How to make alphabetically section headers in table view with a mutable data source

Question

I store strings of a view controller in a string array. I import this string array as a Data Source in my table view. This all works smoothly. But now I would like to sort t

Answer 1

I did it within one loop, not few (Swift 4):

struct ContactData {
    let longName: String
    let phones: [String]
    let thumbnailImageData: Data?
}
var contacts = [ContactData]()
var tableViewSource = [Character : [ContactData]]()
var headerTitles = [Character]()

func createContactsData(completionHandler: @escaping () -> Swift.Void) {
    contacts = extractContacts() // convert CNContact to custom ContactData
    tableViewSource.removeAll()
    var prevChar: Character?
    var currentBatch: [ContactData]!
    contacts.forEach { contact in
        guard let firstChar = contact.longName.first else {
            return
        }
        if prevChar != firstChar {
            if prevChar != nil {
                tableViewSource[prevChar!] = currentBatch
            }
            prevChar = firstChar
            currentBatch = [ContactData]()
        }
        currentBatch.append(contact)
    }

    let allKeys = Array(tableViewSource.keys)
    let sortedSymbols = allKeys.sorted(by: {$0 < $1})
    headerTitles = sortedSymbols
    completionHandler()
}