Three ways to completion blocks in Swift

Variant1_inline.swift

    func startScrolling() {
        scrollLimit = Int(contentImageView.bounds.size.width - scrollView.bounds.size.width)
        scrollByFraction(.Start)
        UIView.animateWithDuration(15,
                                   delay: 0,
                                   options: [.Autoreverse,.Repeat,.BeginFromCurrentState],
                                   animations: {self.scrollByFraction(.End)},
                                   completion: {_ in self.centerContent()})
    }

Variant2_innner_functions.swift

    func startScrolling() {
        scrollLimit = Int(contentImageView.bounds.size.width - scrollView.bounds.size.width)
        scrollByFraction(.Start)
        func animations() {
            scrollByFraction(.End)
        }
        func onAnimationComplete(completed:Bool) {
            centerContent()
        }

        UIView.animateWithDuration(15,
                                   delay: 0,
                                   options: [.Autoreverse,.Repeat,.BeginFromCurrentState],
                                   animations: animations,
                                   completion: onAnimationComplete)
    }

Variant3_methods.swift

    func scrollToLimit() {
        scrollByFraction(.End)
    }

    func onAnimationComplete(completed:Bool) {
        centerContent()
    }

    func startScrolling() {
        scrollLimit = Int(contentImageView.bounds.size.width - scrollView.bounds.size.width)
        scrollByFraction(.Start)
        UIView.animateWithDuration(15,
                                   delay: 0,
                                   options: [.Autoreverse,.Repeat,.BeginFromCurrentState],
                                   animations: scrollToLimit,
                                   completion: onAnimationComplete)
    }