LiveData.getValue()返回Room的null [英] LiveData.getValue() returns null with Room

问题描述

Java POJO对象

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;
    }
}

我的查询方法

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

访问数据库

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

在下一行中，我正在检查sections.getValue()，尽管我在数据库中有数据，但始终为空，后来我在onChanged()方法中获取了值.

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){

    }
});

但是当我从查询中省略LiveData时，我正在按预期方式获取数据. 查询方法:

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

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

访问数据库:

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

推荐答案

我通过这种方法解决了这个问题

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;
    }

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