房间数据库 Livedata getValue() returns null

Question

我试图使用 LiveData 和视图模型从房间中获取自定义对象数据列表。而使用Livedata的getValue()方法，returnsnull但是获取list直接显示的是实际数据。如何在视图模型 class.

中使用 LiveData 获取期间列表 class

Entity classes

@Entity
public class Period {

@PrimaryKey
@NonNull
String header;
@TypeConverters(WritterConverter.class)
ArrayList<Writter> writters;

public Period(String header, ArrayList<Writter> writters) {
    this.header = header;
    this.writters = writters;
}

public String getHeader() {
    return header;
}

public ArrayList<Writter> getWritters() {
    return writters;
}


}

@Entity
public class Writter {

String birth;
String death;
String name;
ArrayList<String> novels;

public Writter(){}

public Writter(String birth, String death, String name, ArrayList<String> novels) {
    this.birth = birth;
    this.death = death;
    this.name = name;
    this.novels = novels;
}

public String getBirth() {
    return birth;
}

public String getDeath() {
    return death;
}

public String getName() {
    return name;
}

public ArrayList<String> getNovels() {
    return novels;
}
}

Converter Class

public class WritterConverter {

@TypeConverter
public static ArrayList<Writter> fromString(String value){
    Type listType = new TypeToken<ArrayList<Writter>>(){}.getType();
    return new Gson().fromJson(value, listType);
}

@TypeConverter
public static String fromArrayList(ArrayList<Writter> list){
    Gson gson = new Gson();
    String json = gson.toJson(list);
    return json;
}

}

DAO class

@Dao
@TypeConverters({WritterConverter.class})
public interface DAO {

@Insert(onConflict = OnConflictStrategy.REPLACE)
void insertPeriod(Period period);

@Query("SELECT * FROM Period WHERE header LIKE :header")
Period getPeriodList(String header);

@Query("SELECT * FROM Period")
LiveData<List<Period>> getAllPeriodList();

}

Database

@Database(version = 1, entities = {Period.class})
public abstract class MyDatabase extends RoomDatabase{

static MyDatabase databaseInstance;

public static MyDatabase getDatabaseInstance(Context context) {
    if (databaseInstance == null)
        databaseInstance = Room
                .databaseBuilder(context, MyDatabase.class, "testDatabase1")
                .fallbackToDestructiveMigration()
                //     .addMigrations(FROM_1_TO_2)
                .build();

    return databaseInstance;
}

abstract public DAO dao();



}

Get Data method in viewmodel class

 private void getDataFromDatabase() {

    new AsyncTask<Void, Void, LiveData<List<Period>>>() {
        @Override
        protected LiveData<List<Period>> doInBackground(Void... voids) {
            LiveData<List<Period>> periodList = MyApplication.getDatabase().dao().getAllPeriodList();
          //  Period periodList = MyApplication.getDatabase().dao().getWriterList(EndPoints.THE_OLD_ENGLISH_PERIOD);
            return periodList;
        }

        @Override
        protected void onPostExecute(LiveData<List<Period>> aVoid) {
            super.onPostExecute(aVoid);
         //  Period period = aVoid.getValue().get(0);
            Log.d("header", aVoid.getValue().get(0).getHeader());
        }
    }.execute();

}

Answer 1

通常 Room 处理查询同步但与它一起使用 LiveData 这些查询在 ASynchronous 中执行方式，所以当您调用此行时：

LiveData<List<Period>> periodList = MyApplication.getDatabase().dao().getAllPeriodList();

，你实际上得到了一个 empty 容器，它将在执行并完成查询后得到 updated。因此，要获得查询结果，甚至要让查询首先得到 executed，您必须 observe 返回的 LiveData.

现在通常的情况是 将 Room 从 ViewModel 提供的 LiveData 传递给您的 ViewModel View 然后观察 LiveData 在 View.

中的变化
内部 ViewModel:

public LiveData<List<Period>> getAllPeriods() { return MyApplication.getDatabase().dao().getAllPeriodList(); }

里面查看:

viewModel.getAllPeriods().observe(this, new Observer<List<Period>>() { @Override public void onChanged(@Nullable List<Period> periods) { updateList(periods); } });

Answer 2

事实是，

Asynctask 在后台的另一个线程上运行异步
Livedata 就像漂浮在数据库上以查看更改一样工作。

所以完成 Asyntask 的获取操作需要时间。您可以通过在 10 秒延迟的处理程序中测试 Asynctask 来查看它。所以最好直接在view model中调用fetch操作，发送到activity/fragment as livedata.observe.

房间数据库 Livedata getValue() returns null

Room database Livedata getValue() returns null

android

dao

viewmodel

android-room

android-livedata