基于Ramis回答的Swift 3解决方案:

extension String {
    func withReplacedCharacters(_ characters: String, by separator: String) -> String {
        let characterSet = CharacterSet(charactersIn: characters)
        return components(separatedBy: characterSet).joined(separator: separator)
    }
}

尝试根据Swift 3的命名约定，提出一个合适的函数名。

你可以用这个:

let s = "This is my string"
let modified = s.replace(" ", withString:"+")    

如果你在你的代码中添加这个扩展方法:

extension String
{
    func replace(target: String, withString: String) -> String
    {
       return self.stringByReplacingOccurrencesOfString(target, withString: withString, options: NSStringCompareOptions.LiteralSearch, range: nil)
    }
}

斯威夫特3:

extension String
{
    func replace(target: String, withString: String) -> String
    {
        return self.replacingOccurrences(of: target, with: withString, options: NSString.CompareOptions.literal, range: nil)
    }
}

这在swift 4.2中很容易做到。只需使用replacingOccurrences(of: " "， with: "_")进行替换

var myStr = "This is my string"
let replaced = myStr.replacingOccurrences(of: " ", with: "_")
print(replaced)

基于Ramis回答的Swift 3解决方案:

extension String {
    func withReplacedCharacters(_ characters: String, by separator: String) -> String {
        let characterSet = CharacterSet(charactersIn: characters)
        return components(separatedBy: characterSet).joined(separator: separator)
    }
}

尝试根据Swift 3的命名约定，提出一个合适的函数名。

如果你不想使用Objective-C NSString方法，你可以使用split和join:

var string = "This is my string"
string = join("+", split(string, isSeparator: { $0 == " " }))

split(string, isSeparator:{$0 == " "})返回一个字符串数组(["This"， "is"， "my"， "string"])。

join用一个+连接这些元素，产生所需的输出:"This+is+my+string"。

下面是Swift 3的示例:

var stringToReplace = "This my string"
if let range = stringToReplace.range(of: "my") {
   stringToReplace?.replaceSubrange(range, with: "your")
}