はじめに

やりたいことはタイトルの通りです。
1番前にいる時に前の要素を取得しようとすると、1番後ろの要素を取得したいです
1番後ろにいる時に前の要素を取得しようとすると、1番前の要素を取得したいです

実装

struct XXX {
    let id: String
}

let array: [XXX] = [XXX(id: "000"), XXX(id: "001"), XXX(id: "002"), XXX(id: "003"), XXX(id: "004")]

let selectedItem = XXX(id: "003")

func currentItemIndex(selected: XXX, in array: [XXX]) -> Int? {
    if let index = array.firstIndex(where: { $0.id == selected.id }) {
        return index
    } else {
        return nil
    }
}

func previousItem(selected: XXX, in array: [XXX]) -> XXX? {
    if let currentIndex = currentItemIndex(selected: selected, in: array) {
        let nextIndex = (currentIndex + 1) % array.count
        return array[nextIndex]
    }
    return nil
}

func nextItem(selected: XXX, in array: [XXX]) -> XXX? {
    if let currentIndex = currentItemIndex(selected: selected, in: array) {
        let previousIndex = (currentIndex - 1 + array.count) % array.count
        return array[previousIndex]
    }
    return nil
}

print(nextItem(selected: selectedItem, in: array)) // XXX(id: "002")
print(previousItem(selected: selectedItem, in: array)) // XXX(id: "004")

前の要素を取得する

func previousItem(selected: XXX, in array: [XXX]) -> XXX? {
    if let currentIndex = currentItemIndex(selected: selected, in: array) {
        let nextIndex = (currentIndex + 1) % array.count
        return array[nextIndex]
    }
    return nil
}

次の要素を取得する

func nextItem(selected: XXX, in array: [XXX]) -> XXX? {
    if let currentIndex = currentItemIndex(selected: selected, in: array) {
        let previousIndex = (currentIndex - 1 + array.count) % array.count
        return array[previousIndex]
    }
    return nil
}

おわり

実現できました