[FIXED] How do i fix "Job was Cancelled" exception?

Issue

In my Fragment, I'm trying to fetch data from Firebase Database using coroutines where data is retrieving properly. Here is my code

 @ExperimentalCoroutinesApi    //Fragment Class code
  override fun onStart() {
    super.onStart()
    checkOutViewModel.viewModelScope.launch {
        try{
            if (isActive){
                checkOutViewModel.getCartDataFromFirebaseNetwork().collect{
                    tempList.add(it)
                }
            }
        }catch (ex : Exception){
            Log.d("exception message",ex.cause?.message!!)  //Fatal Exception: Main
        }
        orderListAdapter?.submitList(tempList)
        binding.progress.visibility = View.GONE
        binding.recycler.visibility = View.VISIBLE
    }
}

 @ExperimentalCoroutinesApi        //Viewmodel class code
   suspend  fun getCartDataFromFirebaseNetwork()= firebaseNetwork.getCartFromFirebase()

 


 @ExperimentalCoroutinesApi      //Repository class code
   suspend fun getCartFromFirebase() = callbackFlow<Cart> {
        ensureActive()
        val counterList = myFlow.toList()
        val itemList = myFlow.mapBasketToItemsList().toList()
        val pairs = myFlow.mapBasketListToQuantity().toList()
        if(itemList.isNotEmpty() && pairs.isNotEmpty()){
            for ((current,item) in itemList.withIndex()) {
                val cart = Cart(counterList[current].basketId!!,item.id!!,item.url!!,item.name!!,pairs[current].first,pairs[current].second,counterList[current].itemCounter!!,pairs[current].second)
                offer(cart)
            }
            channel.close()
        }
}

  @ExperimentalCoroutinesApi
 val myFlow = callbackFlow<Basket> {
    databaseReference.child("Cart").child(getCurrentUserUid())
        .addListenerForSingleValueEvent(object : ValueEventListener {
            override fun onCancelled(p0: DatabaseError) {

            }

            override fun onDataChange(dataSnapshot: DataSnapshot) {
                if (dataSnapshot.exists()) {
                    for (data in dataSnapshot.children) {
                        val basket = Basket()
                        basket.basketId = data.key
                        basket.itemId = data.child("itemId").value as String
                        basket.itemCounter = data.child("itemCounter").value as String
                        basket.itemWeight = data.child("itemWeight").value as String
                            offer(basket)
                    }
                    channel.close()
                }
            }
        })
    awaitClose()
}

  @ExperimentalCoroutinesApi
  private  fun Flow<Basket>.mapBasketToItemsList() : Flow<Items>  = map{basket ->
         suspendCoroutine<Items> {continuation ->
             databaseReference.child("Items").child(basket.itemId!!)
                 .addListenerForSingleValueEvent(object : ValueEventListener {
                     override fun onCancelled(p0: DatabaseError) {

                     }
                     override fun onDataChange(dataSnapshot: DataSnapshot) {
                         if (dataSnapshot.exists()) {
                           val   items = dataSnapshot.getValue(Items::class.java)!!
                             continuation.resume(items)
                         }
                     }
                 })
         }
}

 @ExperimentalCoroutinesApi
  private fun Flow<Basket>.mapBasketListToQuantity() : Flow<Pair<String,String>> = map{basket ->
    suspendCoroutine<Pair<String,String>> {continuation ->
        databaseReference.child("Quantities").child(basket.itemId!!)
            .child(basket.itemWeight!!)
            .addListenerForSingleValueEvent(object : ValueEventListener {
                override fun onCancelled(p0: DatabaseError) {

                }
                override fun onDataChange(dataSnapshot: DataSnapshot) {
                    if (dataSnapshot.exists()) {
                        val key = dataSnapshot.key
                        val value = dataSnapshot.value as String
                        val myPair = Pair(key!!, value)
                        continuation.resume(myPair)
                    }
                }
            })
    }
}

Edited: This is my Navigation Flow of Fragments

OnBoarding-Authentication-MainFragment-CheckItemListFragment

 override fun onStart() {  //OnBoarding Fragment
    super.onStart()
    try {
        if(viewModel.checkAuth()){
            updateUI()
        }
    }catch (ex : Exception){
        println("In onBoarding Fragment")
        Log.d("exception message",ex.cause?.message!!)
    }
}

override fun onStart() {  //Authentication Fragment
    super.onStart()
    try {
        if(mAuth.currentUser == null){
            showShortToast("Please Login")
        }else{
            updateUI()
        }
    }catch (ex : Exception){
        println("In authentication Fragment")
        Log.d("exception message",ex.cause?.message!!)
    }
}

 override fun onStart() {  //MainFragment
    super.onStart()
    try {
        if(mainFragmentViewModel.checkSignIn() == null)
            findNavController().navigateUp()
        binding.toolbar.add_to_cart.setOnClickListener {
            it.findNavController().navigate(R.id.action_mainFragment_to_checkoutItemsList)
        }
    }catch (ex : Exception){
        println("In Main Fragment")
        Log.d("exception",ex.message!!)
    }
}

    @ExperimentalCoroutinesApi
override fun onStart() {  //CheckItemList Fragment
    super.onStart()
    try {
        binding.addToCart.setOnClickListener {
            checkOutViewModel.viewModelScope.launch {
                val message = orderListAdapter?.getList()?.let { it1 -> checkOutViewModel.submitFinalCart(it1) }
                if(message!!){
                    findNavController().navigate(R.id.action_checkoutItemsList_to_finalCarts)
                }
            }
        }
        viewLifecycleOwner.lifecycleScope.launch {
            ensureActive()
            checkOutViewModel.getCartDataFromFirebaseNetwork().collect {
                tempList.add(it)
                orderListAdapter?.submitList(tempList)
                binding.progress.visibility = View.GONE
                binding.recycler.visibility = View.VISIBLE
            }
        }
    }catch (ex : Exception){
        println("In checkItemList Fragment")
        Log.d("exception message",ex.cause?.message!!)
    }
}

Edited : My Logcat is :-

--------- beginning of crash
07-10 21:18:40.605 30715-30715/com.example.groceryapp E/AndroidRuntime: FATAL 
EXCEPTION: main
Process: com.example.groceryapp, PID: 30715
f.d
    at com.example.groceryapp.checkout.CheckoutItemsList$e.a(:73)
    at f.z.k.a.a.b(:33)
    at kotlinx.coroutines.u0.run(:334)
    at kotlinx.coroutines.z0.k(:68)
    at kotlinx.coroutines.r0.b(:354)
    at f.z.i.b(:42)
    at com.example.groceryapp.f.a$r$a$b.a(:262)
    at com.google.firebase.database.m$a.a(:179)
    at com.google.firebase.database.u.a0.a(:75)
    at com.google.firebase.database.u.i0.d.a(:63)
    at com.google.firebase.database.u.i0.g$a.run(:55)
    at android.os.Handler.handleCallback(Handler.java:742)
    at android.os.Handler.dispatchMessage(Handler.java:95)
    at android.os.Looper.loop(Looper.java:157)
    at android.app.ActivityThread.main(ActivityThread.java:5603)
    at java.lang.reflect.Method.invoke(Native Method)
    at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:774)
    at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:652)

This is how i used my Try Catch block in every Fragments to handle this issue but it is also not working at all. I'm also using isActive method to check whether the job is still active or not before retrieving any data. I get "Fatal Exception: Main, Job was Cancelled" if i pressed back button before recyclerview shows the data. This exception only comes if i use callback flow. Is there any way to handle this issue or is it a bug in callback flow?. So far I couldn't find any possible answer that will solve my issue. Please tell me how do i fix it?

Solution

I faced with the same issue before and here is my solution with an extension of SendChannel

fun <T> SendChannel<T>.offerCatching(element: T): Boolean {
     return runCatching { offer(element) }.getOrDefault(false)
}

and when emit event just call offerCatching

Answered By - Steve.P