Figuring out syntax for Grand Central Dispatch in Swift

Question

I have the following code:

dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0)) {

    // Do stuff in the backgroud

    dispatch_async(dispatch_get_main_queue()) {

        // Do stuff on the UI thread

    }
}

However it will not compile. The inner call to dispatch_async returns the following compile error:

Cannot invoke 'init' with an argument list of type '(dispatch_queue_t!, () -> () -> $T3)'

I can't seem to figure out how to write this so that it works like I used to be able to in Objective C. Thanks for any ideas!

Answer 1

Closures in Swift can have implicit returns if they only contain a single expression (see: Implicit Returns from Single-Expression Closures). Your inner closure most likely has a single expression in it to update the UI. The compiler is using the result of that expression as the return value for the closure which makes the closure's signature not match the signature dispatch_async wants. Since dispatch_async wants a closure that returns () (or Void), the fix is to just add an explicit return at the end of your closure:

dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0)) {

    // Do stuff in the backgroud

    dispatch_async(dispatch_get_main_queue()) {

        // Do stuff on the UI thread

        return
    }
}