Swift, return value from asynchronous block

Question

Very new to iOS development and I've just started dabbling in GCD, I've read several articles on SO and have consulted Apple's documentation, however I am struggling to understand how to set a return value from an "expensive" operation.

The below function will loop over sum 100K users (as an example), doing this on the main thread would obviously halt the GUI for several seconds, therefore in my UserList class, I propose to execute this block asynchronously on my its thread:

let queue = dispatch_queue_create("me.alexsims.user_queue", DISPATCH_QUEUE_CONCURRENT)

public func getUserById(userId: String, completionHandler: (result: User) -> ())
{
    dispatch_async(queue)
    {
        for user in self.users {
            if user.getUserId() == userId
            {
                completionHandler(result: user)
            }
        }
        // Return an empty user object
        completionHandler(result: User())
    }
}

Now from my understanding, the result variable should be returned and I should then be able to access the User() object which is stored in there from my main thread

However, when I return to my controller and run a test:

var list   = UserList()
var a_user = User()

a_user = list.getUserByID(userId: "xyz", completionHandler: { (result) -> () in
    println(result)
})

this throws the error Could not find an overload for 'println' that accepts the supplied arguments, I'm guessing thats because I am not on the main queue? So I try:

a_user = list.getUserById(userId: "xyz", completionHandler: { (result) -> () in
    dispatch_async(dispatch_get_main_queue()) {
        println(result)
    }
})

However, I still get the same error. What am I doing wrong here?

SOLVED As Daniel spotted, the problem was providing the optional parameter to the list.getUserByID call. After changing it to:

a_user = list.getUserById("abcde", completionHandler: { (result) -> () in
    println(result)
})

The println error vanished.

thats not a threading issue, the system can't print out your custom object — Daniel Galasko
– Daniel Galasko, Commented Nov 18, 2014 at 13:42

Community · Accepted Answer · 2020-06-20 09:12:55Z

4

Your Swift class User needs to implement the Printable protocol so that it can be used by println.

Something like this:

Class User: Printable {
  let name = "Bob"
  var description: String {
    return "User: \(name)"
  }
}

Edit

A quick search of your println error turned this up. Not sure why the error falls through to println but heres a fix:

Try removing the first parameter of your function call, in Swift you don't need to type the first parameter to avoid redundancy:

a_user = list.getUserById(userId: "xyz...

Should be:

a_user = list.getUserById("xyz...

edited Jun 20, 2020 at 9:12

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answered Nov 18, 2014 at 13:43

Daniel Galasko

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6 Comments

Daniel Galasko Over a year ago

Which part exactly? Your issue has nothing to do with what thread you are on:) will update my post to be more clear

Halfpint Over a year ago

The only problem is, my User class inherits from a Person class, I tried setting "Printable" on my Person class to fix this, but I still get the could not find overload for println that accepts the supplied arguments error

Daniel Galasko Over a year ago

@Alex check my answer again

Halfpint Over a year ago

Jheeze, removing userId fixed the problem, how strange...thanks for your help Daniel :)

Daniel Galasko Over a year ago

@Alex anytime man, crazy odd error. Perhaps update your question to include the error? So weird. But all the best with iOS man:)

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