func randomStringWithLength (len : Int) -> NSString {
 
    let letters : NSString = "abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789"
 
    var randomString : NSMutableString = NSMutableString(capacity: len)
 
    for (var i=0; i < len; i++){
        var length = UInt32 (letters.length)
        var rand = arc4random_uniform(length)
        randomString.appendFormat("%C",letters.characterAtIndex(Int(rand)))
    }
 
    return randomString
}

Swift 3.0更新

func randomString(length: Int) -> String {
 
    let letters : NSString = "abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789"
    let len = UInt32(letters.length)
 
    var randomString = ""
 
    for _ in 0 ..< length {
        let rand = arc4random_uniform(len)
        var nextChar = letters.character(at: Int(rand))
        randomString += NSString(characters: &nextChar,length: 1) as String
    }
 
    return randomString
}