CODEWITHSUNDEEP
iosswiftgrand-central-dispatch
vichhai
vichhai

Reputation: 2568

Cannot invoke 'dispatch_once' with an argument list of type

I try to use dispatch_once, but I got this kind of error

'dispatch_once' error

var onceToken : dispatch_once_t = 0
    dispatch_once(&onceToken, { () -> Void in
        self.myCollectionView.scrollToItemAtIndexPath(NSIndexPath.indexAtPosition(1), atScrollPosition: UICollectionViewScrollPosition.Left, animated: false)
    })

Upvotes: 0

Views: 261

Answers (1)

zrzka
zrzka

Reputation: 21229

First of all, you can't use onceToken in this way. As I wrote in my comment, read this.

Swift compiler errors/warnings are misleading sometimes. They're improving them, but ... When this kind of error happens and I don't see a problem in my code, I'm going to add simple return at the end of my closure (to match closure type signature). Like this ...

dispatch_once(&onceToken, { () -> Void in
  self.myCollectionView.scrollToItemAtIndexPath(NSIndexPath.indexAtPosition(1),
  atScrollPosition: UICollectionViewScrollPosition.Left, animated: false)
  return
})

... this makes compiler happier and now you see your real problem ...

Cannot invoke 'indexAtPosition' with an argument list of type '(Int)'

... and that's because you're calling method indexAtPosition, which is not class method, on NSIndexPath class. And you have to pass NSIndexPath object there.

If you would like to scroll to the first item, you have to call it in this way:

dispatch_once(&onceToken) {
  let indexPath = NSIndexPath(forRow: 0, inSection: 0)
  self.myCollectionView.scrollToItemAtIndexPath(indexPath, atScrollPosition: .Left, animated: false)
}

Upvotes: 3

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