When `launch` is used we use dispatchers `IO` and `Default`

Question

When `launch` is used we use dispatchers `IO` and `Default` to run code asynchronously Why is `async` run asynchronously if you do not run on one of those dispatchers It s possible I m misunderstanding something

Circusmagnus · Answer

asynchronously does not mean `in different threads` It just means that they do not depend on each other are not synchronized in any way They may still run in parallel OR be just interleaved on single thread suspend functions may well suspend their execution waiting for something This does not block a thread So during this suspension other functions coroutines may be run on this thread And when they finish our suspend function may resume in the very same thread Considering code ```suspend fun one delay 100 suspend fun two callSomeOtherSuspendFun singleThreadScope launch doSth one doSomeOtherThing singleThreadScope async two ``` ``` We are on single thread The launch will start first But soon enough it will suspend for 100 milliseconds From this moment our async may start executing It too cuould get suspended because it is calling suspending function making room for launch to resume its execution So conceptually both launch and async are executing at the same time We cannot know which one will finish first They are asynchronous even when run on single thread ```

Circusmagnus · Answer

That way if you are on android with Room and Retrofit you do not have to think about threading at all Just launch your data fetching coroutines in `viewmodelScope` which is run on Dispatchers Main They will soon enough suspend waiting for network calls db queries to complete And while they are suspended other data fetching corotuines may run on Main Thread What is more UI can run on Main thread too uninterrupted As long as you are not calling any blocking functions you are free to go on Main Thread and just forget about threading completely blocking function function which will execute for a considerable time bloking a thread suspend functions exposed by Room or Retrofit are not blocking and can be called on Main Thread no problem

Circusmagnus · Answer

So IMHO best practice is to avoid wrap blocking functions in `withContext ` wrap non blocking callback APIs in `suspendCancellableCorotuine` which will make coroutine suspend until called back by callback `launch` and `async` without any Dispatcher specified If execution needs to be switched to different thread assume that `suspend fun` which you are calling has taken care of that via `withContext ` or `suspendCancellableCoroutine`

streetsofboston · Answer

Yup Asynchronous = Parallel You can run asynchronously on a dispatcher with only one thread The current call may suspend and start executing resuming an other suspended call When that one is done it may resume the first call again Like a queue of tasks served by a single thread being served and run one at a time With multiple threads in a dispatcher this not only happens asynchronously but they may run in parallel as well Asynchronous is not finishing one call function and starting or resuming another one Doesn t say anything about being parallel or not Parallel is calls run at the same time Often this implies they run asynchronously as well but not necessarily