Updating Views asynchronously

Question

I am trying to populate a recyclerview with data from the web which I want to fetch asynchronously.

I have a function loadData() which is called onCreateView() which first makes a loading Indicator visible, then calls the suspend function loading the data and then tries to notify the view adapter to update.

But at this point I get the following exception:

android.view.ViewRootImpl$CalledFromWrongThreadException: Only the original thread that created a view hierarchy can touch its views.

which surprised me as my understanding was that only my get_top_books() function was called on a different thread and previously when I was showing the loading indicator I was apparently on the right thread.

So why is this run-time exception raised?

My code:

class DiscoverFragment: Fragment() {

    lateinit var loadingIndicator: TextView
    lateinit var viewAdapter: ViewAdapter
    var books = Books(arrayOf<String>("no books"), arrayOf<String>("no books"), arrayOf<String>("no books"))

    override fun onCreateView(inflater: LayoutInflater, container: ViewGroup?, savedInstanceState: Bundle?): View? {
        val viewFrame = layoutInflater?.inflate(R.layout.fragment_discover, container, false)

        val viewManager = GridLayoutManager(viewFrame!!.context, 2)
        viewAdapter = ViewAdapter(books)
        loadingIndicator = viewFrame.findViewById<TextView>(R.id.loading_indicator)

        val pxSpacing = (viewFrame.context.resources.displayMetrics.density * 8f + .5f).toInt()

        val recyclerView = viewFrame.findViewById<RecyclerView>(R.id.recycler).apply {
            setHasFixedSize(true)
            layoutManager = viewManager
            adapter = viewAdapter
            addItemDecoration(RecyclerViewDecorationSpacer(pxSpacing, 2))
        }

        loadData()

        return viewFrame
    }

    fun loadData() = CoroutineScope(Dispatchers.Default).launch {
        loadingIndicator.visibility = View.VISIBLE
        val task = async(Dispatchers.IO) {
            get_top_books()
        }
        books = task.await()
        viewAdapter.notifyDataSetChanged()
        loadingIndicator.visibility = View.INVISIBLE
    }
}

Consider using `GlobalScope.launch(Dispatchers.Default)` instead of `CoroutineScope(Dispatchers.Default).launch`. — Dominic Fischer, Apr 05 '19 at 15:08
Also consider `books = withContext(Dispatchers.IO) { get_top_books() }`, since you don't need the concurrency. — Dominic Fischer, Apr 05 '19 at 15:23

score 13 · Accepted Answer · answered Apr 05 '19 at 14:57

13

After calling books = task.await() you are outside UI thread. It is because you use CoroutineScope(Dispatchers.Default). Change it to Dispatchers.Main:

fun loadData() = CoroutineScope(Dispatchers.Main).launch {
        loadingIndicator.visibility = View.VISIBLE
        val task = async(Dispatchers.IO) {
            get_top_books()
        }
        books = task.await()
        viewAdapter.notifyDataSetChanged()
        loadingIndicator.visibility = View.INVISIBLE
    }

answered Apr 05 '19 at 14:57

Andrew Churilo

2,229
1
17
26

Thanks for the Answer. So the ```await()``` call removed me from the UI thread? If I change the Dispatcher.Default to Dispatchers.Main I get the following runtime exception: ```java.lang.IllegalStateException: Module with the Main dispatcher had failed to initialize ``` ```Caused by: java.lang.AbstractMethodError: abstract method "java.lang.String kotlinx.coroutines.internal.MainDispatcherFactory.hintOnError()"``` – Joschka Goes Apr 05 '19 at 18:59
1

@JoschkaGoes Have you added `org.jetbrains.kotlinx:kotlinx-coroutines-android:1.1.0` to your dependencies? – Dominic Fischer Apr 05 '19 at 20:40
@DominicFischer I just checked and I was on 1.0.1 and upgrading to 1.1.0 solved my issue! Thanks. – Joschka Goes Apr 05 '19 at 20:50

Andrew Churilo · Answer 2 · 2019-04-05T14:57:47.840

3

After calling books = task.await() you are outside UI thread. You should run all UI related code in the main thread. To do this you can use Dispatchers.Main.

CoroutineScope(Dispatchers.Main).launch {
    viewAdapter.notifyDataSetChanged()
    loadingIndicator.visibility = View.INVISIBLE
}

Or using Handler

Handler(Looper.getMainLooper()).post { 
    viewAdapter.notifyDataSetChanged()
    loadingIndicator.visibility = View.INVISIBLE
}

Or you can use Activty instance to call runOnUiThread method.

activity!!.runOnUiThread {
    viewAdapter.notifyDataSetChanged()
    loadingIndicator.visibility = View.INVISIBLE
}

edited Apr 05 '19 at 14:57

answered Apr 05 '19 at 13:46

Andrew Churilo

2,229
1
17
26

Thanks the ```Handler``` method works, but I get an runtime exception if I use Dispatchers.Main – Joschka Goes Apr 05 '19 at 20:46

score 2 · Answer 3 · answered Apr 05 '19 at 20:57

2

Changing the Dispatchers.Default to Dispatchers.Main and upgrading my version of kotlinx-coroutines-android to 1.1.1 did the trick.

Changing

val task = async(Dispatchers.IO) {
    get_top_books()
}
books = task.await()

to

books = withContext(Dispatchers.IO) {
    get_top_books()
}

is also a bit more elegant. Thanks to everyone who responded especially @DominicFischer who had the idea to check my dependencies.

answered Apr 05 '19 at 20:57

Joschka Goes

103
1
6

Remember that suspend function `withContext()` should be called only from a coroutine or another suspend function – Boken Dec 30 '20 at 16:16

score 0 · Answer 4 · answered Apr 05 '19 at 13:39

0

UI related statements should be performed in the UI thread, I can't tell from the linked code only, but maybe you are changing UI in thsi function get_top_books().

Just put the related UI code in the UI thread like this

runOnUiThread(
    object : Runnable {
        override fun run() {
            Log.i(TAG, "runOnUiThread")
        }
    }
)

answered Apr 05 '19 at 13:39

Amine

2,241
2
19
41

```get_top_books()``` is currently just a function wich creates an empty data class of type ```Books``` and then sleeps for some time so that should not be the issue. And is there a way to call ```runOnUiThread()``` from my fragment? Quickly looking that up gave me the impression that this is an Activity thing. – Joschka Goes Apr 05 '19 at 19:24

Updating Views asynchronously

4 Answers4