没有LiveData的房间

Question

我正在尝试Room数据库。 我不想观察我的数据，我只想一次从数据库中获取数据。如何使用MVVM实现此目标？

我面临的问题：如果我尝试在没有AsyncTask的情况下获取数据，它将给出：无法访问主线程上的数据库，因为它可能会长时间锁定UI（如预期的那样），并且如果我使用AsyncTask，则该方法返回null List，因为在AsyncTask完成之前该方法将返回。

道课：

@Query("SELECT * FROM student_table where StudentName = :studentName")List<Student> getStudentWithSameName(String studentName);

存储库：

public List<Student> getAllStudentWithSameName(String studentName) {
    new GetAllStudentWithSameNameAsyncTask(studentDao).execute(studentName);
    return studentsWithSameName;
}



private class GetAllStudentWithSameNameAsyncTask extends AsyncTask< String,Void, List<Student> > {

    StudentDao studentDao;

    public GetAllStudentWithSameNameAsyncTask(StudentDao studentDao) {
        this.studentDao = studentDao;
    }

    @Override
    protected List<Student> doInBackground(String... strings) {
        List<Student> students = studentDao.getStudentWithSameName(strings[0]);
        return students;
    }

    @Override
    protected void onPostExecute(List<Student> students) {
        studentsWithSameName = students;
        super.onPostExecute(students);
    }
}

ViewModel：

public List<Student> getStudentWithSameName(String studentName) {
    studentsWithSameName = studentRepository.getAllStudentWithSameName(studentName);
    return studentsWithSameName;
}

MainActivity：

viewModel = ViewModelProviders.of(this).get(StudentViewModel.class);
List<Student> students = viewModel.getStudentWithSameName("Bill");

Answer 1

class WordViewModel(application: Application) : AndroidViewModel(application) {

    private var parentJob = Job()
    private val coroutineContext: CoroutineContext
        get() = parentJob + Dispatchers.Main
    private val scope = CoroutineScope(coroutineContext)

    private val repository: WordRepository
    val allWords: LiveData<List<Word>>

    init {
        val wordsDao = WordRoomDatabase.getDatabase(application).wordDao()
        repository = WordRepository(wordsDao)
        allWords = repository.allWords
    }

    fun insert(word: Word) = scope.launch(Dispatchers.IO) {
        repository.insert(word)
    }

    override fun onCleared() {
        super.onCleared()
        parentJob.cancel()
    }
}

IMHO：比AsyncTask好。
您可以跟随Google的codelabs并学习如何使用Kotlin Coroutines。

Answer 2

在我的YourClassDao.kt中：

---

// this returns a LiveData object
@Query("SELECT * FROM my_table WHERE my_field = :myId")
fun getMyObject(myId: String): LiveData<List<YourClass>>

成为：

// this returns a normal object
@Query("SELECT * FROM my_table WHERE my_field = :myId")
suspend fun getMyObject(myId: String): List<YourClass>
就我而言，我像这样使用函数：

// using the second (suspend fun) version from above
fun useMyData() {
    val database = AppDatabase.getInstance(context).YourClassDao()  // context could be an activity, for example.
    val getDataJob = GlobalScope.async { database.getMyObject("someId") }
    getDataJob.invokeOnCompletion { cause ->
        if (cause != null) {
            //something happened and the job didn't finish successfully.  Handle that here
            Unit
        } else {
            val myData = getDataJob.getCompleted()

            // ITEM 1
            // ***************************
            // do something with your data
            // ***************************

            Unit  // this is just because the lambda here has to return Unit
        }
    }
    // ITEM 2
}
请记住，这仍然适用于异步数据，因此我代码中的ITEM 2可能在ITEM 1之前发生。 

Answer 3