Coroutines Rule Set
The coroutines rule set analyzes code for potential coroutines problems.
GlobalCoroutineUsage
Report usages of GlobalScope.launch
and GlobalScope.async
. It is highly discouraged by the Kotlin documentation:
> Global scope is used to launch top-level coroutines which are operating on the whole application lifetime and are > not cancelled prematurely.
> Application code usually should use an application-defined CoroutineScope. Using async or launch on the instance > of GlobalScope is highly discouraged.
Active by default: No
Debt: 10min
Noncompliant Code:
fun foo() {
GlobalScope.launch { delay(1_000L) }
}
Compliant Code:
val scope = CoroutineScope(Dispatchers.Default)
fun foo() {
scope.launch { delay(1_000L) }
}
fun onDestroy() {
scope.cancel()
}
InjectDispatcher
Always use dependency injection to inject dispatchers for easier testing. This rule is based on the recommendation https://developer.android.com/kotlin/coroutines/coroutines-best-practices#inject-dispatchers
Active by default: Yes - Since v1.21.0
Requires Type Resolution
Debt: 5min
Configuration options:
dispatcherNames
(default:['IO', 'Default', 'Unconfined']
)The names of dispatchers to detect by this rule
Noncompliant Code:
fun myFunc() {
coroutineScope(Dispatchers.IO)
}
Compliant Code:
fun myFunc(dispatcher: CoroutineDispatcher = Dispatchers.IO) {
coroutineScope(dispatcher)
}
class MyRepository(dispatchers: CoroutineDispatcher = Dispatchers.IO)
RedundantSuspendModifier
suspend
modifier should only be used where needed, otherwise the function can only be used from other suspending
functions. This needlessly restricts use of the function and should be avoided by removing the suspend
modifier
where it's not needed.
Active by default: Yes - Since v1.21.0
Requires Type Resolution
Debt: 5min
Noncompliant Code:
suspend fun normalFunction() {
println("string")
}
Compliant Code:
fun normalFunction() {
println("string")
}
SleepInsteadOfDelay
Report usages of Thread.sleep
in suspending functions and coroutine blocks. A thread can
contain multiple coroutines at one time due to coroutines' lightweight nature, so if one
coroutine invokes Thread.sleep
, it could potentially halt the execution of unrelated coroutines
and cause unpredictable behavior.
Active by default: Yes - Since v1.21.0
Requires Type Resolution
Debt: 5min
Noncompliant Code:
suspend fun foo() {
Thread.sleep(1_000L)
}
Compliant Code:
suspend fun foo() {
delay(1_000L)
}
SuspendFunWithCoroutineScopeReceiver
Suspend functions that use CoroutineScope
as receiver should not be marked as suspend
.
A CoroutineScope
provides structured concurrency via its coroutineContext
. A suspend
function also has its own coroutineContext
, which is now ambiguous and mixed with the
receiver`s.
See https://kotlinlang.org/docs/coroutines-basics.html#scope-builder-and-concurrency
Active by default: No
Requires Type Resolution
Debt: 10min
Noncompliant Code:
suspend fun CoroutineScope.foo() {
launch {
delay(1.seconds)
}
}
Compliant Code:
fun CoroutineScope.foo() {
launch {
delay(1.seconds)
}
}
// Alternative
suspend fun foo() = coroutineScope {
launch {
delay(1.seconds)
}
}
SuspendFunWithFlowReturnType
Functions that return Flow
from kotlinx.coroutines.flow
should not be marked as suspend
.
Flows
are intended to be cold observable streams. The act of simply invoking a function that
returns a Flow
, should not have any side effects. Only once collection begins against the
returned Flow
, should work actually be done.
See https://kotlinlang.org/docs/flow.html#flows-are-cold
Active by default: Yes - Since v1.21.0
Requires Type Resolution
Debt: 10min
Noncompliant Code:
suspend fun observeSignals(): Flow<Unit> {
val pollingInterval = getPollingInterval() // Done outside of the flow builder block.
return flow {
while (true) {
delay(pollingInterval)
emit(Unit)
}
}
}
private suspend fun getPollingInterval(): Long {
// Return the polling interval from some repository
// in a suspending manner.
}
Compliant Code:
fun observeSignals(): Flow<Unit> {
return flow {
val pollingInterval = getPollingInterval() // Moved into the flow builder block.
while (true) {
delay(pollingInterval)
emit(Unit)
}
}
}
private suspend fun getPollingInterval(): Long {
// Return the polling interval from some repository
// in a suspending manner.
}