CODEWITHSUNDEEP
androidandroid-livedata
Matt Spriggs
Matt Spriggs

Reputation: 41

Android: Able to return ArrayList but NOT a LiveData array list

I am unable to get a LiveData ArrayList from a Room database but I am able to retrieve a standard ArrayList and cannot figure out why.

I have run this code in debug mode and the ArrayList returns a size of 4, which it should. The LiveData ArrayList, when get value is used returns null. I have run the LiveData query both within an executor and outside of the executor and it returns null.

Declarations

public LiveData<List<CourseEntity>> courseEntities;
private List<CourseEntity> courseData = new ArrayList<>();

Code outside of executor

public void loadData(final int termId) {

        courseEntities = courseRepository.getCourseByTermId(termId);
        courseData = courseEntities.getValue();

}

Code inside executor

public void loadData(final int termId) {
        executor.execute(new Runnable() {
            @Override
            public void run() {
                courseEntities = courseRepository.getCourseByTermId(termId);
                courseData = courseEntities.getValue();

            }
        });
    }

Code using just an ArrayList

public void loadData(final int termId) {
        executor.execute(new Runnable() {
            @Override
            public void run() {

                courseData = courseRepository.getCourseByTerm(termId);
            }
        });
    }

Queries from Dao

@Query("SELECT * FROM course " +
            "WHERE term_id = :termIdSelected ORDER BY course_start" )
    LiveData<List<CourseEntity>> getCourseByTermId(int termIdSelected);

    @Query("SELECT * FROM course WHERE term_id = :termIdSelected ORDER BY course_start")
    List<CourseEntity> getCourseByTerm(int termIdSelected);

This produces a null value for the LiveData instead of a value of 4 like the plain ArrayList produces. The only difference being the LiveData wrapper for the result. Any wisdom someone can share would be most appreciated.

Upvotes: 1

Views: 987

Answers (1)

CommonsWare
CommonsWare

Reputation: 1007286

When you have a Room @Dao return a LiveData (or an RxJava type like Observable or Single), the generated implementation will do the actual work on a background thread. So, when getCourseByTermId() returns, the work will not yet have begun, so the LiveData will not have results yet.

Reactive types, like LiveData, are meant to be observed. So, your activity/fragment/whatever would observe() the LiveData and react to the result when it is delivered.

Upvotes: 1

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