Kotlin coroutines - gracefully handling errors from suspend functions

user_11357623 0
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Trying to implement graceful handling of the errors with suspend functions that are called from async methods, How to catch the error thrown by a suspend method.

suspend fun findById(id: Long): User? {
    throw Exception("my exception") // intentionally throwing to simulate error situation.
    return userModel.findById(id) // IO, may throw an error
}

Caller piece, launching with IO thread

GlobalScope.launch(Dispatchers.IO) {
    try {

        var userAsync: Deferred<User?>? = null
        arguments?.getLong("id")?.let {
            userAsync = async { viewModel?.findById(it) } // async for efficiency as i've other async methods too.
        }

        val data = userAsync?.await()

        withContext(Dispatchers.Main) {
            user = data // data binding, populating UI fields of user
        }
    } catch (exception: Exception) {
        withContext(Dispatchers.Main) { fault(exception) }
    }
}

fault method

private fun fault(exception: Exception) {
    Log.d("User", "fault: ${exception.localizedMessage}") // expecting output
}

Currently runtime is crashing, want to implement graceful handling of errors.

Attempt 2

Tried placing try catch within the async block but it didn't like it.

var userAsync: Deferred<UserVO?>? = null
arguments?.getLong("id")?.let {
    userAsync = async {
        try {
            delegate?.findById(it)
        } catch (e: Exception) {
            print(e)
        }
    }
}
kotlin-coroutines kotlin error-handling try-catch android
1 Answers

I would use a CoroutineExceptionHandler to make your coroutines fail gracefully:

1) Define the handler:

val exceptionHandler = CoroutineExceptionHandler { context, error ->
    // Do what you want with the error
    Log.d(TAG, error)
}

2) Refactor your findById function to be executed within an IO context and make your ui code main safe:

suspend fun findById(id : Int) : User? = withContext(Dispatchers.IO){
    when(id){
        0 -> throw Exception("not valid")
        else -> return@withContext User(id)
    }
}
  1. Launch your job within MainScope (and so update the ui), passing exceptionHandler to launch coroutine builder in order to catch the exception:
val exceptionHandler = CoroutineExceptionHandler { _, error ->
     // Do what you want with the error
    Log.d(TAG, error)
}

MainScope().launch(exceptionHandler) {
     val user = delegate?.findById(userId)
        user?.let {
            Timber.v(it.toString())
        }
}
user_62751740
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