Kotlin launch coroutine skips code line where Google Volley retrieves information from a server

Question

I have this code in a fragment:

@ExperimentalCoroutinesApi
override fun onViewCreated(view: View, savedInstanceState: Bundle?) {
    super.onViewCreated(view, savedInstanceState)

    create_adoption_btn.setOnClickListener {
        val temp = Intent(activity!!.baseContext, AdoptionCreationActivity::class.java)
        activity!!.startActivityFromFragment(this, temp, 1)
    }

    val mLayoutManager = GridLayoutManager(activity!!.baseContext, 1)
    recycler_view.layoutManager = mLayoutManager
    recycler_view.itemAnimator = DefaultItemAnimator()
    //recycler_view.adapter = adapter
    //AppController.instance!!.getAdoptionList().await()

    GlobalScope.launch(Dispatchers.Main, CoroutineStart.DEFAULT) {
        Log.i("TestAdapter", "Beginning fetch")
        val adapter = AlbumsAdapter(activity!!, AppController.instance!!.getAdoptionList()) //Skips this line, but still executes it
        Log.i("TestAdapter", "Adapter: ${adapter.itemCount}")
        recycler_view.adapter = adapter
        adapter.notifyDataSetChanged()
        Log.i("TestAdapter", "Adapter updated on thread")
    }
}

And this for a class that extends Application

class AppController : Application() {

private var adoptionCardList: MutableList<AdoptionCard> = mutableListOf()

override fun onCreate() {
    super.onCreate()
    instance = this
}

fun getAdoptionList(): MutableList<AdoptionCard> {
    if(adoptionCardList.count() == 0) {
        val service = GetVolley()
        val apiController = ApiController(service)
        val path = "adoptions/read.php"
        apiController.get(path, JSONArray()){ response ->
            if (response != null) {
                var x = 0
                while(x <= response.length() - 1){
                    val jsonObject = (response[x] as JSONObject)
                    adoptionCardList.add(AdoptionCard(
                        jsonObject.getInt("id"),
                        jsonObject.getString("adoption_title"),
                        jsonObject.getString("user_id").toBigInteger(),
                        jsonObject.getString("adoption_created_time")))
                    x+=1
                }
            }
        }
    }
    return adoptionCardList
}

private val requestQueue: RequestQueue? = null
    get() {
        if (field == null) {
            return Volley.newRequestQueue(applicationContext)
        }
        return field
    }

fun <T> addToRequestQueue(request: Request<T>, tag: String) {
    request.tag = if (TextUtils.isEmpty(tag)) TAG else tag
    requestQueue?.add(request)
}

fun <T> addToRequestQueue(request: Request<T>) {
    request.tag = TAG
    requestQueue?.add(request)
}

fun cancelPendingRequests(tag: Any) {
    if (requestQueue != null) {
        requestQueue!!.cancelAll(tag)
    }
}

companion object {
    private val TAG = AppController::class.java.simpleName
    @get:Synchronized var instance: AppController? = null
        private set
}

The "launch" coroutine should wait until Volley retrieves all information from the server but it just skips that line and the Recycler View doesn't update, since the MutableList is empty. If I reload the Fragment, it will do this successfully since there's an already stored list. I read all documentation I could on Kotlin Coroutines and questions asked but I can't make this work. Could anyone help me?

The debug: Debug log

On the first load, as you can see, the adapter has 0 elements, so the view gets nothing; on the second load, it already has 3 elements, so the Recycler view loads those 3.

ApiController:

class ApiController constructor(serviceInjection: RESTapi): RESTapi {
    private val service: RESTapi = serviceInjection

    override fun get(path: String, params: JSONArray, completionHandler: (response: JSONArray?) -> Unit) {
        service.get(path, params, completionHandler)
    }
}

Interface:

interface RESTapi {
    fun get(path: String, params: JSONArray, completionHandler: (response: JSONArray?) -> Unit)
}

GetVolley class:

class GetVolley : RESTapi {
val TAG = GetVolley::class.java.simpleName
val basePath = "http://192.168.0.161/animals/"

override fun get(path: String, params: JSONArray, completionHandler: (response: JSONArray?) -> Unit) {
    val jsonObjReq = object : JsonArrayRequest(Method.GET, basePath + path, params,
        Response.Listener<JSONArray> { response ->
            Log.d(TAG, "/get request OK! Response: $response")
            completionHandler(response)
        },
        Response.ErrorListener { error ->
            VolleyLog.e(TAG, "/get request fail! Error: ${error.message}")
            completionHandler(null)
        }) {
        @Throws(AuthFailureError::class)
        override fun getHeaders(): Map<String, String> {
            val headers = HashMap<String, String>()
            headers["Content-Type"] = "application/json"
            return headers
        }
    }

    AppController.instance?.addToRequestQueue(jsonObjReq, TAG)
}

Answer 1

Your problem here is that Volley is async by default. What this means is that it creates a new thread to run the call on. Since you're using coroutines, this is pointless. You'll need to force it over on the active thread and do a sync call instead.

This part:

AppController.instance?.addToRequestQueue(jsonObjReq, TAG)

Adds it to a request queue. This means it doesn't execute it instantly, but queues it with other requests (if there are any), and launches it on a separate thread. This is where your problem lies. You need to use a sync request instead. Async simply means "not on this thread", regardless of which thread. So since you're using a different one (coroutine), you'll need to force it to be sync. This makes it sync with the active thread, not the main thread.

_{I'm not sure if this will even work with coroutines, but since it's async, it should be fine.}

In order to block the thread, you can use a RequestFuture<JSONArray> as a replacement for the callbacks. You still need to add it to the request queue, but you can call .get on the RequestFuture, which blocks the thread until the request is complete, or it times out.

val future = RequestFuture.newFuture<JSONArray>() // The future
val jsonObjReq = object : JsonArrayRequest(Method.GET, basePath + path, params,
        future, // This is almost identical as earlier, but using the future instead of the different callback
        future) {
    @Throws(AuthFailureError::class)
    override fun getHeaders(): Map<String, String> {
        val headers = HashMap<String, String>()
        headers["Content-Type"] = "application/json"
        return headers
    }
}
AppController.instance?.addToRequestQueue(jsonObjReq, TAG);// Adds it to the queue. **This is very important**
try {
    // Timeout is optional, but I highly recommend it. You can rather re-try the request later if it fails
    future.get(30, TimeUnit.SECONDS).let { response ->
        completionHandler(response)
    }
}catch(e: TimeoutException){
    completionHandler(null)
    // The request timed out; handle this appropriately.
}catch(e: InterruptedException){
    completionHandler(null)
    // The request timed out; handle this appropriately.
}catch(e: ExecutionException){
    completionHandler(null)
    // This is the generic exception thrown. Any failure results in a ExecutionException
}
// The rest could be thrown by the handler. I don't recommend a generic `catch(e: Exception)`

This will block the thread until the response is received, or it times out. The reason I added a timeout is in case it can't connect. It's not that important since it's a coroutine, but if it times out, it's better handling it by notifying the user rather than trying over and over and loading forever.

Answer 2

The problem arises in your apiController.get() call, which returns immediately and not after the network operation is complete. You supply your response callback to it. It will run eventually, once the REST call has got its response.

This is how you should adapt your function to coroutines:

suspend fun getAdoptionList(): MutableList<AdoptionCard> {
    adoptionCardList.takeIf { it.isNotEmpty() }?.also { return it }
    suspendCancellableCoroutine<Unit> { cont ->
        ApiController(GetVolley()).get("adoptions/read.php", JSONArray()) { response ->
            // fill adoptionCardList from response
            cont.resume(Unit)
        }
    }
    return adoptionCardList
}

This is now a suspend fun and it will suspend itself in case the adoption list isn't already populated. In either case the function will ensure that by the time it returns, the list is populated.

I would also advise you to stop using GlobalScope in order to prevent your network calls running in the background, possibly holding on to the entire GUI tree of your activity, after the activity is destroyed. You can read more about structured concurrency from Roman Elizarov and you can follow the basic example in the documentation of CoroutineScope.