Error handling in Kotlin coroutines - How to display error on the UI?

Question

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:

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

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.

Sergio · Accepted Answer · 2022-01-02 07:16:25Z

2

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
         }
 }

answered Jan 2, 2022 at 7:16

Sergio

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Comments

dersu · Accepted Answer · 2022-01-03 15:31:04Z

1

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"))}
             }
        }
    }

answered Jan 3, 2022 at 15:31

dersu

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1 Comment

Joffrey Over a year ago

You can remove all withContext(Main) if you remove the dispatcher from the launch call. It will be used by default. If you really need getRequest and/or processJSON to run on other dispatchers, you could wrap them in withContext with the dispatcher you want. Note that withContext returns the value of the last expression in the lambda, so you can get a result out of it: val departures = withContext(Dispatchers.Default) { processJSON(...) }

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