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LiveData does not update data from one activity to another - Android

I have 2 Activity

viz

  • List Activity

  • More details Activity

The list Activity

shows a list of items, and the detail is Activity

displayed when you click on an item from the list. In ListActivity

we are watching fetching feeds from the database, and as soon as we do that, we update the user interface.

List page

feedViewModel.getFeeds().observe(this, Observer { feeds ->
      feeds?.apply {
            feedAdapter.swap(feeds)
            feedAdapter.notifyDataSetChanged()
      }
})

We now have a page DetailActivity

that updates the feed (item) and Activity

, but the changes are not reflected in the ListActivity

.

Detail page

override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        feedViewModel.setFeedId(id)
        feedViewModel.updateFeed()
}

Channel View Model

class FeedViewModel(application: Application) : AndroidViewModel(application) {


    private val feedRepository = FeedRepository(FeedService.create(getToken(getApplication())),
            DatabaseCreator(application).database.feedDao())

    /**
     * Holds the id of the feed
     */
    private val feedId: MutableLiveData<Long> = MutableLiveData()

    /**
     * Complete list of feeds
     */
    private var feeds: LiveData<Resource<List<Feed>>> = MutableLiveData()

    /**
     * Particular feed based upon the live feed id
     */
    private var feed: LiveData<Resource<Feed>>

    init {
        feeds = feedRepository.feeds
        feed = Transformations.switchMap(feedId) { id ->
            feedRepository.getFeed(id)
        }
    }

    /**
     * Get list of feeds
     */
    fun getFeeds() = feeds

    fun setFeedId(id: Long) {
        feedId.value = id
    }

    /**
     * Update the feed
     */
    fun updateFeed() {
        feedRepository.updateFeed()
    }

    /**
     * Get feed based upon the feed id
     */
    fun getFeed(): LiveData<Resource<Feed>> {
        return feed
    }

}

For simplicity, some code has been abstracted. If necessary, I can add them to track the problem

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2 answers


After a lot of research and some idea from this answer from another question. I figured out the problem.

Question

DatabaseCreator(application).database.feedDao()

did not create a single instance of the database, due to which the first Activity

had a different instance where it was LiveData

listening for changes, and the second Activity

had a different instance where after the data was updated the callback was ignored.



Decision

Use dagger or any other dependency injection to ensure that only one instance of the database and DAO is created.

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I faced this same problem in multiple steps for the same objects. I found it helpful to code the Database Database instance as a Singleton like: this (step 5)

Example:

public abstract class MyDatabase extends RoomDatabase {
    private static MyDatabase mMyDatabase;
    public static MyDatabase getInstance(Context context) {
        if(mMyDatabase==null){
            mMyDatabase = Room.databaseBuilder(context.getApplicationContext(), MyDatabase.class, "app_database").build();
            return mMyDatabase;
        }
    }
}

And in each ViewModel you have (class extends from AndroidViewModel for Context parameter):



    public MyViewModel(@NonNull Application application) {
        super(application);
        mMyDatabase = MyDatabase.getInstance(application.getApplicationContext());
}

Now, in my case, every time I edit a value in the config actions, it is reflected in other actions.

Hope for this help.

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