Swift: Returning a result from inside dispatch_async

Question

So I have a block of code here which doesn't work. This is due to the fact that it found nil while trying to unwrap an optional value. Which is because it gets initialized inside the asynchronous method. My question is, how do I hold off on returning the function until it has fetched the result?

struct Domain {
    var name: String?
    var tld: String?
    var combined: String {
        get {
            return self.name!+self.tld!
        }
    }
    var whoIs: String {
        get {
            if self.whoIs.isEmpty {
                var result: String?
                dispatch_async(dispatch_get_global_queue( DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), {
                    let whois_url = NSURL(string: SEARCH_URL+self.combined+"/whois")
                    result = NSString(contentsOfURL: whois_url!, encoding: NSUTF8StringEncoding, error: nil)
                    print(result!)
                })
                return result!
            }
            return self.whoIs
        }
    }
}

Sergey Kalinichenko · Accepted Answer · 2015-01-22 02:30:13Z

1

If you want to wait for the results of the block, simply replace dispatch_async with dispatch_sync:

dispatch_sync(dispatch_get_global_queue( DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), {
    let whois_url = NSURL(string: SEARCH_URL+self.combined+"/whois")
    result = NSString(contentsOfURL: whois_url!, encoding: NSUTF8StringEncoding, error: nil)
    print(result!)
})

This would ensure that the method does not return until the content of URL is fetched into the result.

answered Jan 22, 2015 at 2:30

Sergey Kalinichenko

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1 Comment

smat88dd Over a year ago

You have to make sure that you dont dispatch_sync from the same thread as to where you are dispatching the closure -> deadlock

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