Kotlin Launch vs. Async: Picking the Best Coroutine Approach

When developing modern applications, especially in Android development, efficient asynchronous programming is vital to manage complex tasks like network requests or database operations without blocking the main thread.

In Kotlin, launch and async are two commonly used coroutine builders. Both have their strengths and specific use cases, but understanding the difference between them is key to selecting the right approach for your needs.

In this blog, we’ll dive deep into Kotlin coroutines, explore how launch and async differ, and examine scenarios to use each effectively.

Understanding Kotlin Coroutines and Their Benefits

Before we get into the launch and async comparison, it’s essential to grasp what Kotlin coroutines are. A coroutine is a lightweight framework for asynchronous programming, allowing you to write suspendable code that is easier to read compared to traditional callbacks.

Kotlin coroutines are:

• Lightweight: Unlike traditional threads, coroutines consume minimal memory and CPU resources because they are not bound to native threads.

• Non-blocking: They allow tasks to run without blocking the main thread, improving app responsiveness.

• Easy to manage: Features like structured concurrency help you manage concurrent operations predictably.

The Building Blocks: Launch and Async

Kotlin offers two primary coroutine builders for creating a new coroutine:

• Launch function: Ideal for fire-and-forget tasks where the result of the operation is not needed.

• Async block: Returns a Deferred object that represents a future result, allowing the caller to await its value.

The Launch Function: Fire and Forget

The launch function is used for fire-and-forget tasks. It doesn’t return a result and is primarily employed when the output isn’t required. For instance, updating the UI or writing to logs.

Example: Launch in Action

Kotlin

1import kotlinx.coroutines.*
2
3fun main() = runBlocking {
4    launch {
5        println("Executing a task in launch on thread: ${Thread.currentThread().name}")
6    }
7    println("This runs independently on thread: ${Thread.currentThread().name}")
8}

​

In the above code, the launch function creates a new coroutine, running the block on the calling thread (or another dispatcher, if specified). The execution of the coroutine is non-blocking, meaning it does not prevent other tasks from running on the main thread.

When to Use Launch

• UI updates in Android development.

• Logging or sending analytics data.

• Tasks that do not need to provide a return value.

Note: Since launch doesn’t return a result, proper exception handling mechanisms like try-catch blocks or CoroutineExceptionHandler are necessary to handle exceptions.

The Async Block: Await the Result

The async block is designed for tasks where you require a result that will be available in the future. It returns a Deferred instance, which can be awaited using the await keyword.

Example: Async for Network Requests

Kotlin

1import kotlinx.coroutines.*
2
3fun main() = runBlocking {
4    val deferred = async {
5        fetchDataFromNetwork()
6    }
7    println("Fetched data: ${deferred.await()}")
8}
9
10suspend fun fetchDataFromNetwork(): String {
11    delay(1000) // Simulate network delay
12    return "Network response"
13}

​

Here, async initiates the network call, and the await keyword suspends the current coroutine until the result is available. This makes async ideal for running parallel tasks whose results are needed after their completion.

Async Returns: Key Points

• Returns a Deferred object.

• Allows suspend function called await for retrieving the result.

• Deferred objects ensure type-safety for the returned result.

• Requires structured concurrency to avoid leaking coroutines.

Key Differences Between Launch and Async

Understanding the difference between launch and async ensures you choose the right approach:

Feature	Launch	Async
Returns a result	No	Yes (Deferred)
Use case	Fire-and-forget tasks	Tasks requiring a result
Exception propagation	Exceptions in launch coroutines are handled using CoroutineExceptionHandler.	Propagates via await()
Typical application	Logging, UI updates	Network calls, computations

Exception Handling in Coroutines

Proper error handling is crucial for robust applications. In launch, you must handle exceptions explicitly. However, in async, exceptions are deferred until the await function is invoked.

Example: Handling Exceptions in Launch

Kotlin

1launch {
2    try {
3        // Some code that may throw an exception
4    } catch (e: Exception) {
5        println("Exception caught: ${e.message}")
6    }
7}

​

Example: Handling Exceptions in Async

Kotlin

1val deferred = async {
2    throw Exception("Something went wrong!")
3}
4try {
5    deferred.await()
6} catch (e: Exception) {
7    println("Exception caught: ${e.message}")
8}

​

In both cases, managing exceptions ensures your application doesn’t crash unexpectedly during critical operations.

Parallel Execution with Async

One of the significant advantages of async is its ability to execute tasks in parallel. You can run multiple network requests simultaneously and await their results.

Example: Parallel Tasks with Async

Kotlin

1fun main() = runBlocking {
2    val task1 = async { fetchData("Task 1") }
3    val task2 = async { fetchData("Task 2") }
4
5    println("Results: ${task1.await()} and ${task2.await()}")
6}
7
8suspend fun fetchData(name: String): String {
9    delay(1000) // Simulate delay
10    return "$name completed"
11}

​

Here, both tasks run concurrently, optimizing the program’s performance.

Best Practices for Using Launch and Async

1. Use launch for fire-and-forget tasks: Logging or sending analytics data doesn’t require results.

2. Leverage async for obtaining results: Use it for network calls, database queries, or computations.

3. Handle exceptions appropriately: Whether in launch or async, ensure exception handling is implemented.

4. Avoid blocking the main thread: Always use coroutines with appropriate dispatchers to keep the UI responsive in Android projects.

5. Ensure structured concurrency: Use structured coroutine builders such as launch or async within a defined scope to prevent resource leaks.

Final Notes

Choosing between Kotlin’s launch and async builders depends on your application’s requirements. While launch is perfect for fire-and-forget tasks, async shines when you need a resulting deferred object for further operations. Both play crucial roles in asynchronous programming, making Kotlin coroutines a powerful toolset for developers. By understanding their nuances and implementing best practices, you can create efficient, maintainable, and responsive Android projects.

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