5 Fatal Coroutine Mistakes Nobody Tells You About

Coroutines are the preferred option for asynchronous programming in Kotlin if you ask me. Still, you need to know what you're doing, else it can quickly backfire for you.
This video shows you 5 common mistakes you rarely hear about when it comes to using coroutines.
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00:00 - Introduction
00:40 - Mistake 1
05:51 - Mistake 2
08:26 - Mistake 3
11:11 - Mistake 4
14:26 - Mistake 5

[@SamRamezanli]

I don't know how to express how helpful this video was. blogs and tutorials often tell us what to do, but it's important to understand what not to do as well. Looking forward to more videos like this

[@PhilippLackner]

Glad it helped! 🙏

[@pizzapizzadesu]

great timing. i was just about to make the "mistake 1" one for retrieving images. Really appreciated

[@it-6411]

Pay attention, that when you use .awaitAll() function in the list of deferred items and if any of these will throw exception, you'll get exception for the whole operation. So, you should use .awaitAll() only in case, when you except full success of all async calls. In other case to handle exceptions for specific items, you can use list.map { it.await() } and try/catch block.

[@PhilippLackner]

Correct, thanks for mentioning :)

[@it-6411]

@@PhilippLackner you’re welcome)

[@abdremo]

OK🤙

[@ChrisAthanas]

Like this? suspend fun getUserFirstNames(userIds: List): List { // Alternative solution - executed in parallel, but allows for each item to cause exception val firstNames4 = coroutineScope { userIds.map { userId -> async { try { getFirstNameWithExceptions(userId + 1000) } catch (e: Exception) { println("Exception in getUserFirstNames: $e") "Error for id=${userId+1000}: $e" } }.also { } } }.also { }.awaitAll() return firstNames4 } suspend fun getFirstNameWithExceptions(userId: Int): String { delay(500) println("getFirstName: $userId") if (userId > 1005) { // simulate an error throw Exception("userId > 505") } return "John $userId" } // note: I am using the `.also{}` blocks in order to let the IDE show the type at that point in execution

[@suleyman_sezer]

thank you for your correctly idea.

[@codinginflow]

This changes everything!

[@RanbirSingh-dl9co]

Above dialogue is said by Bulma from Dragon ball Super when Goku arrive when frizza attacked earth.

[@-Kimma-]

This is so good and clear! This also explains some minor issues I have to understand some of the functionality with coroutines. I'm going to use this video as code snippets when coding my coroutines 😀

[@marksunghunpark]

It's parellel dispatch and you can even use it for async mapper. You can make the code shorter if you want: val firstNames = userIds.map { userId -> async { getFirstName(userId) } }.awaitAll()

[@pradiptapriya7546]

Hi, do you mind answering some of my questions regarding this async? When using async and awaitAll() to get data, is there a chance that the order of name lists retrieved will be changing every time the operation is done? Because I thought since it's being done parallel, there's a time when some data in API are obtained faster than others, making the order of the list changing every time based on how the network performance is right? Or the list will be always the same?

[@marksunghunpark]

@@pradiptapriya7546 It's same with async { getFirstName(userIds[0]) } + async { getFirstName(userIds[1]) } + async { getFirstName(userIds[2]) }... So, it should be the same order.

[@pradiptapriya7546]

@@marksunghunpark I see! Thank you very much! With this, I can improve my code a bit!

[@dev_jeongdaeri]

Async mapper 엄청나네요!! 감사합니다!!👍👍👍👍

[@phantuananh2163]

Dope 😍

[@yash1307]

Sir why are you not becoming a Google Developer Expert? Because you provide an amazing and helpful content...

[@ssgojekblue]

So true!

[@protaties]

*JetBrains Developer Expert

[@royce-rich]

Free google sales person = GDE

[@anujmukul9284]

that doesnt give money!

[@jose-naves]

Pure gold. Thanks a lot!

[@anupdey99]

Excellent content as always! Please make a series about those mistakes. Appreciate your hard work. 🙌

[@spitze3768]

Great videos! They are very informative and have help me a lot in my shift to kotlin. Keep up to good work!

[@adamjacob5482]

Love your videos, love your work. Always great content! Thank you very much :)

[@Shasha-zs4dx]

Kudos to you for making this video...Learned very tricky points related to coroutines by watching this..Thank you mate.keep making these awesome videos

[@behnawm]

Great stuff Philipp! Keep it up!

[@hakimbouatou2558]

Amazing content as always!

[@sohaybbahi8983]

Great video, keep on creating great content !

[@LogicPhalanx]

Mistake 3 is actually main safe but I get your point. Delay doesn't block the calling thread.

[@asadnaqvi8901]

The content you are providing is not available on highly paid sites. You are doing great work. Keep it up buddy 🎉🎉

[@TiagoDvl]

Really nice model of videos. Keep up, man.

[@a.rohimsama7222]

Thank you for sharing! super helpful! love it!

[@gaurav7557]

Awesome Man, Thanks a ton for this

[@RexTorres]

Thanks for this awesome video. I learned A LOT. I use coroutines a lot so I will need to review those.

[@IvanVasheka]

Thanks for the video!

[@Lucien-ow5ne]

Great video, thank you!

[@cristianovecchi]

Great content, thanks!

[@dev_jeongdaeri]

Thanks for super awesome tutorial!!👍👍👍👍❤️

[@GeolseuDeiGamers]

Thank you so much, your videos are awesome

[@Ayor88]

as always with your videos, very interesting for self learners !

[@imashnake_7151]

Wow you look so much healthier now, great work!

[@stanleykou5643]

Excellent examples!

[@user-me1ry6lg6d]

what an explanation Philip . You earned a like .

[@dedeandres455]

amazing video and good explaination Thanks Philipp

[@rahul_spawar]

This video is very helpful , please make more videos like this

[@usmallas2]

We need more videos like this one!

[@z-h-d]

nice video to watch before sleep ) easy listening, very clear and positive 👍

[@sergeyplotnikov5031]

Really useful content/ Thank you very much!!!

[@datel666]

Nice video! Thanks

[@safionweb]

Amazing Content!

[@otoS97]

Amazing video.I would love more of videos like this. It tests the knowledge and also reminds you details. Really good content.

[@shutanovac]

Your best video as far as I'm concerned 👍

[@skandarsouissi7282]

i'm addicted to your videos man, i can't sleep.

[@arash1684]

very useful thanks 👍

[@joaquinalanalvidrezsoto5054]

Sehr tolles Video. You should make more like this :D

[@saquibsiddique2641]

Really very helpful thanks for sharing @Philipp

[@PhilippLackner]

Glad it was helpful!

[@nickolajarjous2639]

Excellent video

[@user-us3pn5rt2g]

Can you please, make several hours of such kind examples) so amazing stuff

[@chandlerbing2820]

Love all your videos. Please do more Android studio projects. Thank you so much!!!!!!!

[@MrGfpf]

Great tips! Thanks in advance! Brazil!

[@bjugdbjk]

Even without async just with the launch, we can have parallel execution because both are non-suspending calls !! Async is specifically useful when you want to get something out from a coroutine !!

[@mustafaammar551]

Thank you Bro

[@thanosfisherman]

Mistake 4. If you make this suspending function return coroutineScope ( The suspend function you also used for mistake 2) or even withContext. Then the cancellation exception will be actually propagated to the outer scopes even if you explicitly catch in inside riskyTask e.g suspend fun riskyTask() = withContext() {}

[@ssgojekblue]

I'm refactoring all my Mistake 5 now 😃

[@zeredantech2966]

And in example 1 there is an interesting moment: Since we use coroutineScope{}, we dont need to await all deffered elements, because, With the rules of "Structured Concurrency", the Deffered, which is being launched under the hood in coroutineScope{} will not be completed until all children get completed, so this call of coroutineScope will wait until all async{} will give us the result. After this, we can use non-suspend method getCompleted() to just collect values.

[@ramasubramanian3154]

please make this a weekly video series.🙏

[@solo-ps9vb]

Very helpful. Using coroutines means coding in a new way.

[@amirhosseinghafoorian9985]

nice issues were mentioned here , especially the defered one

[@HosseinKurd]

Amazing and useful

[@PhilippLackner]

Thanks a lot

[@thegreatwarrior4989]

Very informative vedio👍👍😉😉😉

[@antonychepel5797]

Thanks for video! So usefull. What is that IDE? Visual Code or something else?

[@TheKuoala]

One thing worth mentioning for Mistake1 is the fact that coroutineScope will suspend until all the async calls finish. Therefore reading the result code like this could be confusing since you aren't really waiting for anything outside of the coroutineScope block.

[@ByronicGuitarist]

Great video. Rather than checking if an exception is a CancellationException in a try-catch block, you could use runCatching instead which does not catch CancellationExceptions

[@otoS97]

Everyone is providing video ideas so I would say it too: Running android Junit and Android tests on Gitlab's CI on every commit. If you're had any experience with this it would be a great video. Maybe with docker maybe without it.

[@ChrisAthanas]

@PhilippLackner @17:36 you mention needing to cancel the long-living CoroutineScope, but did you mean any Jobs that were started with the long-living CoroutineScope, and not cancelling the CoroutineScope itself? I dont see any way to cancel the CoroutineScope, just jobs associated with the scope. Please advise.

[@amnsatija]

best corotuines video ever

[@crayolaksh]

Hi, mistake 2 can also be handled with a withTimeout block right? it'll throw an exception if the operation within takes longer than specified time. Please correct if I'm wrong

[@user-ew8wc2mc1o]

Hello, thanks for the amazing videos but one question always concerns me: isn't sometimes the overhead of creating a lot of coroutines defies the purpose of performing tasks in parallel? or wouldn't it be a problem?

[@dk6024]

In the first example, you could have mapped over the incoming IDs and avoided some code and mutability.

[@7xFuryPlayz]

my question is ? will its good or bad to use repository class in the last mistake ? i think so viewModel should not interact with your api , repository class should interact with your api and give data to viewModel and then viewModel give data to activity or fragment ui right ???

[@christianmeazza2309]

I'm glad you're here to tell me about those mistakes, you are the best. I see the next video coming: "5 Fatal Kotlin Flows Mistakes Nobody Tells You About" :D

[@grimonce]

There's something weird with the first example... isn't appending to the list synchronous in here? Why didn't we include it in the async block? I don't know the first thing about Kotlin, I use different tools, but to me this didn't fix anything, unless using "coroutineScope" makes appending asynchronous somehow... Also won't adding the firstNames to the async block become a race condition? Is the memory reserved for the result ahead of time thanks to the type DeferredString?

[@silvestreramirez4515]

But for example if I have one thousand client IDs this works? Can I have one thousand async task at the same time running?

[@path_ethics]

How to create and manage cancelation of our own scope, for example in case we want to have a scope binded to our whole application?

[@zekininadresi]

I think using launch would be better as async would bring all parent the scope down if once job fails to retrieve the name (unless it's supervised).

[@aleksandr.liublinskii]

In mistake #1 since we have multiple coroutines spawned by async, and they all access the mutableListOf concurrently, shouldn’t we use thread safe data structure, for example COW list instead?

[@kemasdimas]

Wow thank you, didn't realize I made a lot of inefficient coroutine calls in my app! 😅 Does your paid course cover this extensively? Will need to check it out

[@PhilippLackner]

My paid courses cover lots of topics and best practices in a bigger practical project 😄

[@kemasdimas]

@@PhilippLackner bought it, thanks!

[@zeyadabdo1964]

Do I need to call suspend functions of Retrofit and Room on a background thread?

[@l_a_h797]

Very helpful explanations! At 4:07, with what keystrokes did you move the whole for loop up together? Ah, looks like Shift+Cmd+Up on Mac. Very useful! (Unfortunately, the video title is misleading clickbait. These are not mistakes that "nobody tells you about." Making your suspend functions main-safe, for example, is a principle thoroughly discussed in Android documentation about coroutines.) On mistake #5, the point is valid that the lifecycleScope is too short-lived for the purpose of posting data to an API, and using the viewModelScope is an improvement. But the ViewModel too can be destroyed (for example if the user navigates to another activity) while data is being posted to the API, and that is not what the user wants either. Android docs recommend that "business-oriented operations," such as uploading users' data, should survive even process death. For that, WorkManager is recommended.

[@_checkit]

#5 so let's say that button is a "save and close" button for the current activity. What then? You pass the "finish()" method as callback to the viewmodel method?

[@AdrianTache]

I think mistake 2 is misleading, because that code should be safe as written. But I agree, with components that aren't coroutine safe, like long running calculations, you should do a check before continuing them whenever it makes sense.

[@johnf419]

Agreed, I think the example was not realistic

[@GrahamFirst]

In mistake 3, shouldn't "networkCall" just return a string (or throw an exception)? I don't see the point of wrapping it in a Result in this case.

[@alishanvaliani9952]

Hi! Make video about Maps Compose, Please!

[@jpvs0101]

Thanks for saved me from Mistake 5

[@ivanstefanenko438]

Thanks for the videos, they are really awesome! One question about Mistake 2: what did you mean when said we will stay in while loop if will not check for is job active? Did you mean while loop will continue executing even after we cancel job? Probably I've caught smth wrong, so will appreciate it if you clarify this thing!

[@PhilippLackner]

Yes it will go on after canceling

[@DanieleSegato]

That code is long running and has no suspended calls in the middle, it's regular code. Meaning it's your job to check. If you had the same loop with a suspending function to create a random you wouldn't need to check cause it would be checked automatically at the next suspending call.

[@GakisStylianos]

@@DanieleSegato actually having a suspend function doesn't automatically fix this. If that suspend function doesn't also cooperate with cancelation then it will still keep going on forever. All suspending functions inside kotlinx.coroutines do cooperate with cancellation but that may not be true for your own suspending functions!

[@ivanstefanenko438]

Roger that! Thank you all for the answears!

[@purplehazer417]

Hello from Poland:)

[@deepeshkumar1704]

Can i update UI like async(first) method?

[@aherbel]

For Mistake #1, doesn't creating a new coroutineScope inside a suspend function loose the parent scope? I mean, if some error is thrown inside that coroutineScope, the parent will not get cancelled...

[@bobbybeat1]

The coroutineScope function creates a CoroutineScope that inherits the context of the parent and forwards cancellations/exceptions to the parent scope. More info: kotlin.github.io/kotlinx.coroutines/kotlinx-coroutines-core/kotlinx.coroutines/coroutine-scope.html

[@samandar7632]

My Android Studio (Bumble bee); not implementing github dependencies! Who resolved this problem?

[@Monarch_943]

Damn this was a very good video! it seems I'v been making alot of mistakes :/

[@PhilippLackner]

Time to stop making them😁

[@chibuezefelixanyanwu300]

Can you please handle paginate data with retrofit

[@zanyking]

Be careful while using async in web backend development, if the function is going to be triggered by client request, and you expect high QPS on this function, then you have to create your own thread pool for this async to use Never use default thread pool, because it’s small and might exhausted and cause other client requests await

[@ackerman6992]

Why it necessary to call async function inside coroutine scope? 🤔 We can call a suspend function in other suspend function right? Or we call coroutine scope so that the function inside coroutine scope works independently?

[@skullkrum20]

Usually your coroutines code runs sequentially. We use async so that we can immediately loop and call async again for the new iteration. This way you start all jobs initially and then wait for all of them to finish with "awaitAll".

[@ackerman6992]

@@skullkrum20 yes i undestood that, but i was asking why we need to call async inside coroutine scope.

[@skullkrum20]

@@ackerman6992 Its for that reason. If you don't you will run everything sequentially. Usually you don't need to if you're fine with that. Good name btw, I'm in love by Shingeki no Kyogin. :D

[@ackerman6992]

@@skullkrum20 ohk got it, Thanks 😂

[@bjugdbjk]

@@ackerman6992 coroutinescope, in this context , this is required because u r launching the coroutines with async, and to control them u need coroutinescope here !! And inside a suspend function we can't launch a coroutine !!

[@Nekrull]

You're right that suspend functions should not be exposed to the UI and I'm with you 200%, but have you seen Google's paging library? that thing not only uses suspend functions to submit data to the adapters, but it also seems to run load queries on the UI by passing the paging source to the list's adapter, it's actually the only library that I promised that I will never use

[@PhilippLackner]

I don't like the paging library either, but the load queries are done in the viewmodel afaik

[@Nekrull]

@@PhilippLackner I will test it out but I don't think it does it on the view model, you post a paging source (which in effect is an object that contains suspending functions to load the data) to your paging adapter through the fragment or activity's lifecycle scope, so it's pretty much impossible to run something on viewmodelscope, though I guess you could create the adapter on viewmodel and let the observer attach it to the paged list, but adapters in the view model are a bitch to unit test without robolectric

[@psistarpsi80]

I wish you'd make a video like this for Lua. Lua coroutines continuously befuddled me.

[@MrBicelis]

What should I do if I want a suspend function in viewmodel (Room function to update entity) to finish the work even after viewmodel gets destroyed?

[@PhilippLackner]

Use an application scope instead of viewmodelscope

[@MrBicelis]

@@PhilippLackner thanks, Phillip! You mean you would launch such a coroutine with GlobalScope? I thought it's a bad practice to use it, so I was looking for ways to avoid it. Even looking into using WorkManager... 🙂 For more context: my activity is used for reordering and/or adding items to a list, after user presses back/up button, I want to update the list in a Room database. I was using viewModelScope, but it wouldn't manage to save in time before the viewmodel gets destroyed. Btw, amazing content Phillip!

[@danyelsh874]

Awesome

[@s-w]

I see a Philipp Lackner video, I click.

[@StreetsOfBoston]

Nice video! But Mistake #3 is not a mistake at all: The function "networkCall" *suspends* for 3 seconds. It does not *block* for 3 seconds. Since "networkCall" is not blocking, there is no need to fix it by using a "withContext". If you wrote/coded "Thread.sleep(3000L)" instead, then this example would have been a real mistake, since you could have *blocked* (not suspended) the main thread.

[@etiennebeaulac8148]

The issue is not that he thought it was blocking, but that the function was not main-safe, which is a mistake. Changing the context to Dispatchers.IO was the right thing to do. Of course, as he mentions, if you use Retrofit you don't need to worry about this since it does it for you.

[@StreetsOfBoston]

@@etiennebeaulac8148 If the "networkCall()"'s body contained "Thread.sleep(3000L)" (or has some actual blocking call like waiting for a socket), then switching the context to Dispatchers.IO would have made perfect sense Written as it is, the code in Mistake -#3, is entirely main safe, since there is no blocking code, only suspending code. Replacing 'delay' with 'Thread.sleep' will make it blocking and not main-safe. It's a bit of nit-picking, but I think details are important sometimes 🙂

[@codingwithsam4992]

Hey Phillip, Its Samuel Laskar again. Can you please share some ways to make money by developing android apps. As playstore is already very saturated and freelance sites pay very less

[@PhilippLackner]

I'll soon upload such a video, yes

[@jatinvashisht4293]

@@PhilippLackner patiently waiting for it :)

[@maskedredstonerproz]

I would imagine use a different store, either that or make courses

[@rustamibrahimli2113]

Can you tell me please: What is your Color theme? This is such a beautiful theme

[@bjugdbjk]

shld be Xcode-dark

[@javlontursunov6527]

Is what you are saying applicable to Java as well ?

[@PhilippLackner]

not in this video

[@dontforgettovote531]

what is the name of the theme you're using in android studio?

[@bjugdbjk]

shld be Xcode-dark

[@dontforgettovote531]

@@bjugdbjk thanks m8