android - 如何在使用 Room 时立即获取查询结果？

Question

我希望一次获得 Room 数据库中所有记录的总数。但是，通常 Room 使用后台线程来异步查询记录。
如果我使用 getTotalOfVoiceAsLiveData()在代码 A 中，它将返回 LiveData<Long>,你知道 LiveData 变量是惰性的，可能结果为空。
如果我使用 getTotalOfVoice()在代码 A 中，我会收到错误，因为我不能使用 return在 viewModelScope.launch{ } .
如何使用 Room 数据库一次获取所有记录的总数？
代码 A

class HomeViewModel(val mApplication: Application, private val mDBVoiceRepository: DBVoiceRepository) : AndroidViewModel(mApplication) {

    fun getTotalOfVoice():Long {
        viewModelScope.launch {
            return mDBVoiceRepository.getTotalOfVoice()   //It will cause error
        }
    }

    fun getTotalOfVoiceAsLiveData(): LiveData<Long>{
        return mDBVoiceRepository.getTotalOfVoiceAsLiveData() //It's lazy, maybe the result is null.
    }

}


class DBVoiceRepository private constructor(private val mDBVoiceDao: DBVoiceDao){
    suspend  fun getTotalOfVoice() = mDBVoiceDao.getTotalOfVoice()

    fun getTotalOfVoiceAsLiveData() = mDBVoiceDao.getTotalOfVoiceAsLiveData()
}



@Dao
interface DBVoiceDao{
   @Query("SELECT count(id) FROM voice_table")
   suspend fun getTotalOfVoice(): Long

   //When Room queries return LiveData, the queries are automatically run asynchronously on a background thread.
   @Query("SELECT count(id) FROM voice_table")
   fun getTotalOfVoiceAsLiveData(): LiveData<Long>
}

添加内容
托比:谢谢!
为什么直接获取数据对您很重要？
我需要根据记录的总数生成一个文件名，例如“untitled0”、“untitled1”、“untitled2”...
如果我可以一次得到查询结果，我可以很容易地使用下面的代码。
再次添加
当我单击开始按钮时，我希望根据查询记录的总数按文件名录制声音。您知道添加或删除记录时，记录总数会发生变化!
代码 B

fun getTotalOfVoice():Long {
    //Get the query result at once
    ...
} 

fun createdFileanme(){
    return "untitled"+getTotalOfVoice().toString()
}

btnStart.setOnClickListener{
    recordVoice(createdFileanme())  //I  will record voice by filename
}

fun addRecord(){
    ...
}

fun deleteRecord(){
    ...
}

新增内容
谢谢!
我认为 'You should also move all of that into the viewmodel class, without LiveData '是个好办法，可以看图A和 How can I get the value of a LivaData<String> at once in Android Studio? .
你同意吗？
图片A

Answer 1

Question: at once meaning synchronous or what ? if yes, what happens if the function to get the result has to take a longer time? like network call? well you can decide to do that on another thread.

我认为您可以使用可变对象并使用 postValue 函数将结果分派(dispatch)给观察者。它应该如下所示:

class HomeViewModel(val mApplication: Application, private val mDBVoiceRepository: DBVoiceRepository) : AndroidViewModel(mApplication) {
    private val voices = MutableLiveData<Long>()

    fun getTotalOfVoiceAsLiveData(): LiveData<Long> {
        voices.postValue(mDBVoiceRepository.getTotalOfVoiceAsLiveData().value)
        return voices;
    }
}

在 Fragment 中使用它如下所示:

override fun onActivityCreated(savedInstanceState: Bundle?) {
        super.onActivityCreated(savedInstanceState)
        if (activity != null) {
            val viewModel = ViewModelProvider(requireActivity())
            viewModel.get(HomeViewModel::class.java).getTotalOfVoiceAsLiveData().observe(viewLifecycleOwner, Observer { voices: Long ? ->
                voices // Sound of music ? be very free to use ...
            })
        }
    }

快乐编码。