Getting the Next Element in a Swift Array

After.swift

extension Array {
    func after(item: T) -> T? {
        if let index = find(self, item) where index + 1 < count {
            return self[index + 1]
        }
        
        return nil
    }
}

A slightly modified version of @mvarie's solution which optionally allows wrapping back to the start or end of the collection:

extension Collection where Element: Equatable {
    
    func element(after element: Element, wrapping: Bool = false) -> Element? {
        if let index = self.firstIndex(of: element){
            let followingIndex = self.index(after: index)
            if followingIndex < self.endIndex {
                return self[followingIndex]
            } else if wrapping {
                return self[self.startIndex]
            }
        }
        return nil
    }
}

extension BidirectionalCollection where Element: Equatable {
    
    func element(before element: Element, wrapping: Bool = false) -> Element? {
        if let index = self.firstIndex(of: element){
            let precedingIndex = self.index(before: index)
            if precedingIndex >= self.startIndex {
                return self[precedingIndex]
            } else if wrapping {
                return self[self.index(before: self.endIndex)]
            }
        }
        return nil
    }
}