Apparently I have failed to understand the logic of exception handling with kotlin coroutines. Could you please help me? On my Android app I have a button which fires the fetch function.

This function does 3 things:

1. retrieves a json file from a server (getRequest) - might fail

2. Processes the json file (processJSON) and stores 4 values temporarily inside the instance of the class Departures (departures). - might fail for another reason

3. populates 4 TextView with those values. (uiUpdate) I would like all of those fields to be populated with one understandable error message generated by the catch clauses of these failed functions. How can I achieve this?

    fun fetch(stopID: String, attr: String, value: String) {
            lifecycleScope.launch(Dispatchers.IO) {
                try {
                    val result = getRequest(stopID)
                    val departures = processJSON(result, attr, value)
                    withContext(Dispatchers.Main) {uiUpdate(stopID, departures)}
                } catch (e: Exception) {
                }
            }
        }
   

If I remove the try-catch here, the app crashes in case of an exception. If I put the uiUpdate into the shown catch clause it crashes again.

CodePudding user response：

By default lifecycleScope uses Dispatchers.Main.immediate dispatcher. You can remove Dispatchers.IO from passing it to lifecycleScope.launch and just call suspend functions, wrapped into try-catch:

 fun fetch(stopID: String, attr: String, value: String) = lifecycleScope.launch {
         try {
             val result = getRequest(stopID)
             val departures = processJSON(result, attr, value)
             uiUpdate(stopID, departures)
         } catch (e: Exception) {
             // You can update UI here as well
         }
 }

CodePudding user response：

Thank you Sergey! The following function really does what I intended to achieve. I removed the try-catch from inside the functions and added the respective @Throws expressions before the declaration of these functions.

 fun fetch(stopID: String, attr: String, value: String) {
          lifecycleScope.launch(Dispatchers.Default) {
              try {
                 val result = getRequest(stopID)
                 val departures = processJSON(result, attr, value)
                 withContext(Dispatchers.Main) {uiUpdate(stopID, departures)}
             } catch (e: UnknownHostException) {
                  withContext(Dispatchers.Main) {uiUpdate(stopID, Departures("Error2","Error2"))}
             } catch (e: JSONException) {
                  withContext(Dispatchers.Main) {uiUpdate(stopID, Departures("Error3","Error3"))}
             } catch (e: Exception) {
                 withContext(Dispatchers.Main) {uiUpdate(stopID, Departures("Error4","Error4"))}
             }
        }
    }