UITextField that automatically formats a percentage as a user types. It limits the input, and can return a value without any string formatting.

PercentageField.swift

//
//  PercentageField.swift
//
//  Created by Khan Winter on 8/16/20.
//

import UIKit

class PercentageField: UITextField {

    public var value: Double {
        get {
            let max = String(string.numbers)
            let divider: Double = pow(Double(10), Double(2))
            return abs(Double(max.numbers.integer) / divider) / 100
        }
        set(newVal) {
            text = String(format: "%.2f", newVal * 100) + "%"
        }
    }
    
    override func awakeFromNib() {
        super.awakeFromNib()
        addTarget(self, action: #selector(editingChanged), for: .editingChanged)
        keyboardType = .numberPad
        text = "%"
        editingChanged()
    }
    
    convenience init() {
        self.init()
        addTarget(self, action: #selector(editingChanged), for: .editingChanged)
        keyboardType = .numberPad
        text = "%"
        editingChanged()
    }
    
    @objc func editingChanged() {
        if !string.contains("%") {
            text?.removeLast()
        }
        let max = String(string.numbers)
        let divider: Double = pow(Double(10), Double(2))
        text = String(format: "%.2f", abs(Double(max.numbers.integer) / divider)) + "%"
        self.value = abs(Double(max.numbers.integer) / divider) / 100
    }
}

extension UITextField {
    var string: String { return text ?? "" }
}

extension String {
    var numbers: String { return components(separatedBy: CharacterSet(charactersIn: "0123456789").inverted).joined() }
    var integer: Int { return Int(numbers) ?? 0 }
}