Kotlin coroutines `runBlocking`

Question

I am learning Kotlin coroutines. I\'ve read that runBlocking is the way to bridge synchronous and asynchronous code. But what is the performance gain if the

Answer 1

Actually you use runBlocking to call suspending functions in "blocking" code that otherwise wouldn't be callable there or in other words: you use it to call suspend functions outside of the coroutine context (in your example the block passed to async is the suspend function). Also (more obvious, as the name itself implies already), the call then is a blocking call. So in your example it is executed as if there wasn't something like async in place. It waits (blocks interruptibly) until everything within the runBlocking-block is finished.

For example assume a function in your library as follows:

suspend fun demo() : Any = TODO()

This method would not be callable from, e.g. main. For such a case you use runBlocking then, e.g.:

fun main(args: Array) {
  // demo() // this alone wouldn't compile... Error:() Kotlin: Suspend function 'demo' should be called only from a coroutine or another suspend function
  // whereas the following works as intended:
  runBlocking {
    demo()
  } // it also waits until demo()-call is finished which wouldn't happen if you use launch
}

Regarding performance gain: actually your application may rather be more responsive instead of being more performant (sometimes also more performant, e.g. if you have multiple parallel actions instead of several sequential ones). In your example however you already block when you assign the variable, so I would say that your app doesn't get more responsive yet. You may rather want to call your query asynchronously and then update the UI as soon as the response is available. So you basically just omit runBlocking and rather use something like launch. You may also be interested in Guide to UI programming with coroutines.