stringByTrimmingCharactersInSetというエラーが直りません

Swiftを学習していて、ファイルをダウンロードし、起動する際にエラーが多発してしまい、原因はswift2で書かれているためでしてので、Swift4に書き換えているのですが、stringByTrimmingCharactersInSet（下記のコードの（１）の部分）という書き換えがどうしてもできずお力を借りれたらと思います。
一応、

if let text = optionalTitle,
            case let text.trimmingCharacters(in characterSet.whitespace).isEmpty == false

のように書き換えてみたのですがエラーが多発し、直らなかったのでご存知の方いましたらお教えください。

import UIKit

typealias ActionButtonItemAction = (ActionButtonItem) -> Void

public class ActionButtonItem: NSObject {

var action: ActionButtonItemAction?
var view: UIView!
var text: String {
    get {
        return self.label.text!
    }

    set {
        self.label.text = newValue
    }
}

private var label: UILabel!
private var button: UIButton!
private var image: UIImage!
private var labelBackground: UIView!
private let viewSize = CGSize(width: 200, height: 35)
private let buttonSize = CGSize(width: 35, height: 35)
private let backgroundInset = CGSize(width: 10, height: 10)

public init(title optionalTitle: String?, image: UIImage?) {
    super.init()

    self.view = UIView(frame: CGRect(origin: CGPoint(x: 0, y: 0), size: self.viewSize))
    self.view.alpha = 0
    self.view.isUserInteractionEnabled = true
    self.view.backgroundColor = UIColor.clear

    self.button = UIButton(type: .custom) as UIButton
    self.button.frame = CGRect(origin: CGPoint(x: self.viewSize.width - self.buttonSize.width, y: 0), size: buttonSize)
    self.button.layer.shadowOpacity = 1
    self.button.layer.shadowRadius = 2
    self.button.layer.shadowOffset = CGSize(width: 1, height: 1)
    self.button.layer.shadowColor = UIColor.gray.cgColor
    self.button.addTarget(self, action: Selector(("buttonPressed:")), for: .touchUpInside)

    if let unwrappedImage = image {
        self.button.setImage(unwrappedImage, for: .normal)
    }

    (1)if let text = optionalTitle where text.stringByTrimmingCharactersInSet(NSCharacterSet.whitespaceCharacterSet()).isEmpty == false {
        self.label = UILabel()
        self.label.font = UIFont(name: "HelveticaNeue-Medium", size: 13)
        self.label.textColor = UIColor.darkGrayColor()
        self.label.textAlignment = .Right
        self.label.text = text
        self.label.addGestureRecognizer(UITapGestureRecognizer(target: self, action: Selector("labelTapped:")))
        self.label.sizeToFit()

        self.labelBackground = UIView()
        self.labelBackground.frame = self.label.frame
        self.labelBackground.backgroundColor = UIColor.whiteColor()
        self.labelBackground.layer.cornerRadius = 3
        self.labelBackground.layer.shadowOpacity = 0.8
        self.labelBackground.layer.shadowOffset = CGSize(width: 0, height: 1)
        self.labelBackground.layer.shadowRadius = 0.2
        self.labelBackground.layer.shadowColor = UIColor.lightGrayColor().CGColor

        // Adjust the label's background inset
        self.labelBackground.frame.size.width = self.label.frame.size.width + backgroundInset.width
        self.labelBackground.frame.size.height = self.label.frame.size.height + backgroundInset.height
        self.label.frame.origin.x = self.label.frame.origin.x + backgroundInset.width / 2
        self.label.frame.origin.y = self.label.frame.origin.y + backgroundInset.height / 2

        // Adjust label's background position
        self.labelBackground.frame.origin.x = CGFloat(130 - self.label.frame.size.width)
        self.labelBackground.center.y = self.view.center.y
        self.labelBackground.addSubview(self.label)

        // Add Tap Gestures Recognizer
        let tap = UITapGestureRecognizer(target: self, action: Selector("labelTapped:"))
        self.view.addGestureRecognizer(tap)

        self.view.addSubview(self.labelBackground)
    }

    self.view.addSubview(self.button)
}

// MARK: - Button Action Methods

func buttonPressed(sender: UIButton) {
    if let unwrappedAction = self.action {
        unwrappedAction(self)
    }
}

// MARK: - Gesture Recognizer Methods
func labelTapped(gesture: UIGestureRecognizer) {
    if let unwrappedAction = self.action {
        unwrappedAction(self)
    }
}

}