你可以这样尝试，先从 String 创建数组，然后从它创建块数组，然后从数组中加入字符串.

You can try like this way, first create array from String, then make chunk array from it and then join the string from array.

let newString = "1,2,3,4,5,6,7,8,9,10,11,12"
let array = newString.components(separatedBy: ",") // ["1", "2", "3", "4", "5", "6", "7", "8", "9", "10", "11", "12"]
let chunkSize = 4
let chunksArray = stride(from: 0, to: array.count, by: chunkSize).map {
    Array(array[$0..<min($0 + chunkSize, array.count)])
}
let subArray = chunksArray.map { $0.joined(separator: ",") }
// ["1,2,3,4", "5,6,7,8", "9,10,11,12"]

您可以像这样将最后两个动作与单个动作合并.

You can merge last two action with single like this way.

let subArray = stride(from: 0, to: array.count, by: chunkSize).map { 
    array[$0..<min($0 + chunkSize, array.count)].joined(separator: ",") 
}
// ["1,2,3,4", "5,6,7,8", "9,10,11,12"]