Suspend function 'delay' should be called only from a coroutine or another suspend functio-CodePudding

Hi am trying to run following kotlin code:

Code Snippet 1

This is not working with the error: function 'delay' should be called only from a coroutine or another suspend function

package kotlinx.coroutines.guide.exampleBasic02

import kotlinx.coroutines.*

fun main() = runBlocking { // this: CoroutineScope
    launch {echoWorldWrapper() }
    println("Hello")
}


suspend fun echoWorldWrapper() {

   echoWorld()
}

  fun echoWorld() {
    delay(1000L)
    println("World!")
}

However following works: Code Snippet 2:

package kotlinx.coroutines.guide.exampleBasic02

import kotlinx.coroutines.*

fun main() = runBlocking { // this: CoroutineScope
    launch {echoWorldWrapper() }
    println("Hello")
}


suspend fun echoWorldWrapper() {

    delay(1000L)
    println("World!")
}

In my production code I can only follow Code Snippet 1. Is there any workaround for approach 1 to work. I am new to Kotlin and couldn't find any other answer to this question. Thanks.

CodePudding user response：

It is not possible to call a suspend function from a function that is not also a suspend function. suspend carries with it under the hood the state of a coroutine in a Continuation object. delay() cannot operate without that coroutine to work with.

The functionality of delay() is to suspend a coroutine for a period of time and then resume it. It doesn't make any sense for it to be called from a non-suspend function, because that means there is no coroutine to suspend.

The alternative for a non-suspend function is to use Thread.sleep(), which will block the thread for a period of time. Example:

//Use withContext to make it acceptable to call a blocking function in a coroutine
suspend fun echoWorldWrapper() = withContext(Dispatchers.IO) {
   echoWorld()
}

// A blocking function:
fun echoWorld() {
    Thread.sleep(1000L)
    println("World!")
}