Getting "Suspension functions can be called only within coroutine body" when calling withContext(Dispatchers.Main) inside a lambda

Question

I am creating a coroutine using the script below:

fun bar(completion: () -> Unit) {   GlobalScope.launch(Dispatchers.IO) {     val lambda = {       withContext(Dispatchers.Main) { //Suspension functions can be called only within coroutine body         completion()       }     }     foo(lambda)   } }  fun foo(lambda: () -> Unit) {   //...do something heavy   lambda() } 

But I am getting the error Suspension functions can be called only within coroutine body when I call withContext(Dispatchers.Main) because the lambda changes the context. I cannot change foo(lambda: () -> Unit) to foo(lambda: suspend () -> Unit) because it's from an external library.

Any idea of what I can do to call withContext(Dispatchers.Main) inside the lambda created inside the launch context?

• Kotlin version: 1.3.41
• Coroutines: 1.3.0-RC

Thank you!

Answer 1

You can rewrite your bar function like this:

fun bar(completion: () -> Unit) {     GlobalScope.launch(Dispatchers.IO) {         suspendCoroutine<Unit> {             val lambda = {                 it.resume(Unit)             }             foo(lambda)         }         withContext(Dispatchers.Main) {             completion()         }     } }  

Answer 2

 fun bar(completion: suspend CoroutineScope.() -> Unit) {     GlobalScope.launch(Dispatchers.IO) {         val lambda : suspend CoroutineScope.() -> Unit =  {             withContext(Dispatchers.Main) { //Suspension functions can be called only within coroutine body                 completion()             }         }         foo(this,lambda)     } }  private suspend fun foo( coroutineScope: CoroutineScope , lambda : suspend CoroutineScope.() -> Unit)  {     lambda.invoke(coroutineScope) }  

also like this

 fun bar(completion: suspend CoroutineScope.() -> Unit) {     GlobalScope.launch(Dispatchers.IO) {         foo(this){             withContext(Dispatchers.Main) { //Suspension functions can be called only within coroutine body                 completion()             }         }     } }  private suspend fun foo( coroutineScope: CoroutineScope , lambda : suspend CoroutineScope.() -> Unit)  {     lambda.invoke(coroutineScope) }