Early Exit from Coroutines: Kotlin
When working with coroutines in Android, you might encounter scenarios where you want to exit early—for example, if cached data is available, avoiding unnecessary network calls. In this tutorial, we'll explore how to cleanly exit a lifecycleScope.launch
block when a certain condition is met (e.g., data exists in the database).
1. The Problem: Unnecessary Execution
Consider this common scenario:
- Check if data exists in a local database (e.g., Room, Paper, etc.).
- If found, process it and skip the network call.
- If not found, proceed with downloading.
A naive implementation might look like this:
```kotlin
lifecycleScope.launch {
val fromDB = getFromDB() // Returns Result
// This still executes even if we got data from DB! val result = downloadFile() // ... handle download
}
``
**Problem:** Even if we get data from the DB, the
downloadFile()` call still executes, wasting resources.
2. Solution: Early Exit with return@launch
To exit the coroutine early when we have the data, we use return@launch
:
kotlin
lifecycleScope.launch {
val fromDB = getFromDB()
fromDB.onSuccess { data ->
processResult(data)
showData()()
return@launch // ← Exits the coroutine here
}
// Only reaches here if DB fetch failed
val result = downloadFile()
// ... handle download
}
✅ Key Benefit: The coroutine stops immediately after return@launch
, preventing unnecessary work.
3. Alternative Approaches
*A. Using when
with Result
(More Explicit)
If you prefer structured control flow:
```kotlin lifecycleScope.launch { when (val result = getFromDB()) { is Result.Success -> { processResult(result.value) showData()() return@launch } is Result.Failure -> { / Proceed / } }
val downloadResult = downloadFile() // ... handle download
} ```
B. Using runCatching
(Kotlin-Style Error Handling)
If your getFromDB()
throws exceptions:
```kotlin lifecycleScope.launch { runCatching { getFromDB() } .onSuccess { data -> processResult(data) showData()() return@launch }
val downloadResult = downloadFile() // ... handle download
} ```
4. Best Practices
-
Avoid Side Effects
-
Ensure
return@launch
doesn’t break expected behavior (e.g., leaving resources open). -
Use Named Lambdas for Clarity
-
If working with nested coroutines, label them for better readability:
kotlin
lifecycleScope.launch(dispatcher) outerScope@ {
someOperation().onSuccess {
return@outerScope // Exits the outer coroutine
}
}
-
Consider
also
/let
for Chaining -
If you need to perform an action before exiting:
```kotlin lifecycleScope.launch { getFromDB() .onSuccess { data -> processResult(data) showData()() } .also { if (it.isSuccess) return@launch } // Exits after processing
downloadFile()
} ```
5. Last word
Using return@launch
inside lifecycleScope.launch
provides a clean way to exit early when a condition is met (e.g., cached data is available). This improves efficiency by avoiding unnecessary operations.
Key Takeaways:
✔ Use return@launch
to exit a coroutine early.
✔ Prefer structured error handling (Result
, runCatching
, when
).
✔ Ensure cleanup logic isn’t skipped unintentionally.
Would you like me to expand on any part? Let me know in the comments! 🚀