LiveData.getValue() returns 空房间

Question

Java POJO 对象

public class Section {

    @ColumnInfo(name="section_id")
    public int mSectionId;

    @ColumnInfo(name="section_name")
    public String mSectionName;

    public int getSectionId() {
        return mSectionId;
    }

    public void setSectionId(int mSectionId) {
        this.mSectionId = mSectionId;
    }

    public String getSectionName() {
        return mSectionName;
    }

    public void setSectionName(String mSectionName) {
        this.mSectionName = mSectionName;
    }
}

我的查询方法

@Query("SELECT * FROM section")
LiveData<List<Section>> getAllSections();

正在访问数据库

final LiveData<List<Section>> sections = mDb.sectionDAO().getAllSections();

在下一行我正在检查 sections.getValue()，尽管我在数据库中有数据，但它总是给我 null，后来我在 onChanged() 方法中获取值。

sections.observe(this, new Observer<List<Section>>() {
    @Override
    public void onChanged(@Nullable List<Section> sections){

    }
});

但是当我从查询中省略 LiveData 时，我得到了预期的数据。查询方式：

@Query("SELECT * FROM section")
List<Section> getAllSections();

正在访问数据库：

final List<Section> sections = mDb.sectionDAO().getAllSections();

Answer 1

if sections.getValue() is null I have to call api for data and insert in into the database

您可以使用 onChange 方法处理此问题：

sections.observe(this, new Observer<List<Section>>() {
    @Override
    public void onChanged(@Nullable List<Section> sections){
         if(sections == null || sections.size() == 0) {
             // No data in your database, call your api for data
         } else {
             // One or more items retrieved, no need to call your api for data.
         }
    }
});

但是您最好将此 Database/Table 初始化逻辑放到存储库 class 中。查看 Google's sample. See DatabaseCreator class.

Answer 2

我通过这个方法解决了这个问题

    private MediatorLiveData<List<Section>> mSectionLive = new MediatorLiveData<>();
    .
    .
    .

    @Override
    public LiveData<List<Section>> getAllSections() {
        final LiveData<List<Section>> sections = mDb.sectionDAO().getAllSections();

        mSectionLive.addSource(sections, new Observer<List<Section>>() {
            @Override
            public void onChanged(@Nullable List<Section> sectionList) {
               if(sectionList == null || sectionList.isEmpty()) {
                  // Fetch data from API
               }else{
                  mSectionLive.removeSource(sections);
                  mSectionLive.setValue(sectionList);
               }
            }
        });
        return mSectionLive;
    }

Answer 3

我解决了类似的问题如下

在您的 ViewModel 中 class

private LiveData<List<Section>> mSections;

@Override
public LiveData<List<Section>> getAllSections() {

    if (mSections == null) {
        mSections = mDb.sectionDAO().getAllSections();
    }

    return mSections;
}

这都是必需的。切勿更改 LiveData 的实例。

Answer 4

On the next line I am checking sections.getValue() which is always giving me null although I have data in the DataBase and later I am getting the value in the onChanged() method.

这是正常行为，因为 return LiveData 的查询正在异步工作。那一刻该值为空。

所以调用这个方法

LiveData<List<Section>> getAllSections();

稍后你会在这里得到结果

sections.observe(this, new Observer<List<Section>>() {
@Override
public void onChanged(@Nullable List<Section> sections){

}
});

来自文档：

Room does not allow accessing the database on the main thread unless you called allowMainThreadQueries() on the builder because it might potentially lock the UI for a long periods of time. Asynchronous queries (queries that return LiveData or RxJava Flowable) are exempt from this rule since they asynchronously run the query on a background thread when needed.

Answer 5

LiveData 是一个异步查询，你得到了 LiveData 对象，但它可能不包含任何数据。您可以使用额外的方法来等待数据被填充然后提取数据。

public static <T> T getValue(LiveData<T> liveData) throws InterruptedException {
    final Object[] objects = new Object[1];
    final CountDownLatch latch = new CountDownLatch(1);

    Observer observer = new Observer() {
        @Override
        public void onChanged(@Nullable Object o) {
            objects[0] = o;
            latch.countDown();
            liveData.removeObserver(this);
        }
    };
    liveData.observeForever(observer);
    latch.await(2, TimeUnit.SECONDS);
    return (T) objects[0];
}

Answer 6

如果您需要在代码中从数据库中同步获取数据，我建议创建另一个没有 LiveData 的查询。

DAO:

@Query("SELECT COUNT(*) FROM section")
int countAllSections();

视图模型：

Integer countAllSections() {
    return new CountAllSectionsTask().execute().get();
}

private static class CountAllSectionsTask extends AsyncTask<Void, Void, Integer> {

    @Override
    protected Integer doInBackground(Void... notes) {
        return mDb.sectionDAO().countAllSections();
    }
}

Answer 7

对于遇到此问题的任何人。如果您正在调用 LiveData.getValue() 并且您始终得到 null。您可能忘记调用 LiveData.observe()。如果您忘记这样做，getValue() 将始终 return null 特别是对于 List<> 数据类型。

LiveData.getValue() returns 空房间

LiveData.getValue() returns null with Room

android

android-room

android-livedata

android-architecture-components