Update UI async call with coroutines

Question

I've got to update the UI with an async call to the Room Database, but when I do I've got this error : android.view.ViewRootImpl$CalledFromWrongThreadException: Only the original thread that created a view hierarchy can touch its views.

// FavoritesPresenter.kt

GlobalScope.launch {
    favoritesView.showFavorites(ProductProvider.getAllProducts() as ArrayList<Product>)
}

// ProductProvider.kt

fun getAllProducts() : MutableList<Product> {
    return dao.getAllProducts()
}

// ProductDao.kt

@Query("SELECT * FROM product")
fun getAllProducts(): MutableList<Product>

What I need is to update my UI though my ProductProvider, as I'll use for all my entities I need a reliable solution.

Answer 1

Room 2.1 (currently in alpha) adds support for Kotlin coroutines . You can do the following:

1. Mark functions in ProductDao and ProductProvider as suspend :

    // ProductDao.kt @Query("SELECT * FROM product") suspend fun getAllProducts(): List<Product> // ProductProvider.kt suspend fun getAllProducts(): List<Product> = dao.getAllProducts() 

2. Create local scope for a coroutine in FavoritesPresenter :

    class FavoritesPresenter { private var favoritesView: FavoritesView? = null private val provider: ProductProvider = ...// initialize it somehow private var job: Job = Job() private val scope = CoroutineScope(job + Dispatchers.Main) fun getProducts() { scope.launch { favoritesView?.showFavorites(provider.getAllProducts()) } } fun attachView(view: FavoritesView) { this.favoritesView = view } fun detachView() { job.cancel() // cancel the job when Activity or Fragment is destroyed favoritesView = null } interface FavoritesView { fun showFavorites(products: List<Product>) } } 

3. Use FavoritesPresenter in Activity or Fragment :

    class MainActivity : AppCompatActivity(), FavoritesPresenter.FavoritesView { lateinit var presenter: FavoritesPresenter override fun onCreate(savedInstanceState: Bundle?) { super.onCreate(savedInstanceState) // ... presenter = FavoritesPresenter() presenter.attachView(this) presenter.getProducts() } override fun onDestroy() { presenter.detachView() super.onDestroy() } override fun showFavorites(products: List<Product>) { // use products to update UI } } 

To use Dispatchers.Main import:

implementation 'org.jetbrains.kotlinx:kotlinx-coroutines-android:1.1.1'

Answer 2

You should fetch from Room using an IO Coroutine, and switch to a Main (UI) Coroutine to update the view.

Try:

GlobalScope.launch(Dispatchers.IO) {
            val products = ProductProvider.getAllProducts() as ArrayList<Product>
            withContext(Dispatchers.Main) {
                favoritesView.showFavorites(products)
            }
        }

Make sure to have the Android Coroutine library installed so that Main Dispatcher correctly recognises the Android Main Thread.

api "org.jetbrains.kotlinx:kotlinx-coroutines-android:1.1.1"

Answer 3

It would be better not use GlobalScope, instead use your own CoroutineContext, for example:

class YourActivity : CoroutineScope {
    private lateinit var job: Job

    // context for io thread
    override val coroutineContext: CoroutineContext
        get() = Dispatchers.IO + job

    override fun onCreate(savedInstanceState: Bundle?) {
    super.onCreate(savedInstanceState)

    job = Job()
  }

    fun toDoSmth() {
        launch {
            // task, do smth in io thread
            withContext(Dispatchers.Main) {
              // do smth in main thread after task is finished
            }                  
        }
    }

   override fun onDestroy() {
      job.cancel()
      super.onDestroy()
   }
}