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"。

你测试过这个吗?

var test = "This is my string"

let replaced = test.stringByReplacingOccurrencesOfString(" ", withString: "+", options: nil, range: nil)

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")
} 

你可以测试这个:

let newString = test。stringByReplacingOccurrencesOfString(" "， withString: "+"，选项:nil，范围:nil)

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。

修改现有可变字符串的类别:

extension String
{
    mutating func replace(originalString:String, withString newString:String)
    {
        let replacedString = self.stringByReplacingOccurrencesOfString(originalString, withString: newString, options: nil, range: nil)
        self = replacedString
    }
}

使用:

name.replace(" ", withString: "+")