What does the suspend function mean in a Kotlin Coroutine?

Question

I\'m reading Kotlin Coroutine and know that it is based on suspend function. But what does suspend mean?

Coroutine or function gets

Answer 1

Suspending functions are at the center of everything coroutines. A suspending function is simply a function that can be paused and resumed at a later time. They can execute a long running operation and wait for it to complete without blocking.

The syntax of a suspending function is similar to that of a regular function except for the addition of the suspend keyword. It can take a parameter and have a return type. However, suspending functions can only be invoked by another suspending function or within a coroutine.

suspend fun backgroundTask(param: Int): Int {
     // long running operation
}

Under the hood, suspend functions are converted by the compiler to another function without the suspend keyword, that takes an addition parameter of type Continuation. The function above for example, will be converted by the compiler to this:

fun backgroundTask(param: Int, callback: Continuation): Int {
   // long running operation
}

Continuation is an interface that contains two functions that are invoked to resume the coroutine with a return value or with an exception if an error had occurred while the function was suspended.

interface Continuation {
   val context: CoroutineContext
   fun resume(value: T)
   fun resumeWithException(exception: Throwable)
}