Update UI async call with coroutines

问题

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.

回答1:

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

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()

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>)
    }
}

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'

回答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"

回答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()
   }
}

来源：https://stackoverflow.com/questions/54671817/update-ui-async-call-with-coroutines

标签

android

kotlin

coroutine