What does main-safe in Kotlin Coroutines?

Question

I'm learning Coroutines of Kotlin. The Text A is from https://codelabs.developers.google.com/codelabs/kotlin-coroutines/#8

Does it mean the system will run them in background thread automatically when it need ?

Does it mean I will never use the code such as withContext(Dispatchers.IO) in both Room and Retrofit operation ?

Text A

Both Room and Retrofit make suspending functions main-safe. It's safe to call these suspend funs from Dispatchers.Main, even though they fetch from the network and write to the database.

score 12 · Answer 1 · answered Oct 24 '20 at 16:04

12

For me as an android developer this simple definition made perfect sens

We consider a function main-safe when it doesn't block UI updates on the main thread.

Found it here https://developer.android.com/kotlin/coroutines

answered Oct 24 '20 at 16:04

Calin

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Enselic · Accepted Answer · 2020-04-01T07:31:18.650

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What does main-safe [mean for] Kotlin Coroutines?

You literally quote the answer yourself:

It's safe to call these suspend funs from Dispatchers.Main, even though they fetch from the network and write to the database.

And the answer to

Does it mean I will never use the code such as withContext(Dispatchers.IO) in both Room and Retrofit operation ?

is: Correct (assuming you configure them properly, e.g. use suspend modifier in Retrofit fun definitions).

edited Apr 01 '20 at 07:31

answered Apr 01 '20 at 07:20

Enselic

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1

But that's confusing. Then "suspend" doesn't guarrantee main-safety. Only the method's implementation does. ???? – Juan José Melero Gómez Nov 27 '20 at 17:08
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@JuanJoséMeleroGómez Correct, "suspend" by itself does not guarantee main-safety. A suspend fun can still block the UI thread if it is badly written. Roman talks about this explicitly here for example: https://elizarov.medium.com/blocking-threads-suspending-coroutines-d33e11bf4761 – Enselic Nov 27 '20 at 20:31

score 3 · Answer 3 · answered Aug 03 '21 at 07:25

Please check this answer for the exact reason why the API calls works with retrofit without changing the dispatcher to IO, referencing https://stackoverflow.com/a/61216333/4354001 :

It works because Retrofit's suspend implementation delegates to Call.enqueue. This means it already executes on its own background executor by default instead of using the caller's Dispatcher.

What does main-safe in Kotlin Coroutines?

3 Answers3