Android 全新组件：Paging Library 基于RecyclerView的分页加载框架

//paging
implementation 'android.arch.paging:runtime:1.0.0'
// RxKotlin
implementation 'io.reactivex.rxjava2:rxkotlin:2.2.0'
// RxAndroid
implementation 'io.reactivex.rxjava2:rxandroid:2.0.2'
// Lifecycle
implementation 'android.arch.lifecycle:extensions:1.1.1'

package per.lijuan.pagingdome.paging
import android.arch.lifecycle.MutableLiveData
import android.arch.paging.DataSource
import java.util.concurrent.Executor
/**
 * 一个简单的数据源工厂，它提供了一种观察上次创建的数据源的方式，这使得我们能够将其网络请求状态等返回到UI界面
 * Created by juan on 2018/05/23.
 */
class StudentDataSourceFactory(private val retryExecutor: Executor): DataSource.Factory<String, StudentBean>() {
    val sourceLiveData=MutableLiveData<StudentDataSource>()
    override fun create(): DataSource<String, StudentBean> {
        val source= StudentDataSource(retryExecutor)
        sourceLiveData.postValue(source)
        return source
    }
}

package per.lijuan.pagingdome.paging
import android.arch.lifecycle.Transformations.switchMap
import android.arch.paging.LivePagedListBuilder
import android.arch.paging.PagedList
import java.util.concurrent.Executor
/**
 1. Repository实现返回一个直接从网络加载数据的Listing，并使用该名称作为加载上一页/下一页数据的关键
 2. Created by juan on 2018/05/23.
 */
class StudentDataRepository(private val executor: Executor): StudentRepository {
    override fun getStudentList(pageSize: Int): Listing<StudentBean?> {
        val sourceFactory= StudentDataSourceFactory(executor)
        val pagedListConfig = PagedList.Config.Builder()
                .setEnablePlaceholders(false)
                .setInitialLoadSizeHint(pageSize*2)//定义第一页加载项目的数量
                .setPageSize(pageSize)//定义从DataSource中每一次加载的项目数量
                .build()
        val pagedList = LivePagedListBuilder(sourceFactory, pagedListConfig)
                .setFetchExecutor(executor)//设置Executor执行器用于从用于从DataSources中获取PagedLists数据，如果未设置，则默认为Arch组件I/O线程。
                .build()
        val refreshState = switchMap(sourceFactory.sourceLiveData) {
            it.initialLoad
        }
        return Listing<StudentBean?>(
                pagedList =pagedList,
                networkState = switchMap(sourceFactory.sourceLiveData, {
                    it.networkState
                }),
                retry = {
                    sourceFactory.sourceLiveData.value?.retryAllFailed()
                },
                refresh = {
                    sourceFactory.sourceLiveData.value?.invalidate()
                },
                refreshState = refreshState)
    }
}

/**
 3. 给不同的Repository实现共享的通用接口
 4. Created by juan on 2018/05/23.
 */
interface StudentRepository {
    fun getStudentList(pageSize: Int): Listing<StudentBean?>
}

package per.lijuan.pagingdome.paging
import android.arch.lifecycle.MutableLiveData
import android.arch.paging.ItemKeyedDataSource
import android.util.Log
import java.util.concurrent.Executor
/**
 * DataSource负责加载第一页以及后面每一页数据
 * Created by juan on 2018/05/23.
 */
open class StudentDataSource(private val retryExecutor: Executor) : ItemKeyedDataSource<String, StudentBean>(){
    private var TAG: String="paging"
    private var retry:(()->Any)?=null
    private var startPosition:Int = 0
    fun retryAllFailed(){
        val prevRetry=retry
        retry=null
        prevRetry?.let {
            retryExecutor.execute { it.invoke() }
        }
    }
    val networkState by lazy{
        MutableLiveData<Resource<String>>()
    }
    val initialLoad by lazy{
        MutableLiveData<Resource<String>>()
    }
    /**
     * 接收初始加载的数据，在这里需要将获取到的数据通过LoadInitialCallback的onResult进行回调，用于出始化PagedList，并对加载的项目进行计数
     */
    override fun loadInitial(params: LoadInitialParams<String>, callback: LoadInitialCallback<StudentBean>) {
        Log.d(TAG,"loadInitial->mSkip:"+startPosition+",count:"+params.requestedLoadSize)
        networkState.postValue(Resource.loading(null))
        initialLoad.postValue(Resource.loading(null))
        //模拟耗时操作
        val list=loadData(startPosition,params.requestedLoadSize)
        retry=null
        networkState.postValue(Resource.success(null))
        initialLoad.postValue(Resource.success(null))
        callback.onResult(list)
        startPosition+=list.size
    }
    /**
     * 接收加载的数据
     */
    override fun loadAfter(params: LoadParams<String>, callback: LoadCallback<StudentBean>) {
        Log.d(TAG,"loadAfter->mSkip:"+startPosition+",count:"+params.requestedLoadSize)
        networkState.postValue(Resource.loading(null))
        //模拟耗时操作
        val list=loadData(startPosition,params.requestedLoadSize)
        retry=null
        networkState.postValue(Resource.success(null))
        callback.onResult(list)
        startPosition+=list.size
    }
    override fun loadBefore(params: LoadParams<String>, callback: LoadCallback<StudentBean>) {
        Log.d(TAG,"loadBefore")
    }
    override fun getKey(item: StudentBean): String  = item.id!!
    /**
     * 模拟耗时操作，假设这里需要做一些后台线程的数据加载任务。
     */
    private fun loadData(startPosition: Int, limit: Int): List<StudentBean> {
        val list = ArrayList<StudentBean>()
        for (i in 0 until limit) {
            var position=startPosition + i
            val data = StudentBean(position.toString(), "学生@$position")
            list.add(data)
        }
        return list
    }
}

package per.lijuan.pagingdome.paging
import android.arch.lifecycle.LiveData
import android.arch.paging.PagedList
/**
 * UI显示列表和系统其余部分进行交互所必需的数据类
 * Created by juan on 2018/05/23.
 */
data class Listing<T> (
        val pagedList: LiveData<PagedList<T>>,
        val networkState: LiveData<Resource<String>>,
        val refreshState: LiveData<Resource<String>>,
        val refresh: () -> Unit,
        val retry: () -> Unit)

package per.lijuan.pagingdome.paging
/**
 *
 * 一个通用类，用于保存具有其加载状态的值
 * Created by juan on 2018/05/23.
 */
data class Resource<out T>(val status: Status, val data: T?, val message: String?) {
    companion object {
        fun <T> loading(msg: String? = null, data: T? = null): Resource<T> {
            return Resource(Status.LOADING, data, msg)
        }
        fun <T> success(data: T? = null): Resource<T> {
            return Resource(Status.SUCCESS, data, null)
        }
        fun <T> error(msg: String? = null, data: T? = null): Resource<T> {
            return Resource(Status.ERROR, data, msg)
        }
    }
}
enum class Status {
    SUCCESS,
    ERROR,
    LOADING
}

package per.lijuan.pagingdome
import android.app.Application
import android.arch.lifecycle.AndroidViewModel
import android.arch.lifecycle.MutableLiveData
import android.arch.lifecycle.Transformations
import per.lijuan.pagingdome.paging.StudentRepository
/**
 * Created by juan on 2018/05/23.
 */
class StudentViewModel : AndroidViewModel {
    private lateinit var mPostRepository: StudentRepository
    constructor(application: Application, postRepository: StudentRepository):super(application){
        this.mPostRepository=postRepository
    }
    // region 基于Android官方Paging Library的分页加载框架
    private val data = MutableLiveData<String>()
    private val repoResult = Transformations.map(data, {
        mPostRepository.getStudentList(10)
    })
    val posts = Transformations.switchMap(repoResult, { it.pagedList })!!
    val networkState = Transformations.switchMap(repoResult, { it.networkState })!!
    val refreshState = Transformations.switchMap(repoResult, { it.refreshState })!!
    fun refresh() {
        repoResult.value?.refresh?.invoke()
    }
    fun showDatas(subreddit: String): Boolean {
        if (data.value == subreddit) {
            return false
        }
        data.value = subreddit
        return true
    }
    fun retry() {
        val listing = repoResult?.value
        listing?.retry?.invoke()
    }
    fun currentData(): String? = data.value
    // endregion
}

class MainActivity : AppCompatActivity() {
    private lateinit var studentViewModel: StudentViewModel
    private lateinit var mAdapter: StudentAdapter
    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        setContentView(R.layout.activity_main)
        studentViewModel=getViewModel()
        initAdapter()
        initSwipeToRefresh()
        studentViewModel.showDatas("")
    }
    private fun getViewModel(): StudentViewModel {
        return ViewModelProviders.of(this, object : ViewModelProvider.Factory {
            override fun <T : ViewModel?> create(modelClass: Class<T>): T {
                val repo = ServiceLocator.instance()
                        .getRepository()
                return StudentViewModel(application, repo) as T
            }
        })[StudentViewModel::class.java]
    }
    private fun initSwipeToRefresh() {
        swipeRefreshLayout.setColorSchemeColors(Color.BLUE, Color.GREEN, Color.RED, Color.YELLOW)
        swipeRefreshLayout.setOnRefreshListener {
            studentViewModel.refresh()
        }
        studentViewModel.refreshState.observe(this, Observer { resource->
            if (resource==null){
                return@Observer
            }
            when(resource.status){
                Status.LOADING->{
                    swipeRefreshLayout.isRefreshing=true
                }
                Status.SUCCESS->{
                    swipeRefreshLayout.isRefreshing=false
                }
                Status.ERROR->{
                    Toast.makeText(this,resource.message,Toast.LENGTH_LONG).show()
                    swipeRefreshLayout.isRefreshing=false
                }
            }
        })
    }
    private fun initAdapter() {
        val mLayoutManager = LinearLayoutManager(this)
        mLayoutManager.orientation = LinearLayout.VERTICAL
        rv!!.layoutManager = mLayoutManager
        rv.addItemDecoration(DividerItemDecoration(this, DividerItemDecoration.VERTICAL))//添加分割线
        mAdapter= StudentAdapter(this)
        rv.adapter = mAdapter
        studentViewModel.posts.observe(this, Observer((mAdapter::submitList)))
    }
}

class StudentAdapter(private val mContext: Context):PagedListAdapter<StudentBean, StudentViewHolder>(diffCallback){
    override fun onBindViewHolder(holder: StudentViewHolder, position: Int) {
        holder.bindTo(getItem(position),mContext)
    }
    override fun onCreateViewHolder(parent: ViewGroup, viewType: Int): StudentViewHolder = StudentViewHolder(parent)
    companion object {
        private val diffCallback = object : DiffUtil.ItemCallback<StudentBean>() {
            override fun areItemsTheSame(oldItem: StudentBean, newItem: StudentBean): Boolean =
                    oldItem.id == newItem.id
            override fun areContentsTheSame(oldItem: StudentBean, newItem: StudentBean): Boolean =
                    oldItem == newItem
        }
    }
}