There are two ways to define asynchronous functions in Kotlin:

• using the modifier suspend in the function declaration
• creating an extension function on CoroutineScope (or passing it as a parameter)

The suspend modifier is generally used for functions that might take some time to complete. The caller coroutine might be potentially suspended.

Functions that return results immediately but start a coroutine in the background should be written as extension functions on CoroutineScope. At the same time, these functions should not be declared suspend, as suspending functions should not leave running background tasks behind.

Noncompliant Code Example

suspend fun CoroutineScope.f(): Int {
    val resource1 = loadResource1()
    val resource2 = loadResource2()
    return resource1.size + resource2.size
}

Compliant Solution

Using suspend:

suspend fun f(): Int {
    val resource1 = loadResource1()
    val resource2 = loadResource2()
    return resource1.size + resource2.size
}

Using extension on CoroutineScope:

fun CoroutineScope.f(): Deferred<Int> = async {
    val resource1 = loadResource1()
    val resource2 = loadResource2()
    resource1.size + resource2.size
}

See

• Coroutine Context and Scope