[Swift] iOS awesome Coupon Code TextField

CouponCodeTextField.swift

//
//  CouponCodeTextField.swift
//
//  Created by 홍경표 on 2021/06/10.
//

import UIKit

// Not necessary. Replacable with other ways.
import SnapKit // convenient to handle constraint
import Then // convenient to set properties right after initialized
// ------------

final class CouponCodeTextField: UITextField {
    
    private enum Constants {
        static let sectionSpacing: Double = 0.02 // proportional
        static let cursorBlinkInterval: Double = 0.4
        static let blinkColor: UIColor = .green // I think black is better, though. :)
        static let filledLineColor: UIColor = .black
        static let emptyLineColor: UIColor = .lightGray
        static let textColor: UIColor = .gray
    }

    let slotCount: Int
    let sectionCount: Int
    
    private var sectionSizes: [Int] {
        var temp = [Int](repeating: (slotCount / sectionCount), count: sectionCount)
        temp[sectionCount - 1] += slotCount % sectionCount
        return temp
    }
    private var digitLabels: [UILabel] = []
    private var bottomLines: [UIView] = []
    private var cursorColorFlag: Bool = true
    
    private weak var timer: Timer?
    
    init(slotCount: Int, sectionCount: Int = 0) {
        self.slotCount = slotCount
        self.sectionCount = sectionCount
        super.init(frame: .zero)
        
        timer = Timer.scheduledTimer(withTimeInterval: Constants.cursorBlinkInterval, repeats: true, block: { [weak self] timer in
            guard let self = self,
                  let currentIndex = self.text?.count,
                  currentIndex < self.slotCount else { return }
            self.bottomLines[currentIndex].backgroundColor = self.cursorColorFlag ? Constants.blinkColor : Constants.emptyLineColor
            self.cursorColorFlag.toggle()
        })
        
        configure()
    }
    required init?(coder: NSCoder) {
        fatalError("NO WAY")
    }
    deinit {
        timer?.invalidate()
        timer = nil
    }
    
    private func configure() {
        self.tintColor = .clear
        self.textColor = .clear
        self.textAlignment = .center
        self.keyboardType = .asciiCapable
        self.autocapitalizationType = .allCharacters
        
        self.addTarget(self, action: #selector(textDidChange), for: .editingChanged)
        
        let tapRecognizer = UITapGestureRecognizer()
        tapRecognizer.addTarget(self, action: #selector(becomeFirstResponder))
        self.addGestureRecognizer(tapRecognizer)
        
        let container = UIStackView().then {
            $0.axis = .horizontal
            $0.distribution = .equalSpacing
            $0.alignment = .fill
        }
        sectionSizes.forEach { [unowned self] sectionSize in
            let sectionStackView = UIStackView().then {
                $0.axis = .horizontal
                $0.distribution = .fillEqually
                $0.alignment = .fill
            }
            for _ in 0..<sectionSize {
                let label = UILabel().then {
                    $0.textAlignment = .center
                    $0.font = .systemFont(ofSize: UIFont.preferredFont(forTextStyle: .title3).pointSize, weight: .bold)
                    $0.adjustsFontSizeToFitWidth = true
                    $0.textColor = Constants.textColor
                    $0.text = " "
                }
                let bottomLine = UIView()
                bottomLine.backgroundColor = Constants.emptyLineColor
                label.addSubview(bottomLine)
                bottomLine.snp.makeConstraints {
                    $0.height.equalTo(2)
                    $0.leading.trailing.bottom.equalToSuperview()
                }
                
                sectionStackView.addArrangedSubview(label)
                
                digitLabels.append(label)
                bottomLines.append(bottomLine)
            }
            container.addArrangedSubview(sectionStackView)
            sectionStackView.snp.makeConstraints {
                $0.width.equalToSuperview().multipliedBy(Double(sectionSize) / Double(slotCount) - Constants.sectionSpacing)
            }
        }
        
        self.addSubview(container)
        container.snp.makeConstraints {
            $0.edges.equalToSuperview()
        }
        
    }
    
    @objc private func textDidChange() {
        text = text?.uppercased()
            .filter {
                $0.isNumber || $0.isUppercase
            }
            .prefix(slotCount).description
        
        guard let text = text else { return }
        
        var iter = text.makeIterator()
        for i in 0..<slotCount {
            if let c = iter.next() {
                digitLabels[i].text = String(c)
            } else {
                digitLabels[i].text = " "
            }
        }
        
        bottomLines[0..<text.count].forEach {
            $0.backgroundColor = Constants.filledLineColor
        }
        if text.count < slotCount {
            bottomLines[text.count..<slotCount].forEach {
                $0.backgroundColor = Constants.emptyLineColor
            }
        }
    }
    
}