Flow Exception Handling

In this series of articles , we are discussing about Kotlin flows and in this article we will discuss about Flows Exception handling

try/catch operator

• We can simply enclose the collect block with try/catch as follows

suspend fun main(): Unit = coroutineScope {

    launch {
        val brandFlow = brandFlow()
        try {
            brandFlow.collect { name ->
                println(name)
            }
        } catch (ex: Exception) {
            println("Caught Exception $ex")
        }
    }
}


private fun brandFlow() = flow {
    emit("Google Pixel")
    emit("Samsung")
    emit("Apple")

    throw Exception("Anonymous Exception")
}

What happens if an exception is thrown in collect block , one option is to use try/catch block which might make the code more complex .

Call to emit() internally leads to the execution of collect block (Flows under the hood)

OnEach()

• Returns a flow that invokes the given action before each value of the upstream flow is emitted downstream.

• Adding this operator before catch{...} block ensures that all the exception are caught and the collect{...} will be empty as we see in the below code

Exception Transparency

1.A Downstream exception must always be propagated to collector

2.Once an uncaught exception is thrown downstream , the flow isn't allowed to emit additional values

use catch operator instead of try/catch block

catch()

• only catches upstream exception and passes all downstream exceptions

• use this operator to follow the exception transparency principle .

• When flow builder throws exception , the applied operators will be stopped

suspend fun main(): Unit = coroutineScope {

    launch {
        val brandFlow = brandFlow()
        brandFlow
            .onEach { name ->
                println(name)
            }
            .catch { cause ->
                println("Caught Exception : $cause")
            }
            .collect{}
    }
}


private fun brandFlow() = flow {
    emit("Google Pixel")
    emit("Samsung")
    throw Exception("Anonymous Exception")
    emit("Apple")
}

Please run and play with above Kotlin codes to understand it in a better way .Just copy and paste it in a kotlin file .

In the next article , we will discuss about Flow intermediate operators .

Please leave your comments to improve and discuss more

Happy and Enjoy coding