Have trailing text persist in a text field.

PostFixTextField.swift

import UIKit

class PostFixTextField: UITextField {
    @IBInspectable var postfixColor: UIColor?
    @IBInspectable var shouldRemovePostfixOnEditing: Bool = false
    @IBInspectable var postfixText: String = "" {
        willSet {
            removePostFix()
        }
        didSet {
            textWasChanged()
        }
    }

    private var shouldUpdateWithPostfix: Bool = false
    private var originalTextColor: UIColor!

    var originalText: String? {
        return self.text?.replacingOccurrences(of: postfixText, with: "")
    }
    var originalTextCount: Int {
        return originalText?.count ?? 0
    }

    func setText(_ newText: String?) {
        guard let text = newText,
            !text.isEmpty else {
                self.text = nil
                return
        }
        self.text = text
        addPostFix()
        attributeText()
    }

    override func awakeFromNib() {
        super.awakeFromNib()

        self.originalTextColor = textColor ?? .black
        self.addTarget(self, action: #selector(textWillChange), for: .editingDidBegin)
        self.addTarget(self, action: #selector(textChanged), for: .editingChanged)
        self.addTarget(self, action: #selector(textWasChanged), for: .editingDidEnd)
    }

    @objc func textWillChange() {
        if shouldRemovePostfixOnEditing {
            removePostFix()
            textColor = originalTextColor
        } else {
            setCursorPosition(input: self, position: originalTextCount)
        }
    }

    @objc func textChanged() {
        if !shouldRemovePostfixOnEditing {
            textWasChanged()
            setCursorPosition(input: self, position: originalTextCount)
        }
    }

    @objc func textWasChanged() {
        guard originalTextCount > 0 else {
            self.text = nil
            return
        }
        removePostFix()
        addPostFix()
        attributeText()
    }

    private func setCursorPosition(input: UITextField, position: Int) {
        guard let position = input.position(from: input.beginningOfDocument, offset: position) else {
            return
        }
        input.selectedTextRange = input.textRange(from: position, to: position)
    }

    private func addPostFix() {
        guard let text = self.text,
            !text.isEmpty else {
                return
        }
        self.text = text + postfixText
    }

    private func removePostFix() {
        self.text = originalText
    }

    private func attributeText() {
        guard originalTextCount > 0,
            let originalText = originalText,
            let postfixColor = postfixColor else {
                return
        }
        let originalTextAttributes = [NSAttributedStringKey.foregroundColor: originalTextColor ?? .black]
        let originalTextRange = NSRange(location: 0, length: originalText.count)
        let postfixAttributes = [NSAttributedStringKey.foregroundColor: postfixColor]
        let postfixRange = NSRange(location: originalText.count, length: postfixText.count)

        let attributedString = NSMutableAttributedString(string: originalText + postfixText)
        attributedString.setAttributes(originalTextAttributes, range: originalTextRange)
        attributedString.setAttributes(postfixAttributes, range: postfixRange)

        self.attributedText = attributedString
    }

}