flatten的作用之一可以將集合裡的型別為集合的值連線起來。直接看例子：
[[1,2],[3]].flatten()
// 結果為[1,2,3]複製程式碼但是如果陣列元素為String時，有一個特別的方法joined(separator:)，作用也是將集合裡的元素連線起來（只是元素必須是String）
看定義：
extension Array where Element == String {
/// Returns a new string by concatenating the elements of the sequence,
/// adding the given separator between each element.
///
/// The following example shows how an array of strings can be joined to a
/// single, comma-separated string:
///
/// let cast = ["Vivien", "Marlon", "Kim", "Karl"]
/// let list = cast.joined(separator: ", ")
/// print(list)
/// // Prints "Vivien, Marlon, Kim, Karl"
///
/// - Parameter separator: A string to insert between each of the elements
/// in this sequence. The default separator is an empty string.
/// - Returns: A single, concatenated string.
public func joined(separator: String = default) -> String
}複製程式碼顯然針對過去flatten的使用方式，它的真實意圖就是join。所以在swift 3中，將flatten()重新命名為joined()。
這麼一來可讀性也提高了。也增加了一種使用方法：在連線集合內元素時可以指定連線的元素。比如：
let nestedNumbers = [[1, 2, 3], [4, 5, 6], [7, 8, 9]]
let joined = nestedNumbers.join(separator: [-1, -2])
print(Array(joined))
// Prints "[1, 2, 3, -1, -2, 4, 5, 6, -1, -2, 7, 8, 9]"複製程式碼相關連結：
SE0133-Rename flatten() to joined()
A (mostly) comprehensive list of Swift 3.0 and 2.3 changes