swift 4 size of string

sizeOfString.swift

extension String {

    var size: CGSize {
        return size()
    }

    public func size(usingFont font: UIFont?) -> CGSize {
        let f = font ?? UIFont.systemFont(ofSize: 16)
        let attributes = [NSAttributedStringKey.font: f]
        return self.size(withAttributes: attributes)
    }

}