LiveData.getValue() returns null with Room

问题

Java POJO Object

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;
    }
}

My Query method

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

Accessing DB

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

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.

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

    }
});

But when I omit LiveData from the query I am getting the data as expected. Query Method:

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

Accessing DB:

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

回答1:

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.

This is a normal behavior, because queries that return LiveData, are working asynchronous. The value is null that moment.

So calling this method

LiveData<List<Section>> getAllSections();

you will get the result later here

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

}
});

from documentation:

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.

回答2:

I solve this problem through this approach

    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;
    }

回答3:

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

You can handle this at onChange method:

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.
         }
    }
});

But you should better put this Database/Table initialization logic to a repository class. Check out Google's sample. See DatabaseCreator class.

回答4:

I resolved the similar issue as follows

Inside your ViewModel class

private LiveData<List<Section>> mSections;

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

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

    return mSections;
}

This is all required. Never change the LiveData's instance.

回答5:

LiveData is an asynchronous query, you get the LiveData object but it might contain no data. You could use an extra method to wait for the data to be filled then extract the data.

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];
}

回答6:

I would suggest to create another query without LiveData if you need to synchronously fetch data from database in your code.

DAO:

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

ViewModel:

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();
    }
}

来源：https://stackoverflow.com/questions/44428389/livedata-getvalue-returns-null-with-room