Android Paging+BoundaryCallback分页的实现与封装

ViewModel+LiveData Android Architecture Components
Paging Library Paging

Paging

1. Datasource

数据源抽象类，Paging有三种实现
（1）PageKeyedDataSource 按页加载，如请求数据时传入page页码。
（2）ItemKeyedDataSource 按条目加载，即请求数据需要传入其它item的信息，如加载第n+1项的数据需传入第n项的id。
（3）PositionalDataSource 按位置加载，如加载指定从第n条到n+20条。

2. PagedList

PagedList是List的子类，通过Datasource加载数据,并可以设置一次加载的数量以及预加载的数量等。

3.PagedListAdapter

PagedListAdapte是RecyclerView.Adapter的实现，用于展示PagedList的数据。数据源变动时后台线程通过DiffUtil比较前后两个PagedList的差异，然后调用notifyItem...()方法更新RecyclerView。

4. LivePagedListBuilder

将PagedList和LiveData整合成LiveData。

Paging+BoundaryCallback封装

网上许多demo都是基于google example，官方example主要是教我们怎么用，也有很多值得参考的地方，但直接搬项目中去用却明显水土不服，建议先理解了google example中Paging的用法，再自己封装下...

定义列表的几种状态

/**
 * 列表状态
 */
sealed class ListStatus

/**
 * 初始化中
 */
class Initialize : ListStatus()

/**
 * 初始化成功
 */
class InitializeSuccess : ListStatus()

/**
 * 初始化错误
 */
class InitializeError : ListStatus()

/**
 * 列表空
 */
class Empty : ListStatus()

/**
 * 列表往下加载更多中
 */
class LoadMoreIn : ListStatus()

/**
 * 列表往下加载成功
 */
class LoadMoreSuccess : ListStatus()

/**
 * 列表往下加载失败
 */
class LoadMoreError : ListStatus()

/**
 * 列表往下已全部加载完毕
 */
class End : ListStatus()

/**
 * 列表往上加载更多中
 */
class AtFrontLoadMoreIn : ListStatus()

/**
 * 列表往上加载成功
 */
class AtFrontLoadMoreSuccess : ListStatus()

/**
 * 列表往上加载失败
 */
class AtFrontLoadMoreError : ListStatus()

/**
 * 列表往上已全部加载完毕
 */
class AtFrontEnd : ListStatus()

自定义BoundaryCallback

class ListBoundaryCallback(private val listLiveData: MutableLiveData) : PagedList.BoundaryCallback() {
    override fun onZeroItemsLoaded() {
        super.onZeroItemsLoaded()
        listLiveData.value = Empty()
    }

    override fun onItemAtEndLoaded(itemAtEnd: Value) {
        super.onItemAtEndLoaded(itemAtEnd)
        listLiveData.value = End()
    }

    override fun onItemAtFrontLoaded(itemAtFront: Value) {
        super.onItemAtFrontLoaded(itemAtFront)
        listLiveData.value = AtFrontEnd()
    }
}

BoundaryCallback是Datasource中的数据加载到边界时的回调，为PagedList设置BoundaryCallback用来监听加载本地数据的事件，对应回调方法中为ListStatus设置不同状态

公共ViewModel

open class MyAndroidViewModel(application: Application) : AndroidViewModel(application) {
    private fun makePagedListConfig(pageSize: Int = 20): PagedList.Config =
            PagedList.Config.Builder().apply {
                setPageSize(pageSize)
                setInitialLoadSizeHint(pageSize)
                setPrefetchDistance(2)
                setEnablePlaceholders(false)
            }.build()

    fun  makePagedList(dataSourceFactory: DataSource.Factory,
                                   listStatus: MutableLiveData, pageSize: Int = 20): LiveData> =
            LivePagedListBuilder(dataSourceFactory, makePagedListConfig(pageSize))
                    .setBoundaryCallback(ListBoundaryCallback(listStatus))
                    .build()
}

ViewModel中设置PagedList.Config,初始化LivePagedListBuilder

自定义PageKeyedDataSource及相关回调

abstract class ListStatusPageKeyedDataSource(private val status: MutableLiveData) : PageKeyedDataSource() {

    final override fun loadInitial(params: LoadInitialParams, callback: LoadInitialCallback) {
        status.postValue(Initialize())
        onLoadInitial(params, ListStatusPageKeyedLoadInitialCallback(callback, status))
    }

    abstract fun onLoadInitial(params: LoadInitialParams, callback: ListStatusPageKeyedLoadInitialCallback)

    final override fun loadAfter(params: LoadParams, callback: LoadCallback) {
        status.postValue(LoadMoreIn())
        onLoadAfter(params, ListStatusPageKeyedLoadCallback(callback, status))
    }

    abstract fun onLoadAfter(params: LoadParams, callback: ListStatusPageKeyedLoadCallback)

    final override fun loadBefore(params: LoadParams, callback: LoadCallback) {
        status.postValue(AtFrontLoadMoreIn())
        onLoadBefore(params, ListStatusAtFrontPageKeyedLoadCallback(callback, status))
    }

    abstract fun onLoadBefore(params: LoadParams, callback: ListStatusAtFrontPageKeyedLoadCallback)
}

PageKeyedDataSource是数据源, 将这几种回调替换成我们自定义的回调并抽象出来，改变对应Status状态。

class ListStatusPageKeyedLoadInitialCallback(
        private val callback: PageKeyedDataSource.LoadInitialCallback,
        private val listStatus: MutableLiveData) : PageKeyedDataSource.LoadInitialCallback() {

    override fun onResult(data: MutableList, position: Int, totalCount: Int, previousPageKey: Key?, nextPageKey: Key?) {
        callback.onResult(data, position, totalCount, previousPageKey, nextPageKey)
        if (!data.isEmpty()) {
            listStatus.postValue(InitializeSuccess())
        } else {
            // 当列表为空时 BoundaryCallback 会回调 Empty 状态因此这里不用处理
        }
    }

    override fun onResult(data: MutableList, previousPageKey: Key?, nextPageKey: Key?) {
        callback.onResult(data, previousPageKey, nextPageKey)
        if (!data.isEmpty()) {
            listStatus.postValue(InitializeSuccess())
        } else {
            // 当列表为空时 BoundaryCallback 会回调 Empty 状态因此这里不用处理
        }
    }

    fun onError() {
        listStatus.postValue(InitializeError())
    }
}

class ListStatusPageKeyedLoadCallback(
        private val callback: PageKeyedDataSource.LoadCallback,
        private val listStatus: MutableLiveData) : PageKeyedDataSource.LoadCallback() {

    override fun onResult(data: MutableList, adjacentPageKey: Key?) {
        callback.onResult(data, adjacentPageKey)
        listStatus.postValue(LoadMoreSuccess())
    }

    fun onError() {
        listStatus.postValue(LoadMoreError())
    }
}

class ListStatusAtFrontPageKeyedLoadCallback(
        private val callback: PageKeyedDataSource.LoadCallback,
        private val listStatus: MutableLiveData) : PageKeyedDataSource.LoadCallback() {

    override fun onResult(data: MutableList, adjacentPageKey: Key?) {
        callback.onResult(data, adjacentPageKey)
        listStatus.postValue(AtFrontLoadMoreSuccess())
    }

    fun onError() {
        listStatus.postValue(AtFrontLoadMoreError())
    }
}

这几个回调分别对应初始化、向前加载、向后加载

为请求创建DataSource

class BourseListDataSource(status: MutableLiveData) : ListStatusPageKeyedDataSource(status) {
    override fun onLoadInitial(params: LoadInitialParams, callback: ListStatusPageKeyedLoadInitialCallback) {
        RetrofitClient.getInstance().getAPI().getBourseList(0, params.requestedLoadSize).enqueue(object : Callback>> {
            override fun onResponse(call: Call>>?, response: Response>>) {
                val httpResponse = HttpResponseProcess.responseProcess(response)
                callback.onResult(httpResponse.data?.list
                        ?: emptyList().toMutableList(), null, 1)
            }

            override fun onFailure(call: Call>>?, throwable: Throwable) {
                val errorResponse = HttpResponseProcess.responseProcess>(throwable)
                callback.onError()
            }
        })
    }

    override fun onLoadAfter(params: LoadParams, callback: ListStatusPageKeyedLoadCallback) {
        RetrofitClient.getInstance().getAPI().getBourseList(params.key * params.requestedLoadSize, params.requestedLoadSize).enqueue(object : Callback>> {
            override fun onResponse(call: Call>>?, response: Response>>) {
                val httpResponse = HttpResponseProcess.responseProcess(response)
                callback.onResult(httpResponse.data?.list
                        ?: emptyList().toMutableList(), params.key + 1)
            }

            override fun onFailure(call: Call>>?, throwable: Throwable) {
                val errorResponse = HttpResponseProcess.responseProcess>(throwable)
                callback.onError()
            }
        })
    }

    override fun onLoadBefore(params: LoadParams, callback: ListStatusAtFrontPageKeyedLoadCallback) {

    }

    class Factory(private val status: MutableLiveData) : DataSource.Factory() {
        override fun create(): DataSource = BourseListDataSource(status)
    }
    
}

 @FormUrlEncoded
 @POST("market/getBourseList")
 fun getBourseList(@Field("start") start: Int, @Field("size") size: Int): Call>>

接口的DataSource中各回调直接通过Retrofit联网请求数据（没有数据持久化需求），getBourseList api需传的参数为起始位置start与数量size, params.requestedLoadSize就是前面PagedList.Config中所配置PageSize
onLoadInitial中传的start为0
onLoadAfter 中传的start为params.key * params.requestedLoadSize(页码乘每页数量，如果api分页需要的是页码，直接传params.key就可以)
onLoadBefore跟onLoadAfter一样，不需要向上加载的话就置空
请求成功后将数据及下一页页码传给callback的onResult，请求失败直接调用callback的onError

为页面创建ViewModel

class BourseListViewModel(application: Application) : MyAndroidViewModel(application) {
    val listStatus = MutableLiveData()
    val list = makePagedList(BourseListDataSource.Factory(listStatus), listStatus)
}

ok，就这些...

页面扩展公共方法

fun Fragment.bindList(adapter: AssemblyPagedListAdapter, hint: HintView?, listStatus: MutableLiveData, list: LiveData>, emptyFragment: Fragment? = null,
                      isFastHidden: Boolean = false, onClickListener: View.OnClickListener? = null) {
    list.observe(this, Observer { adapter.submitList(it) })

    listStatus.observe(this, Observer {
        when (it) {
            null -> adapter.loadMoreFinished(false)
            is Initialize -> hint?.loading()?.show(childFragmentManager)
            is InitializeSuccess -> if (isFastHidden) hint?.fastHidden() else hint?.hidden()
            is InitializeError -> hint?.error(onClickListener)?.show(childFragmentManager)
            is End -> adapter.loadMoreFinished(true)
            is Empty -> if (emptyFragment != null) hint?.empty(emptyFragment)?.show(childFragmentManager) else hint?.empty()?.show(childFragmentManager)
        }
    })
}

这个仅供参考，AssemblyPagedListAdapter和HintView都是自定义的，大家知道什么意思就行...

页面

class MyListFragment : BaseFragment(){
    private val viewModel by bindViewModel(BourseListViewModel::class)
    private val adapter = AssemblyPagedListAdapter().apply {
        addItemFactory(BourseItem.Factory())
        addMoreItem(LoadMoreItem.Factory())
    }
    override fun onCreateView(inflater: LayoutInflater, container: ViewGroup?, savedInstanceState: Bundle?): View? {
        return inflater.inflate(R.layout.fm_list, container, false)
    }
    override fun initViews(view: View, savedInstanceState: Bundle?) {
        recyclerView.layoutManager = LinearLayoutManager(context, LinearLayoutManager.VERTICAL, false)
        recyclerView.adapter = adapter
    }
    override fun loadData() {
        bindList(adapter, hintView, viewModel.listStatus, viewModel.list as LiveData>)
    }
}

收工，可以看到页面其实只需要初始化View与ViewModel，然后直接bindList就可以了