlow(PageState.Loading)
    override val refreshDataAction: () -> Unit = {
        adapter.refresh()
    }
}
class MainActivity : BaseActivity(), BaseActivity.PagingPageState {


    private val loadStateListener = object : Function1<CombinedLoadStates, Unit> {
        override fun invoke(combinedLoadStates: CombinedLoadStates) {
            when (val state = combinedLoadStates.refresh) {
                is LoadState.Loading -> {
              
                    lifecycleScope.launch {
                        pageState.emit(PageState.Loading)
                    }
                }
                is LoadState.Error -> {
                    // Show error message or handle error state
                    val errorMessage = state.error.message ?: "Unknown error"
                    // Display the error message to the user
                    lifecycleScope.launch {
                        pageState.emit(PageState.Error(Throwable(errorMessage)))
                    }
                }
                is LoadState.NotLoading -> {
                    // Hide loading animation or loading indicator
                    lifecycleScope.launch {
                        if (adapter.itemCount == 0) {
                            pageState.emit(PageState.Empty)
                        } else {
                            pageState.emit(PageState.NotLoading)
                        }
                    }




                }
            }
        }
    }


    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        setContentView(binding.root)
        initPageState(binding.root)

        initView()
        addDataObserve()

    }


    override val pageState: MutableStateFlow<PageState> = MutableStateFlow(PageState.Loading)
    override val refreshDataAction: () -> Unit = {
        adapter.refresh()
    }
}
class MainActivity : BaseActivity(), BaseActivity.PagingPageState {


    private val loadStateListener = object : Function1<CombinedLoadStates, Unit> {
        override fun invoke(combinedLoadStates: CombinedLoadStates) {
            when (val state = combinedLoadStates.refresh) {
                is LoadState.Loading -> {
              
                    lifecycleScope.launch {
                        pageState.emit(PageState.Loading)
                    }
                }
                is LoadState.Error -> {
                    // Show error message or handle error state
                    val errorMessage = state.error.message ?: "Unknown error"
                    // Display the error message to the user
                    lifecycleScope.launch {
                        pageState.emit(PageState.Error(Throwable(errorMessage)))
                    }
                }
                is LoadState.NotLoading -> {
                    // Hide loading animation or loading indicator
                    lifecycleScope.launch {
                        if (adapter.itemCount == 0) {
                            pageState.emit(PageState.Empty)
                        } else {
                            pageState.emit(PageState.NotLoading)
                        }
                    }




                }
            }
        }
    }


    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        setContentView(binding.root)
        initPageState(binding.root)

        initView()
        addDataObserve()

    }


    override val pageState: MutableStateFlow<PageState> = MutableStateFlow(PageState.Loading)
    override val refreshDataAction: () -> Unit = {
        adapter.refresh()
    }
}

在MainActivity中，我们实现PagingPageState接口，并实现相关变量的获取，其中，重新加载操作为调用adapter.refresh()，刷新整个列表。

在PagingAdapter加载状态的监听中发送对应的页面状态，从而展示相应的状态UI。

在完成上述工作后，我们来看一下加载成功和加载失败的效果：

加载更多的状态处理

在完成页面状态展示的逻辑之后，我们需要来处理加载更多的状态转换。加载更多状态UI的处理其实就是列表Footer UI的处理，跟页面状态处理的流程类似，包含以下几个步骤：

1. 定义状态UI；

2. 定义LoadStateAdapter，根据Footer不同的加载状态展示不同UI；

3. 将LoadStateAdapter关联到RecyclerView上

4. 重新出发加载更多的处理

5. 定义Footer的状态UI

Footer的状态定义不需要太复杂，只要一个 loading圈+ 文字描述

<?xml version="1.0" encoding="utf-8"?>
<androidx.constraintlayout.widget.ConstraintLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:app="http://schemas.android.com/apk/res-auto"
    android:orientation="horizontal"
    android:layout_width="match_parent"
    android:layout_height="40dp">
    <ProgressBar
        android:id="@+id/progressBar"
        style="@style/ProgressBarStyle"
        android:layout_width="24dp"
        android:layout_height="24dp"
        android:layout_gravity="center"
        app:layout_constraintEnd_toStartOf="@+id/textView"
        app:layout_constraintBottom_toBottomOf="parent"
        app:layout_constraintVertical_bias="0.5"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toTopOf="parent"
        app:layout_constraintHorizontal_chainStyle="packed" />
    <TextView
        android:id="@+id/textView"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="@string/loading"
        android:textColor="@color/color_797980"
        android:textSize="18sp"
        android:layout_marginStart="16dp"
        app:layout_constraintTop_toTopOf="parent"
        app:layout_constraintBottom_toBottomOf="parent"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintVertical_bias="0.5"
        app:layout_constraintStart_toEndOf="@+id/progressBar" />
</androidx.constraintlayout.widget.ConstraintLayout>
<?xml version="1.0" encoding="utf-8"?>




<androidx.constraintlayout.widget.ConstraintLayout xmlns:android="http://schemas.android.com/apk/res/android"



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


    android:orientation="horizontal"
    android:layout_width="match_parent"
    android:layout_height="40dp">



    <ProgressBar
        android:id="@+id/progressBar"
        style="@style/ProgressBarStyle"
        android:layout_width="24dp"
        android:layout_height="24dp"
        android:layout_gravity="center"
        app:layout_constraintEnd_toStartOf="@+id/textView"
        app:layout_constraintBottom_toBottomOf="parent"
        app:layout_constraintVertical_bias="0.5"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toTopOf="parent"
        app:layout_constraintHorizontal_chainStyle="packed" />

    <TextView
        android:id="@+id/textView"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="@string/loading"
        android:textColor="@color/color_797980"

        android:textSize="18sp"
        android:layout_marginStart="16dp"
        app:layout_constraintTop_toTopOf="parent"
        app:layout_constraintBottom_toBottomOf="parent"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintVertical_bias="0.5"
        app:layout_constraintStart_toEndOf="@+id/progressBar" />


</androidx.constraintlayout.widget.ConstraintLayout>
<?xml version="1.0" encoding="utf-8"?>




<androidx.constraintlayout.widget.ConstraintLayout xmlns:android="http://schemas.android.com/apk/res/android"



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


    android:orientation="horizontal"
    android:layout_width="match_parent"
    android:layout_height="40dp">



    <ProgressBar
        android:id="@+id/progressBar"
        style="@style/ProgressBarStyle"
        android:layout_width="24dp"
        android:layout_height="24dp"
        android:layout_gravity="center"
        app:layout_constraintEnd_toStartOf="@+id/textView"
        app:layout_constraintBottom_toBottomOf="parent"
        app:layout_constraintVertical_bias="0.5"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toTopOf="parent"
        app:layout_constraintHorizontal_chainStyle="packed" />

    <TextView
        android:id="@+id/textView"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="@string/loading"
        android:textColor="@color/color_797980"

        android:textSize="18sp"
        android:layout_marginStart="16dp"
        app:layout_constraintTop_toTopOf="parent"
        app:layout_constraintBottom_toBottomOf="parent"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintVertical_bias="0.5"
        app:layout_constraintStart_toEndOf="@+id/progressBar" />


</androidx.constraintlayout.widget.ConstraintLayout>

2. 定义LoadStateAdapter

Paging3已经贴心的为我们考虑到添加Footer或者Header的场景，我们只需要继承LoadStateAdapter，并重写onCreateViewHolder和onBindViewHolder方法即可：

class DefaultFooterAdapter(private val retryCallback: () -> Unit = {})
    : LoadStateAdapter<DefaultFooterAdapter.FooterViewHolder>() {
    override fun onCreateViewHolder(parent: ViewGroup, loadState: LoadState): FooterViewHolder {
        return FooterViewHolder(
            LayoutVerticalListFooterBinding.inflate(
                LayoutInflater.from(parent.context),
                parent,
                false
            )
        )
    }
    override fun onBindViewHolder(holder: FooterViewHolder, loadState: LoadState) {
        holder.bindData(loadState)
    }
    inner class FooterViewHolder(private val binding: LayoutVerticalListFooterBinding) : RecyclerView.ViewHolder(binding.root) {
        fun bindData(state: LoadState) {
            // 根据不同的状态，显示不同的内容，比如加载中、加载错误提示、加载完成等
            when (state) {
                is LoadState.Loading -> {
                    binding.progressBar.visibility = View.VISIBLE
                    binding.textView.visibility = View.VISIBLE
                    binding.textView.text = "正在加载中"
                }
                is LoadState.Error -> {
                    binding.progressBar.visibility = View.GONE
                    binding.textView.visibility = View.VISIBLE
                    binding.textView.text = "加载异常"
                }
                is LoadState.NotLoading -> {
                    binding.progressBar.visibility = View.GONE
                    if(state.endOfPaginationReached) {
                        binding.textView.visibility = View.VISIBLE
                        binding.textView.text = "到底了"
                    } else {
                        binding.textView.visibility = View.GONE
                        binding.textView.text = ""
                    }
                }
            }
        }
    }
}
class DefaultFooterAdapter(private val retryCallback: () -> Unit = {})
    : LoadStateAdapter<DefaultFooterAdapter.FooterViewHolder>() {

    override fun onCreateViewHolder(parent: ViewGroup, loadState: LoadState): FooterViewHolder {
        return FooterViewHolder(
            LayoutVerticalListFooterBinding.inflate(
                LayoutInflater.from(parent.context),
                parent,
                false
            )
        )
    }

    override fun onBindViewHolder(holder: FooterViewHolder, loadState: LoadState) {
        holder.bindData(loadState)
    }





    inner class FooterViewHolder(private val binding: LayoutVerticalListFooterBinding) : RecyclerView.ViewHolder(binding.root) {
        fun bindData(state: LoadState) {
            // 根据不同的状态，显示不同的内容，比如加载中、加载错误提示、加载完成等
            when (state) {
                is LoadState.Loading -> {
                    binding.progressBar.visibility = View.VISIBLE
                    binding.textView.visibility = View.VISIBLE
                    binding.textView.text = "正在加载中"
                }
                is LoadState.Error -> {
                    binding.progressBar.visibility = View.GONE
                    binding.textView.visibility = View.VISIBLE
                    binding.textView.text = "加载异常"
                }
                is LoadState.NotLoading -> {
                    binding.progressBar.visibility = View.GONE
                    if(state.endOfPaginationReached) {
                        binding.textView.visibility = View.VISIBLE
                        binding.textView.text = "到底了"
                    } else {
                        binding.textView.visibility = View.GONE
                        binding.textView.text = ""
                    }
                }
            }
        }
    }
}
class DefaultFooterAdapter(private val retryCallback: () -> Unit = {})
    : LoadStateAdapter<DefaultFooterAdapter.FooterViewHolder>() {

    override fun onCreateViewHolder(parent: ViewGroup, loadState: LoadState): FooterViewHolder {
        return FooterViewHolder(
            LayoutVerticalListFooterBinding.inflate(
                LayoutInflater.from(parent.context),
                parent,
                false
            )
        )
    }

    override fun onBindViewHolder(holder: FooterViewHolder, loadState: LoadState) {
        holder.bindData(loadState)
    }





    inner class FooterViewHolder(private val binding: LayoutVerticalListFooterBinding) : RecyclerView.ViewHolder(binding.root) {
        fun bindData(state: LoadState) {
            // 根据不同的状态，显示不同的内容，比如加载中、加载错误提示、加载完成等
            when (state) {
                is LoadState.Loading -> {
                    binding.progressBar.visibility = View.VISIBLE
                    binding.textView.visibility = View.VISIBLE
                    binding.textView.text = "正在加载中"
                }
                is LoadState.Error -> {
                    binding.progressBar.visibility = View.GONE
                    binding.textView.visibility = View.VISIBLE
                    binding.textView.text = "加载异常"
                }
                is LoadState.NotLoading -> {
                    binding.progressBar.visibility = View.GONE
                    if(state.endOfPaginationReached) {
                        binding.textView.visibility = View.VISIBLE
                        binding.textView.text = "到底了"
                    } else {
                        binding.textView.visibility = View.GONE
                        binding.textView.text = ""
                    }
                }
            }
        }
    }
}

上述代码也是常规的重写RecyclerView.Adapter流程，我们在FooterViewHolde.binData方法中，将加载状态和页面UI进行绑定。

3. 将DefaultFooterAdapter和RecyclerView关联

private fun initView() {
   binding.apply {
       list.layoutManager = LinearLayoutManager(this@MainActivity)
        // 直接使用PagingDataAdapter.withLoadStateFooter即可
       list.adapter = adapter.withLoadStateFooter(DefaultFooterAdapter())
       adapter.addLoadStateListener(loadStateListener)
   }
}
private fun initView() {
   binding.apply {
       list.layoutManager = LinearLayoutManager(this@MainActivity)
        // 直接使用PagingDataAdapter.withLoadStateFooter即可
       list.adapter = adapter.withLoadStateFooter(DefaultFooterAdapter())
       adapter.addLoadStateListener(loadStateListener)



   }
}
private fun initView() {
   binding.apply {
       list.layoutManager = LinearLayoutManager(this@MainActivity)
        // 直接使用PagingDataAdapter.withLoadStateFooter即可
       list.adapter = adapter.withLoadStateFooter(DefaultFooterAdapter())
       adapter.addLoadStateListener(loadStateListener)



   }
}

在PagingDataAdapter中，有一个withLoadStateFooter方法：

/**
 * Create a [ConcatAdapter] with the provided [LoadStateAdapter]s displaying the
 * [LoadType.APPEND] [LoadState] as a list item at the start of the presented list.
 *
 * @see LoadStateAdapter
 * @see withLoadStateHeaderAndFooter
 * @see withLoadStateHeader
 */
fun withLoadStateFooter(
    footer: LoadStateAdapter<*>
): ConcatAdapter {
    addLoadStateListener { loadStates ->
        footer.loadState = loadStates.append
    }
    return ConcatAdapter(this, footer)
}
/**
 * Create a [ConcatAdapter] with the provided [LoadStateAdapter]s displaying the
 * [LoadType.APPEND] [LoadState] as a list item at the start of the presented list.
 *
 * @see LoadStateAdapter
 * @see withLoadStateHeaderAndFooter
 * @see withLoadStateHeader
 */
fun withLoadStateFooter(
    footer: LoadStateAdapter<*>
): ConcatAdapter {
    addLoadStateListener { loadStates ->
        footer.loadState = loadStates.append
    }
    return ConcatAdapter(this, footer)
}
/**
 * Create a [ConcatAdapter] with the provided [LoadStateAdapter]s displaying the
 * [LoadType.APPEND] [LoadState] as a list item at the start of the presented list.
 *
 * @see LoadStateAdapter
 * @see withLoadStateHeaderAndFooter
 * @see withLoadStateHeader
 */
fun withLoadStateFooter(
    footer: LoadStateAdapter<*>
): ConcatAdapter {
    addLoadStateListener { loadStates ->
        footer.loadState = loadStates.append
    }
    return ConcatAdapter(this, footer)
}

该方法会返回一个ConcatAdapter实例，ConcatAdapter将PagingDataAdapter和LoadStateDapter进行了组合，在数据加载时，会讲不同的数据分发给不同的Adapter，比如此处，会将网络加载回来的数据给PagingDataAdapter，将加载状态给到LoadStateAdapter。

另外，在withLoadStateFooter方法中，为LoadStateAdapter添加一个LoadStateListener，并将append类型的LoadState赋值给LoadStateAdapter，从而达到在状态改变时，LoadStateAdapter的onBindViewHolder会被调用。

至此，我们就完成了加载更多的状态处理，但是为了能够在加载异常时重新触发加载，我们还需要额外多做一些工作。

4. 重新触发加载更多

通常用户习惯会反复上拉列表来触发加载更多，因此我们只需要监听Touch事件，计算用户上拉距离，超过阈值时调用PagingDataAdapter.retry()方法，因此我们需要对第三步的使用稍微改造一下：

@SuppressLint("ClickableViewAccessibility")
fun RecyclerView.setAdapterWithDefaultFooter(adapter: MyPagingDataAdapter) {
    this.adapter = adapter.withLoadStateFooter(DefaultFooterAdapter())
    var downEventY = 0f
    setOnTouchListener { _, event ->
        when (event.action) {
            MotionEvent.ACTION_DOWN -> {
                downEventY = event.rawY
            }
            MotionEvent.ACTION_UP -> {
                if (downEventY - event.rawY > 20) {
                    adapter.retry()
                }
            }
        }
        false
    }
}
@SuppressLint("ClickableViewAccessibility")
fun RecyclerView.setAdapterWithDefaultFooter(adapter: MyPagingDataAdapter) {
    this.adapter = adapter.withLoadStateFooter(DefaultFooterAdapter())
    var downEventY = 0f
    setOnTouchListener { _, event ->
        when (event.action) {
            MotionEvent.ACTION_DOWN -> {
                downEventY = event.rawY
            }



            MotionEvent.ACTION_UP -> {
                if (downEventY - event.rawY > 20) {
                    adapter.retry()
                }
            }
        }
        false
    }
}


@SuppressLint("ClickableViewAccessibility")
fun RecyclerView.setAdapterWithDefaultFooter(adapter: MyPagingDataAdapter) {
    this.adapter = adapter.withLoadStateFooter(DefaultFooterAdapter())
    var downEventY = 0f
    setOnTouchListener { _, event ->
        when (event.action) {
            MotionEvent.ACTION_DOWN -> {
                downEventY = event.rawY
            }



            MotionEvent.ACTION_UP -> {
                if (downEventY - event.rawY > 20) {
                    adapter.retry()
                }
            }
        }
        false
    }
}

我们给RecyclerView定义一个扩展函数setAdapterWithDefaultFooter(adapter: MyPagingDataAdapter)，在该方法中，我们首先调用adapter.withLoadStateFooter(DefaultFooterAdapter())设置Footer，接着设置TouchListener，记录ACTION_DOWN事件的纵坐标，当用户抬手时，计算抬手点的纵坐标，从而计算用户滑动列表距离，如果超过20，则触发adapter.retry()重新加载。

至此，我们完成加载更多的Footer状态处理，运行下看个效果吧：

总结

在Paging3网络框架下，处理网络加载状态要比自己写一套稍微方便一些，特别是Footer的处理，主要涉及以下几点：

• 数据加载状态 LoadState ；

• 加载类型 LoadType ；

• 包含所有加载类型的类 CombinedLoadStates

• 加载状态监听 addLoadStateListener ；

• LoadStateAdapter

本文只是对通用的状态处理提出了一套实现方法，更多更复杂场景下的状态展示还需要更加针对性的处理，但是大致上都脱离不了这一套，只是可能会做更多的特殊处理。