Dispatch Group not working for completion block

Question

Trying to get return reply objects from a network call. The session is a class that is using the star scream API. I just can't seem to get this to work. It's only printing out 1 set of results which is the from the first id. What am I missing here?

let myGroup = DispatchGroup()

for i in 0 ..< marketIds.count {
    myGroup.enter()
    self.session.retrieve(withMethod: MarketKeys.key, withParameters: [MarketKeys.id: marketIds[i]], completion: { (results, error) in
        print("results \n")
        print(results!)
        myGroup.leave()
        })
    }
myGroup.notify(queue:.main) {
print("Done")
}

@ReinierMelian actually, it is *not* necessary, he is missing the notifying part... Anyway, I would add a related question. — Ahmad F, Jan 18 '18 at 14:20
You are missing the "notifying" part. Related: https://stackoverflow.com/a/41809248/5501940 — Ahmad F, Jan 18 '18 at 14:20
I would start by making sure that `marketIds.count` is actually greater than 1. Add a print statement after the call to `enter` and make sure you're looping as much as you think you are. (Side note, this should be `for id in marketIds` rather than using subscripting. Subscripting works, but it creates more oppotunities for your app to crash. But this has nothing to do with your problem.) — Rob Napier, Jan 18 '18 at 15:04
I know I was just using the "i" to print the count of the for loop to check — Russell Warwick, Jan 18 '18 at 15:15

score 5 · Answer 1 · answered Jan 18 '18 at 14:20

This article gives you a quick reference guide to simple DispatchGroup use.

An example:

let dispatchGroup = DispatchGroup()

dispatchGroup.enter()
longRunningFunction { dispatchGroup.leave() }

dispatchGroup.enter()
longRunningFunctionTwo { dispatchGroup.leave() }

dispatchGroup.notify(queue: .main) {
    print("Both functions complete ")
}

The notify function is called when all items in the queue have been processed and allows you to react to this accordingly. So the example above will run two long running tasks and then will output "Both functions complete "

score 0 · Answer 2 · answered Jan 18 '18 at 14:13

0

Add this so notification to group can be sent

 myGroup.notify(queue: .main) {
   print("Both functions complete ")
  }

answered Jan 18 '18 at 14:13

Shehata Gamal

98,760
8
65
87

Add this where? – Russell Warwick Jan 18 '18 at 14:15
outside the for loop – Shehata Gamal Jan 18 '18 at 14:16
So notifying isn't the issue. The problem is that "session retrieve" only gets executed once. When in fact it could be getting called 3 times or the count of "marketIds" – Russell Warwick Jan 18 '18 at 14:21

Dispatch Group not working for completion block

2 Answers2