How to Suspend Function in Kotlin

This article will introduce the suspend function in Kotlin Coroutine and how to implement it.

the suspend Function in Kotlin

Almost everything in Kotlin Coroutine revolves around the suspend function. Coroutines are like lightweight threads that can run a block of code concurrently with the rest of the program.

Coroutines are not bound to any specific threads, so we can suspend them in one thread and resume them in another. The suspend function can pause and continue at any given time.

It can allow a massive program to run and complete without interruption.

We can define a suspend function by using the suspend keyword before the function name while declaring it. But to execute a suspend function, we need to invoke it either through another suspend function or from a coroutine; otherwise, it will throw an error.

Although we define a suspend function using the suspend keyword, it is compiled as a standard one behind the screen. The compiled function takes an additional parameter Continuation<T>.

Code:

suspend fun example(param: Int): Int {
    // another running operation
}

The code above becomes,

fun example(param: Int, callback: Continuation<Int>): Int {
    // another running operation
}

Here’s a simple example of the suspend function:

import kotlinx.coroutines.*

fun main() = runBlocking {
    launch { suspendExample() }
    println("Hi")
}

suspend fun suspendExample() {
    delay(2000L)
    println("Welcome!")
}

Output:

Example of the suspend Function in Kotlin

Let’s look at another suspended function in Kotlin to understand better how concurrent threads execute one after the other.

In this example, we will use 2 delay functions to display different print messages on the screen.

import kotlinx.coroutines.*

fun main() = runBlocking {
    suspendExample()
    println("Programming")
}

suspend fun suspendExample() = coroutineScope {
    launch {
        delay(1000L)
        println("Kotlin")
    }
    launch {
        delay(500L)
        println("Welcome to")
    }
    println("Hello, ")
}

Output: