BadgeBar.swift

import UIKit

public class BadgeBarButtonItem: UIBarButtonItem {
  @IBInspectable
  public var badgeNumber: Int = 0 {
    didSet {
      self.updateBadge()
    }
  }
  
  @IBInspectable
  public var maxCount: Int = 99
  
  @IBInspectable
  public var badgeBackgroundColor: UIColor = .red
  
  @IBInspectable
  public var badgeTextColor: UIColor = .white
  
  private let label: UILabel
  private let badgeSize: CGSize = CGSize(width: 16, height: 16)
  private let viewString: String = "view"
  
  required public init?(coder aDecoder: NSCoder) {
    let label = UILabel()
    label.backgroundColor = badgeBackgroundColor
    label.alpha = 0.9
    label.layer.cornerRadius = badgeSize.width / 2
    label.clipsToBounds = true
    label.isUserInteractionEnabled = false
    label.translatesAutoresizingMaskIntoConstraints = false
    label.textAlignment = .center
    label.textColor = badgeTextColor
    label.layer.zPosition = 1
    label.font = UIFont.systemFont(ofSize: 11)
    self.label = label
    
    super.init(coder: aDecoder)
    
    self.addObserver(self, forKeyPath: viewString, options: [], context: nil)
  }
  
  override public func observeValue(forKeyPath keyPath: String?, of object: Any?, change: [NSKeyValueChangeKey : Any]?, context: UnsafeMutableRawPointer?) {
    self.updateBadge()
  }
  
  private func updateBadge() {
    guard let view = self.value(forKey: viewString) as? UIView else { return }
    
    self.label.text = badgeNumber > maxCount ? "\(maxCount)+" : "\(badgeNumber)"
    
    if self.badgeNumber > 0 && self.label.superview == nil {
      view.addSubview(self.label)
      
      let height = NSLayoutConstraint(item: label, attribute: .height, relatedBy: .equal, toItem: nil, attribute: .notAnAttribute, multiplier: 1.0, constant: badgeSize.height)
      let width = NSLayoutConstraint(item: label, attribute: .width, relatedBy: .greaterThanOrEqual, toItem: nil, attribute: .notAnAttribute, multiplier: 1.0, constant: badgeSize.width)
      let centerX = NSLayoutConstraint(item: label, attribute: .centerX, relatedBy: .equal, toItem: view, attribute: .centerX, multiplier: 1.0, constant: -10)
      let centerY = NSLayoutConstraint(item: label, attribute: .centerY, relatedBy: .equal, toItem: view, attribute: .centerY, multiplier: 1.0, constant: -12)
      
      NSLayoutConstraint.activate([height, width, centerX, centerY])
    } else if self.badgeNumber == 0 && self.label.superview != nil {
      self.label.removeFromSuperview()
    }
  }
  
  deinit {
    self.removeObserver(self, forKeyPath: viewString)
  }
}