Flow Cancellation

In this series of articles , we are discussing about Kotlin flows and in this article we will discuss about Flow Cancellation.

There are 2 ways to cancel the flow

1.Cancel the coroutine job

    // cancel the job
    val job = launch {
        sampleFlow()
            .onCompletion { cause ->
                if (cause is CancellationException) {
                    println("Flow cancelled Manually by job ")
                }
            }
            .collect {
                println("Type 1 Received $it")
            }
    }

    delay(550)
    job.cancel()
/**
     * Output :
     * Type 1 Received 1
     * Type 1 Received 3
     * Flow cancelled Manually by job
     */

2.Cancel in scope

// cancel the code in scope
launch {
    sampleFlow()
        .onCompletion { cause ->
            if (cause is CancellationException) {
                println("Flow cancelled Manually in scope")
            }
        }.collect {
            println("Type 2 Received $it")
            if (it == 3) {
                cancel()
            }
        }
}.join()
/**
     * Output :
     * Type 2 Received 1
     * Type 2 Received 3
     * Flow cancelled Manually in scope
     */

Code gist for 2 types of cancelling can be found in https://gist.github.com/vprabhu/ebce4cf922f50325f3f82b400681c0ac

Cooperative Cancellation

All suspend function in coroutines are cancellable
They check for coroutine cancellation internally and throw CancellationException when cancelled
But if a coroutine is not checking for cancellation when working on its job , then it is non-cancellable .

Behaviour of flowOf() when cancelling

flowOf() doesn't check internally if the coroutine is active

        launch {
            flowOf(1,3,5)
                .onCompletion { cause ->
                    if (cause is CancellationException) {
                        println("Flow cancelled $cause")
                    }
                }
                .collect {
                    println("Received $it")
                    if (it == 3) {
                        cancel()
                    }
                }
        }.join()

        /**
         * output :
         * Received 1
         * Received 3
         * Received 5
         */

In the above code , even when you are cancelling the flow if the value is 3 inside the collect{} block , its not stopping the flow.

There are 2 ways to check that

1.ensureActive() in onEach() block

 launch {
        flowOf(1,3,5)
            .onCompletion { cause ->
                if (cause is CancellationException) {
                    println("Flow cancelled $cause")
                }
            }.onEach {
                ensureActive()
            }
            .collect {
                println("Received $it")
                if (it == 3) {
                    cancel()
                }
            }
    }.join()

    /**
     * output : 
     * Received 1
     * Received 3
     * Flow cancelled kotlinx.coroutines.JobCancellationException:
     *   StandaloneCoroutine was cancelled; job=StandaloneCoroutine{Cancelling}@635a732e
     */

2.cancellable()

 launch {
        flowOf(1,3,5)
            .onCompletion { cause ->
                if (cause is CancellationException) {
                    println("Flow cancelled $cause")
                }
            }
            .cancellable()
            .collect {
                println("Received $it")
                if (it == 3) {
                    cancel()
                }
            }
    }.join()

    /**
     * output :
     * Received 1
     * Received 3
     * Flow cancelled kotlinx.coroutines.JobCancellationException:
     *   StandaloneCoroutine was cancelled; job=StandaloneCoroutine{Cancelling}@635a732e
     */

Code gist for 2 types of cancelling in the flowOf() can be found in https://gist.github.com/vprabhu/1a9defc5e8decffc619b5ed746b43d17

When the flow is working on something intensive , we can use ensureActive() inside the function to cancel it. Let me know if you need a sample code .

In the next article , we will discuss about Cold Flow and Hot Flow.

Please leave your comments to improve and discuss more

Happy and Enjoy coding

Vignesh Prabhu