Blog Series: Mastering Kotlin Coroutines

Custom Coroutine Builders and Coroutine Lifecycle States

In this article, we explore creating custom coroutine builders in Kotlin. While launch and async are commonly used, creating your own builders provides a deeper understanding of coroutine internals and the flexibility to address unique use cases. We’ll also delve into CoroutineStart and coroutine lifecycle states to uncover how coroutines function under the hood.

1. Anatomy of a Coroutine Builder

Coroutine builders like launch and async are higher-order functions that:

1. Create a coroutine context.
2. Manage the coroutine’s lifecycle.
3. Handle exceptions and results.

Example: Simplified Coroutine Builder

Let’s create a basic coroutine builder named myLaunch:

fun CoroutineScope.myLaunch(block: suspend CoroutineScope.() -> Unit): Job {
    val job = Job(coroutineContext[Job])
    val scope = CoroutineScope(coroutineContext + job)
    scope.launch {
        try {
            block()
        } catch (e: Throwable) {
            println("Error: ${e.message}")
        } finally {
            println("Coroutine completed")
        }
    }
    return job
}

fun main() = runBlocking {
    myLaunch {
        println("Custom coroutine started")
        delay(1000)
        println("Custom coroutine ended")
    }.join()
}

Key Steps in myLaunch:

1. Create a Job: The coroutine’s lifecycle is tied to the Job.
2. Establish Context: Combine the parent context with the new Job.
3. Launch Coroutine: Execute the block in a safe scope.
4. Handle Completion: Log messages upon success or failure.

2. Deep Dive: CoroutineStart

The CoroutineStart parameter determines when and how a coroutine begins execution. Kotlin provides four options:

1. DEFAULT:

   • Starts immediately upon creation.

2. LAZY:

   • Delays execution until explicitly started or awaited.

3. ATOMIC:

   • Starts immediately and cannot be canceled before execution begins.

4. UNDISPATCHED:

   • Starts in the current thread until the first suspension point.

Example: Comparing CoroutineStart Modes

fun main() = runBlocking {
    val lazyJob = launch(start = CoroutineStart.LAZY) {
        println("Lazy coroutine started")
    }
    println("Lazy coroutine created")
    lazyJob.start() // Explicitly start the coroutine
    lazyJob.join()

    val atomicJob = launch(start = CoroutineStart.ATOMIC) {
        println("Atomic coroutine started")
    }
    atomicJob.join()
}

Output:

Lazy coroutine created
Lazy coroutine started
Atomic coroutine started

3. Custom CoroutineStart Example

You can implement custom behavior by leveraging CoroutineStart in your builders. Let’s modify myLaunch to support LAZY start:

fun CoroutineScope.myLaunch(
    start: CoroutineStart = CoroutineStart.DEFAULT,
    block: suspend CoroutineScope.() -> Unit
): Job {
    val job = Job(coroutineContext[Job])
    val scope = CoroutineScope(coroutineContext + job)
    return scope.launch(start) {
        try {
            block()
        } catch (e: Throwable) {
            println("Error: ${e.message}")
        } finally {
            println("Coroutine completed")
        }
    }
}

fun main() = runBlocking {
    val lazyJob = myLaunch(start = CoroutineStart.LAZY) {
        println("Lazy coroutine execution")
    }
    println("Custom lazy coroutine created")
    lazyJob.start()
    lazyJob.join()
}

Output:

Custom lazy coroutine created
Lazy coroutine execution
Coroutine completed

4. Coroutine Lifecycle States

A coroutine’s lifecycle consists of distinct states that determine its behavior and visibility:

1. Active:

   • The coroutine is running or ready to execute.

2. Cancelling:

   • The coroutine is in the process of being canceled.

3. Cancelled/Completed:

   • The coroutine has finished execution.

Inspecting Lifecycle States

fun main() = runBlocking {
    val job = launch {
        delay(500)
        println("Coroutine work in progress")
    }

    println("Job is active: ${job.isActive}")
    delay(100)
    println("Job is active after delay: ${job.isActive}")
    job.cancelAndJoin()
    println("Job is completed: ${job.isCompleted}")
}

Output:

Job is active: true
Job is active after delay: true
Coroutine work in progress
Job is completed: true

5. Debugging Custom Builders

To debug and profile custom builders, enable debugging probes:

DebugProbes.install()
DebugProbes.dumpCoroutines()

You can also utilize structured logging for tracing:

val loggingContext = CoroutineName("CustomBuilderExample")

fun main() = runBlocking(loggingContext) {
    myLaunch(CoroutineName("ChildCoroutine")) {
        println("Executing custom coroutine with logging")
    }.join()
}

6. Real-World Use Case: Retriable Coroutine Builder

Let’s create a coroutine builder that retries the provided block upon failure:

fun CoroutineScope.retryLaunch(
    retries: Int = 3,
    block: suspend CoroutineScope.() -> Unit
): Job {
    return launch {
        var attempt = 0
        while (attempt < retries) {
            try {
                block()
                return@launch
            } catch (e: Exception) {
                attempt++
                println("Retry $attempt failed: ${e.message}")
            }
        }
        println("All retries failed")
    }
}

fun main() = runBlocking {
    retryLaunch {
        println("Attempting work")
        if (Math.random() > 0.7) {
            println("Work succeeded")
        } else {
            throw RuntimeException("Simulated failure")
        }
    }.join()
}

Conclusion

Creating custom coroutine builders opens up possibilities to tailor coroutine behavior for specific application needs. By understanding and utilizing CoroutineStart and lifecycle states, you can fine-tune coroutine execution for performance and reliability. Experimenting with custom builders like myLaunch and retryLaunch will deepen your mastery of Kotlin coroutines.