App crashes with this error `monarchy.realmThreadLocal.get()` returns null

[ksyamkrishnan]

2019-08-03 13:10:40.732 19654-19677/com.example.android.codelabs.paging E/AndroidRuntime: FATAL EXCEPTION: arch_disk_io_0 Process: com.example.android.codelabs.paging, PID: 19654 java.lang.NullPointerException: Attempt to invoke virtual method 'void io.realm.Realm.refresh()' on a null object reference at com.zhuinden.monarchy.Monarchy$RealmTiledDataSource.isInvalid(Monarchy.java:668) at androidx.paging.WrapperPositionalDataSource.isInvalid(WrapperPositionalDataSource.java:51) at androidx.paging.TiledPagedList.(TiledPagedList.java:89) at androidx.paging.PagedList.create(PagedList.java:203) at androidx.paging.PagedList.access$000(PagedList.java:114) at androidx.paging.PagedList$Builder.build(PagedList.java:362) at androidx.paging.LivePagedListBuilder$1.compute(LivePagedListBuilder.java:201) at androidx.paging.LivePagedListBuilder$1.compute(LivePagedListBuilder.java:166) at androidx.lifecycle.ComputableLiveData$2.run(ComputableLiveData.java:101) at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1167) at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:641) at java.lang.Thread.run(Thread.java:764)

[Zhuinden]

Oh my, that really shouldn't happen 😭 One possible option though is to manually use monarchy.open somewhere so that stopping to observe the LiveData wouldn't kill/create a Realm in the background.

Do you have code that reproduces this so I can debug it a bit?

[ksyamkrishnan]

thanks, I was able to resolve it, the issue was I tired to create LiveData<PagedList> from RealmDataSourceFactory without Monarchy. But now I am getting this error Process: com.example.android.codelabs.paging, PID: 23228 java.lang.IllegalStateException: This Realm instance has already been closed, making it unusable. at io.realm.BaseRealm.checkIfValid(BaseRealm.java:433) at io.realm.RealmQuery.findAll(RealmQuery.java:1765) at com.zhuinden.monarchy.PagedLiveResults.createQuery(PagedLiveResults.java:50) at com.zhuinden.monarchy.Monarchy.createAndObserveRealmQuery(Monarchy.java:195) at com.zhuinden.monarchy.Monarchy$5.run(Monarchy.java:243) at android.os.Handler.handleCallback(Handler.java:873) at android.os.Handler.dispatchMessage(Handler.java:99) at android.os.Looper.loop(Looper.java:193) at android.os.HandlerThread.run(HandlerThread.java:65)

            realm.executeTransaction { realm ->
                var query = realm.where(Repo::class.java).equalTo("name", name)
                datasource = monarchy.createDataSourceFactory({ realm -> query })
                dataSourceFactory = (datasource.map { input ->
                    var repoRealm = Repo()
                    repoRealm.id = input.id
                    repoRealm.name = input.name
                    repoRealm.fullName = input.fullName
                    repoRealm.url = input.url
                    repoRealm.description = input.description
                    repoRealm.stars = input.stars
                    repoRealm.forks = input.forks
                    repoRealm.language = input.language
                } as? DataSource.Factory<Int, Repo>)!!
                pagedList = monarchy.findAllPagedWithChanges(
                        datasource,
                        LivePagedListBuilder<Int, Repo>(dataSourceFactory, 50))
            }
        }```

[ksyamkrishnan]

On debug further, I got his reason java.lang.IllegalStateException: Realm access from incorrect thread. Realm objects can only be accessed on the thread they were created.

[Zhuinden]

You cannot share RealmQuery across threads. Use the Realm instance in the Monarchy lambda to build the RealmQuery instance.

I'm actually not sure where the realm.executeTransaction came from, it's safer to use the monarchy.runTransactionSync method.

[ksyamkrishnan]

ok cool thanks @Zhuinden. Issue got fixed.

         realm -> realm.where(RepoRealm::class.java) }
         dataSourceFactory = dataSourceFactory = (realmDataSourceFactory.map { input ->
         Repo(input.id,input.name,input.fullName,input.description,
          input.url,input.stars,input.forks,input.language)
          } ) as DataSource.Factory<Int, Repo>
          monarchy.findAllPagedWithChanges(
          realmDataSourceFactory,
          LivePagedListBuilder<Int, Repo>(dataSourceFactory, 
          DATABASE_PAGE_SIZE).setBoundaryCallback(boundaryCallback))```

One more doubt can I use realm with and without Monarchywrapper in the same application 

[ksyamkrishnan]

This is not a library issue fixed from app side.

[Zhuinden]

You can use Monarchy with regular Realm, all you need to be aware of is that Monarchy runs stuff on its own thread (unless the method has __Sync in it).

Monarchy is generally safer though in regards that it makes it harder to keep Realm instances lingering around even when they should be closed.