Dealing with multiple completion handlers

Question

I'm trying to coordinate several completion handlers for each element in an array.

The code is essentially this:

var results = [String:Int]()

func requestData(for identifiers: [String])
{
    identifiers.forEach
    {   identifier in

        service.request(identifier, completion: { (result) in
            result[identifier] = result
        })
    }

    // Execute after all the completion handlers finish
    print(result)
}

So each element in the Array is sent through a service with a completion handler, and all the results are stored in an array. Once all of these handlers complete, I wish to execute some code.

I attempted to do this with DispatchQueue

var results = [String:Int]()

func requestData(for identifiers: [String])
{
    let queue = DispatchQueue.init(label: "queue")

    identifiers.forEach
    {   identifier in

        service.request(identifier, completion: { (result) in
            queue.sync
            {
                result[identifier] = result
            }
        })
    }

    // Execute after all the completion handlers finish
    queue.sync
    {
        print(result)
    }
}

but the print call is still being executed first, with an empty Dictionary

Answer 1

If I understand what are you are trying to do correctly, you probably want to use a DispatchGroup

Here is an example:

let group = DispatchGroup()

var letters = ["a", "b", "c"]

for letter in letters {
    group.enter()
    Server.doSomething(completion: { [weak self] (result) in
        print("Letter is: \(letter)")
        group.leave()
    })
}

group.notify(queue: .main) {
    print("- done")
}

This will print something like:

b
c
a
// ^ in some order
- done