Android 带有RecyclerView的房间数据库视图

我是Room和Kotlin的新手，我正在尝试构建一个简单的Room数据库，并将其显示在ViewPager中的RecyclerView中

该活动包含一个ViewPager对象，该ViewPager包含一个片段，该片段包含一个RecyclerView： 活动-->查看页面-->片段-->回收视图

我遇到的问题是，当我尝试接收数据库时（在插入之后），我得到了null

代码：

道：

活动：

class EventActivity : AppCompatActivity()
{
    private lateinit var viewPager: ViewPager

    override fun onCreate(savedInstanceState: Bundle?)
    {
        super.onCreate(savedInstanceState)
        setContentView(R.layout.activity_event)

        viewPager = findViewById(R.id.guests_view_pager)
        val pagerAdapter =  EventPagerAdapter(supportFragmentManager)
        viewPager.adapter = pagerAdapter
    }
}

适配器：

class EventPagerAdapter(fragmentManager: FragmentManager): FragmentPagerAdapter(fragmentManager)
{

    override fun getCount(): Int
    {
        return 1
    }

    override fun getItem(position: Int): Fragment
    {
        return GuestListFragment.newInstance()
    }
}

片段：

class GuestListFragment : Fragment()
{
    private var guestsDataBase: InviterRoomDatabase? = null
    private lateinit var dbWorkerThread: DBWorkerThread

    companion object
    {
        fun newInstance(): GuestListFragment
        {
            val fragment = GuestListFragment()
            return fragment
        }
    }


    override fun onCreateView(inflater: LayoutInflater, container: ViewGroup?, savedInstanceState: Bundle?): View? 
    {

        dbWorkerThread = DBWorkerThread("workerThread")
        dbWorkerThread.start()
        guestsDataBase = InviterRoomDatabase.getDatabase(this.context!!)
        insertGuestToDB(Guest("Joe","052352332",false,0))

        var rootView = inflater.inflate(R.layout.fragment_guest_list, container, false)
        var rv: RecyclerView = rootView.findViewById(R.id.guests_list_recycler_view)
        rv.layoutManager = LinearLayoutManager(context)

        var d = getAllDataFromDB()
        rv.adapter = GuestsRecycleViewAdapter.newInstance(d)

        return rootView
    }


    private fun insertGuestToDB(guest: Guest)
    {
        val task = Runnable {guestsDataBase?.guestsDao()?.insert(guest)}
        dbWorkerThread.postTask(task)
    }

    private fun getAllDataFromDB(): List<Guest>
    {
        var data: List<Guest> = emptyList()
        val task = Runnable { data = guestsDataBase?.guestsDao()?.getAllGuests()!!}

        dbWorkerThread.postTask(task)

        return data
    }
}

class GuestListFragment:Fragment（）
{
私有var guestsDataBase:InviterRoomDatabase？=null
私有lateinit var dbWorkerThread:dbWorkerThread
伴星
{
fun newInstance（）：GuestListFragment
{
val fragment=GuestListFragment（）
返回片段
}
}
覆盖创建视图（充气机：布局充气机，容器：ViewGroup？，savedInstanceState:Bundle？）：视图？
{
dbWorkerThread=dbWorkerThread（“workerThread”）
dbWorkerThread.start（）
guestsDataBase=InviterRoomDatabase.getDatabase（this.context！！）
插入GuestToDB（Guest（“Joe”，“052352332”，假，0））
var rootView=充气机。充气（R.layout.fragment\u guest\u列表，容器，false）
var rv:RecyclerView=rootView.findviewbyd（R.id.guests\u list\u recycler\u视图）
rv.layoutManager=LinearLayoutManager（上下文）
var d=getAllDataFromDB（）
rv.adapter=GuestsRecycleViewAdapter.newInstance（d）
返回根视图
}
私人娱乐insertGuestToDB（客人：客人）
{
val task=Runnable{guestsDataBase？.guestsDao（）？.insert（guest）}
dbWorkerThread.postTask（任务）
}
private fun getAllDataFromDB（）：列表
{
变量数据：List=emptyList（）
val task=Runnable{data=guestsDataBase？.guestsDao（）？.getAllGuests（）！！}
dbWorkerThread.postTask（任务）
返回数据
}
}

您需要理解多线程的概念，才能理解代码无法工作的原因。简而言之，您得到的是一个空列表，因为您试图在主线程中获得该列表，而在工作线程上运行的插入和查询尚未完成

我将注释放在代码中以解释执行顺序。注释中的编号为完成顺序：

---

请注意，这绝对不是解决此问题的最佳方法。这是一个非常常见的问题，有很多不同的解决方案更具可读性、灵活性和可维护性。只是其他解决方案需要更深入的理解，多线程是一个如此广泛的主题，我无法用一个简单的答案解释所有这些问题。不过，我希望您能了解总体思路，并鼓励您探索解决此问题的其他解决方案。

您确定数据库为空吗？当您在INSERT和get函数中使用线程时，可能会得到一个null，因为当您尝试从数据库获取数据时，数据尚未添加..我尝试单独插入并获取所有数据，但我还是得到了空列表对于这种情况，什么是最好的方法？@Itay我想说，使用或的反应式设计可以说是最先进的解决方案。你可能已经很熟悉了。似乎有更多的

Android-room

教程使用其中一个，而不是没有它们的教程。

class EventActivity : AppCompatActivity()
{
    private lateinit var viewPager: ViewPager

    override fun onCreate(savedInstanceState: Bundle?)
    {
        super.onCreate(savedInstanceState)
        setContentView(R.layout.activity_event)

        viewPager = findViewById(R.id.guests_view_pager)
        val pagerAdapter =  EventPagerAdapter(supportFragmentManager)
        viewPager.adapter = pagerAdapter
    }
}

class EventPagerAdapter(fragmentManager: FragmentManager): FragmentPagerAdapter(fragmentManager)
{

    override fun getCount(): Int
    {
        return 1
    }

    override fun getItem(position: Int): Fragment
    {
        return GuestListFragment.newInstance()
    }
}

class GuestListFragment : Fragment()
{
    private var guestsDataBase: InviterRoomDatabase? = null
    private lateinit var dbWorkerThread: DBWorkerThread

    companion object
    {
        fun newInstance(): GuestListFragment
        {
            val fragment = GuestListFragment()
            return fragment
        }
    }


    override fun onCreateView(inflater: LayoutInflater, container: ViewGroup?, savedInstanceState: Bundle?): View? 
    {

        dbWorkerThread = DBWorkerThread("workerThread")
        dbWorkerThread.start()
        guestsDataBase = InviterRoomDatabase.getDatabase(this.context!!)
        insertGuestToDB(Guest("Joe","052352332",false,0))

        var rootView = inflater.inflate(R.layout.fragment_guest_list, container, false)
        var rv: RecyclerView = rootView.findViewById(R.id.guests_list_recycler_view)
        rv.layoutManager = LinearLayoutManager(context)

        var d = getAllDataFromDB()
        rv.adapter = GuestsRecycleViewAdapter.newInstance(d)

        return rootView
    }


    private fun insertGuestToDB(guest: Guest)
    {
        val task = Runnable {guestsDataBase?.guestsDao()?.insert(guest)}
        dbWorkerThread.postTask(task)
    }

    private fun getAllDataFromDB(): List<Guest>
    {
        var data: List<Guest> = emptyList()
        val task = Runnable { data = guestsDataBase?.guestsDao()?.getAllGuests()!!}

        dbWorkerThread.postTask(task)

        return data
    }
}

override fun onCreateView(inflater: LayoutInflater, container: ViewGroup?, savedInstanceState: Bundle?): View? 
{
    // [1] onCreateView starts on main thread.
    dbWorkerThread = DBWorkerThread("workerThread")
    dbWorkerThread.start()
    guestsDataBase = InviterRoomDatabase.getDatabase(this.context!!)
     // [2] calls insertGuestToDB on main thread.
    insertGuestToDB(Guest("Joe","052352332",false,0))

    var rootView = inflater.inflate(R.layout.fragment_guest_list, container, false)
    var rv: RecyclerView = rootView.findViewById(R.id.guests_list_recycler_view)
    rv.layoutManager = LinearLayoutManager(context)

    // [4] calls getAllDataFromDB on main thread.
    var d = getAllDataFromDB()
    // [6] get "d" on main thread. "d" is an empty list. 
    rv.adapter = GuestsRecycleViewAdapter.newInstance(d)

    return rootView
}


private fun insertGuestToDB(guest: Guest)
{
    // [2] insertGuestToDB executed on main thread.
    val task = Runnable {
        guestsDataBase?.guestsDao()?.insert(guest)
        // [7] insertion finished on worker thread.     
    }
    dbWorkerThread.postTask(task)
    // [3] insertGuestToDB finishes on main thread.
}

private fun getAllDataFromDB(): List<Guest>
{
    // [4] getAllDataFromDB on main thread. "data" points to an empty list
    var data: List<Guest> = emptyList()
    val task = Runnable {
        data = guestsDataBase?.guestsDao()?.getAllGuests()!!
        // [8] query finishes on worker thread, but "data" is now lost.
    }
    dbWorkerThread.postTask(task)

    // [5] returns on main thread. "data" still points to an empty list
    return data
}

var rv: RecyclerView

override fun onCreateView(inflater: LayoutInflater, container: ViewGroup?, savedInstanceState: Bundle?): View? 
{

    dbWorkerThread = DBWorkerThread("workerThread")
    dbWorkerThread.start()

    var rootView = inflater.inflate(R.layout.fragment_guest_list, container, false)
    rv= rootView.findViewById(R.id.guests_list_recycler_view)
    rv.layoutManager = LinearLayoutManager(context)

    guestsDataBase = InviterRoomDatabase.getDatabase(this.context!!)
    insertGuestToDbAndUpdateRv(Guest("Joe","052352332",false,0))

    return rootView
}


private fun insertGuestToDbAndUpdateRv(guest: Guest)
{
    val task = Runnable {
        // 1. Insert on a worker thread.
        guestsDataBase?.guestsDao()?.insert(guest)

        // 2. Query on a worker thread.
        var data = guestsDataBase?.guestsDao()?.getAllGuests()!!
        this@GuestListFragment.getActivity().runOnUiThread {
            // 3. Once they are done, update ui on main thread.
            rv.adapter = GuestsRecycleViewAdapter.newInstance(data)
        }
    }
    dbWorkerThread.postTask(task)
}