What does the 'suspend' keyword mean in Kotlin?

The suspend keyword in Kotlin is used to mark a function or lambda expression as potentially pausable and resumable later. Such functions are called coroutines.

Main properties of suspend functions:

• They can only be called from another suspend function or within a coroutine scope.
• Suspending a function does not block the thread in which it runs. Instead, the thread can be used to perform other tasks.
• After completing a suspended operation (e.g., a network request), the function resumes from where it was suspended.
• Behind the scenes, the Kotlin compiler transforms suspend functions into state machines using Continuation-Passing Style (CPS).

Example:

// Example suspend function to simulate a long operation
suspend fun fetchData(): String {
    // Simulate a network request or a long task
    kotlinx.coroutines.delay(1000) // Suspension without blocking the thread
    return "Data received"
}

// Function calling a suspend function
fun main() = kotlinx.coroutines.runBlocking { // runBlocking creates a coroutine scope
    println("Loading data...")
    val data = fetchData() // Call to suspend function
    println("Data: $data")
}