iOS Swift 3 DispatchQueue.main.async() in API response closure

Question

So I'm parsing a json response from an API service and I update the UI in the response parse closure which is supposed to happen asynchronously.

I noticed that - even though the API's response is very fast as I can tell from the console log - the UI doesn't update instantly. It takes a few seconds to update.

So I searched a bit and I found out that putting my UI update code inside DispatchQueue.main.async() fixes it.

Why is that the case? Aren't closures supposed to be async and take care of all that? Am I misunderstanding something? Thanks

Do you know on which thread the API service callout/callback was happening? Also, this might help: stackoverflow.com/questions/25740779/… — Jacob M. Barnard
– Jacob M. Barnard, Commented Jul 24, 2017 at 13:11

Mohammad Sadiq · Accepted Answer · 2017-07-24 12:40:53Z

1

Yes my friend, you are missing something here. Closures are not inherently supposed to be async. Even the normal functions that you defined are closures. Closures are not the magic wand that would take care of all for you. Its just a block of statements which can be passed around.

As you say that the response comes fast but UI does not update automatically, it might be taking time in parsing. Make sure that you are not doing the parsing part on main queue, only UI updations should be on main queue.

answered Jul 24, 2017 at 12:40

Mohammad Sadiq

5,23130 silver badges31 bronze badges

Sign up to request clarification or add additional context in comments.

Comments

mustafa · Accepted Answer · 2017-07-24 12:51:53Z

1

Are you using URLSessions's dataTask(with:completionHandler:) api? If this is the case completionHandler is called from background queue. That's why it takes some time to update UI(Be careful here. You can get random crashes too).

answered Jul 24, 2017 at 12:51

mustafa

15.5k10 gold badges50 silver badges57 bronze badges

Collectives™ on Stack Overflow

iOS Swift 3 DispatchQueue.main.async() in API response closure

2 Answers 2

Comments

Comments

Your Answer

Linked

Hot Network Questions

Collectives™ on Stack Overflow

2 Answers 2

Comments

Comments

Your Answer

Sign up or log in

Post as a guest

Linked

Related