Hello guys is there any explanation for `suspend fun` for du

Question

Hello guys is there any explanation for `suspend fun` for dummies I mean if you would have to explain what is a `suspend` for someone that is completely new on this world of coroutines what would be your answer

Lukas Sztefek · Answer

Do you want to know more about usage or internals

Pablo · Answer

No something like suspend fun are main safe but if you add a wrong body in the function and block the UI something like that

Pablo · Answer

I mean suspend doesn t block the current thread but if you add wrong body to its function it could block

Lukas Sztefek · Answer

<https medium com mobile app development publication understanding suspend function of coroutines de26b070c5ed> This is pretty good explanation It also explains difference between blocking and suspending which is crucial to understand coroutines correctly

Niklas Gürtler · Answer

suspend basically allows the kotlin compiler to internally automagically split that function body into multiple blocks of code where the split points are certain special functions such as delay or waiting for other coroutine s results The individual blocks are run as whole on the specified thread dispatcher but inbetween those blocks anything else can be run on the thread When e g the delay has elapsed the next block of code is run as a whole If you wrongly execute a blocking function that is not a suspend fun like Thread sleep the kotlin compiler will not split the code block there and the thead will actually block and can t be used by anything else

Pablo · Answer

< Lukas Sztefek> what I see is that suspend can pause resume pause resume N times and it won t affect the funcion 2 but if function 1 gets block for isntance Thread Sleep it will block the thread and the function 2 won t stop until function 1 stop blocking

Pablo · Answer

But about suspend what it does internally I know suspend fun you can call them from a coroutine or other suspend fun but more info for dummies

Pablo · Answer

< Niklas Gürtler> That s a good example though but I d like to understand for someone that is not technical for instance what is doing suspend under the hood I mean something like this runs in a coroutine and doesn t block the thread what is inside but if you put wrong stuff like BigInteger findPrime even if it s suspend and supsend doesn t block the current thread it would do etc you got me

Niklas Gürtler · Answer

A suspend fun that does a big calculation will block the thread A coroutine will only give up the thread at certain points such as delay that are explicitly intended to do that

Pablo · Answer

But what I need to explain is this phrase 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

Pablo · Answer

What does it mean that dependning of the threads or coroutines available they can do some part of the suspend method then go to another method then return to the suspended one

Niklas Gürtler · Answer

That s overly general That long running operation needs to be something that is intended to be used that way such as delay or job wait yes exactly the threads involved execute pieces of coroutines as they become ready to execute

Niklas Gürtler · Answer

That s the difference to classic multithreading a thread can be interrupted at any point so that another thread can use the CPU So even if you have multiple threads doing big calculations and just one CPU core the OS can distribute the time and therefore processing power evenly among them by forcibly interrupting them and switching between them With multiple coroutines sharing one thread it s different coroutines have to voluntarily give up control to switch to other coroutines and they don t allow you to share processing time evenly

Pablo · Answer

ok so in general terms a suspend fun is a function that can be paused resumed many times until its execution is finished without blocking the thread

Niklas Gürtler · Answer

Yes but the function needs to explicitly allow suspending at certain points It can block but it doesn t block when it s suspended

Vivek Sharma · Answer

yes suspend function are like normal functions only but as they are called from a coroutine they can suspend themselves till they process and have the result ready and then they can resume to give the result

Vivek Sharma · Answer

<https www youtube com watch v=ne6CD1ZhAI0 amp feature=emb logo> its a video from coroutines codelab take a look you will understand better

Pablo · Answer

Ok got it i m trying to do an example of it and wondering why this code stops the execution ```suspend fun foo String val bar = withContext Dispatchers Default BigInteger probablePrime 4096 Random fun main CoroutineScope Dispatchers Unconfined launch print example1 print example2 print example3 print foo print example4 print example5 print example6 ```

Pablo · Answer

Problem here the execution stops on example 3 and rest are not executed that s because I m using Unconfined

Niklas Gürtler · Answer

execution blocks until probablePrime returns If that takes long

Niklas Gürtler · Answer

`withContext` waits for the stuff inside to complete

Pablo · Answer

I m trying to run this block

Pablo · Answer

And it says provess finished with exit code 0

Pablo · Answer

and last print was example 3

Niklas Gürtler · Answer

oh probably because the outer coroutine doesn t keep the process alive

Niklas Gürtler · Answer

maybe wrap the main inside a `runBlocking`

Niklas Gürtler · Answer

```fun main runBlocking CoroutineScope Dispatchers Unconfined launch print example1 print example2 print example3 print foo print example4 print example5 print example6 join ```

Pablo · Answer

But what if I have more than 1 launch

Niklas Gürtler · Answer

```fun main runBlocking coroutineScope Dispatchers Unconfined launch launch ```

Vivek Sharma · Answer

```fun main = runBlocking lt Unit gt launch for i in 1 10 delay 200 println I am not blocked println doSomething suspend fun doSomething String println doing something delay 2000 Thread sleep 3000 return something done ```

Pablo · Answer

< Vivek Sharma> where you do this I mean do you use a kotlin file where you can run it

Vivek Sharma · Answer

run this code you can see that suspend fun here is not blocking our main thread but if you comment delay and uncomment Thread sleep you will see the difference

Vivek Sharma · Answer

IntelliJ IDEA