Swift等价于你所描述的是字符串插值。如果你考虑JavaScript做“x”+数组，在Swift中等价的是“x\(数组)”。

一般来说，字符串插值和Printable协议之间有一个重要的区别。只有特定的类符合Printable。每个类都可以以某种方式插入字符串。这在编写泛型函数时很有帮助。您不必将自己限制在Printable类上。

对于任何元素类型

extension Array {

    func joined(glue:()->Element)->[Element]{
        var result:[Element] = [];
        result.reserveCapacity(count * 2);
        let last = count - 1;
        for (ix,item) in enumerated() {
            result.append(item);
            guard ix < last else{ continue }
            result.append(glue());
        }
        return result;
    }
}

试试这个:

let categories = dictData?.value(forKeyPath: "listing_subcategories_id") as! NSMutableArray
                        let tempArray = NSMutableArray()
                        for dc in categories
                        {
                            let dictD = dc as? NSMutableDictionary
                            tempArray.add(dictD?.object(forKey: "subcategories_name") as! String)
                        }
                        let joinedString = tempArray.componentsJoined(by: ",")

修改一个可选/非可选字符串数组

//Array of optional Strings
let array : [String?] = ["1",nil,"2","3","4"]

//Separator String
let separator = ","

//flatMap skips the nil values and then joined combines the non nil elements with the separator
let joinedString = array.flatMap{ $0 }.joined(separator: separator)


//Use Compact map in case of **Swift 4**
    let joinedString = array.compactMap{ $0 }.joined(separator: separator

print(joinedString)

这里是flatMap, compactMap跳过数组中的nil值，并附加其他值以给出一个连接字符串。

斯威夫特3

["I Love","Swift"].joined(separator:" ") // previously joinWithSeparator(" ")

Swift 2.0 Xcode 7.0 beta 6以上使用joinWithSeparator()代替join():

var array = ["1", "2", "3"]
let stringRepresentation = array.joinWithSeparator("-") // "1-2-3"

joinWithSeparator被定义为SequenceType的扩展

extension SequenceType where Generator.Element == String {
    /// Interpose the `separator` between elements of `self`, then concatenate
    /// the result.  For example:
    ///
    ///     ["foo", "bar", "baz"].joinWithSeparator("-|-") // "foo-|-bar-|-baz"
    @warn_unused_result
    public func joinWithSeparator(separator: String) -> String
}