StatusDot.swift

import UIKit

@IBDesignable
final class StatusDot: UIView {
    
    convenience init() {
        self.init(frame: .zero)
    }
    
    override init(frame: CGRect) {
        super.init(frame: frame)
        setup()
    }
    
    required init?(coder aDecoder: NSCoder) {
        super.init(coder: aDecoder)
        setup()
    }
    
}

// MARK: - View Lifecylce

extension StatusDot {
    
    override func layoutSubviews() {
        super.layoutSubviews()
        cornerRadius = bounds.width / 2.0
    }
    
}

// MARK: - Setup

fileprivate extension StatusDot {
    
    func setup() {
        backgroundColor = .radicalRed
        forceLayoutUpdate()
    }
    
}

UITabBarController+Badge.swift

import UIKit

extension UITabBarController {
    
    func showBadge(_ show: Bool = true, atIndex index: Int) {
        switch (isShowingBadge(atIndex: index), show) {
        case (true, true): return
        case (true, false): removeBadge(fromIndex: index)
        case (false, true): addBadge(atIndex: index)
        case (false, false): return
        }
    }
    
}


// MARK: - ViewController Lifecycle 

extension UITabBarController {
    
    open override func viewDidLayoutSubviews() {
        super.viewDidLayoutSubviews()
        tabBar.forceLayoutUpdate()
        layoutBadges()
    }
    
}

// MARK: - Layout

fileprivate extension UITabBarController {
    
    var barButtonFrames: [Int : CGRect] {
        let barButtonClassName = "UITabBarButton"
        let imageClassName = "UITabBarSwappableImageView"
        return tabBar.subviews
            .filter { String(describing: type(of: $0)) == barButtonClassName }
            .sorted { (prev, next) in
                return prev.frame.origin.x < next.frame.origin.x
            }
            .map { (button) -> CGRect in
                let imageViewFrame = button.subviews
                    .filter { String(describing: type(of: $0)) == imageClassName }
                    .first!.frame
                
                return button.convert(imageViewFrame, to: tabBar)
            }
            .enumerated()
            .reduce([:]) { (dict, enumerate) -> [Int : CGRect] in
                var dict = dict
                dict[enumerate.offset] = enumerate.element
                return dict
        }
    }
    
    var badges: [Int : StatusDot] {
        return tabBar.subviews
            .flatMap { $0 as? StatusDot }
            .reduce([:]) { (dict, dot) -> [Int : StatusDot] in
                var dict = dict
                dict[dot.tag] = dot
                return dict
        }
    }
    
    func isShowingBadge(atIndex index: Int) -> Bool {
        return badges[index].exists
    }
    
    func addBadge(atIndex index: Int) {
        let badge = StatusDot(frame: CGRect(x: 0, y: 0, width: 16, height: 16))
        badge.tag = index
        tabBar.addSubview(badge)
        layout(badge, atIndex: index)
        
        animateVisibilityOfBadge(badge, toVisible: true)
    }
    
    func removeBadge(fromIndex index: Int) {
        guard let badge = badges[index] else { return }
        animateVisibilityOfBadge(badge, toVisible: false)
    }
    
    func layoutBadges() {
        badges.forEach { layout($0.1, atIndex: $0.0) }
    }
    
    func layout(_ badge: StatusDot, atIndex index: Int) {
        guard let frame = barButtonFrames[index] else {
            removeBadge(fromIndex: index)
            return
        }
        
        let correction = CGPoint(x: -2, y: 4)
        let center = CGPoint(
            x: frame.maxX + correction.x,
            y: frame.origin.y + correction.y
        )
        
        badge.center = center
    }

    func animateVisibilityOfBadge(_ badge: UIView, toVisible isVisible: Bool) {
        let zeroScale = CATransform3DMakeScale(0, 0, 1)
        let from = isVisible ? zeroScale : CATransform3DIdentity
        let to = isVisible ? CATransform3DIdentity : zeroScale
        
        badge.layer.transform = from
        badge.layer.animate(
            .transform(from: from, to: to),
            with: SpringTraits(damping: 14, stiffness: 150),
            completion: {
                if !isVisible { badge.removeFromSuperview() }
            }
        )
    }
    
}