環境

Items	Version
Xcode	8.2
Swift	3.0.2

定義

/// Int拡張
extension Int {
    /// カンマ付け
    var withComma: String {
        let decimalFormatter = DecimalFormatter()
        guard let s = decimalFormatter.string(from: self as NSNumber) else {
            fatalError()
        }
        return s
    }
}

/// String拡張
extension String {
    /// カンマ区切り数字のカンマどり
    var noComma: Int {
        if self.characters.count == 0 {
            return 0
        }
        let decimalFormatter = DecimalFormatter()
        guard let i = decimalFormatter.number(from: self) else {
            preconditionFailure("NumberFormatter.number method failure!")
        }
        return Int(i)
    }
}

/// カンマ編集制御用NumberFormatter拡張クラス
class DecimalFormatter: NumberFormatter {
    override init() {
        super.init()
        self.locale = Locale(identifier: "ja_JP")
        self.numberStyle = .decimal
    }
    
    required init?(coder aDecoder: NSCoder) {
        super.init(coder: aDecoder)
    }
}

呼び出し

let s: String = (1234567).withComma         //"1,234,567"
let i: Int = s.noComma                      //1234567
let i2: Int = "123,456x".noComma            //Error

（参考）ブラッシュアップ前

/// Int拡張
extension Int {
    /// カンマ付け
    var commmaDecStr: String {
        let decimalFormatter = NumberFormatter()
        decimalFormatter.numberStyle = NumberFormatter.Style.decimal
        decimalFormatter.groupingSeparator = ","
        decimalFormatter.groupingSize = 3
        
        return decimalFormatter.string(from: self as NSNumber)!
    }
}

/// String拡張
extension String {
    /// カンマ区切り数字のカンマどり
    var delCommmaFromDecStr: String {
        if self.characters.count > 0 {
            return self.replacingOccurrences(of: ",", with: "")
        } else {
            return "0"
        }
    }
}

I Love Swift!!