import UIKit

class ViewController: UIViewController {

    // Works programmatically or using the interface builder
    @IBOutlet weak var smartImageView: SmartImageView!
    
    fileprivate let image = UIImage(named: "Sample1")

    override func viewDidLoad() {
        super.viewDidLoad()
        smartImageView.delegate = self
        smartImageView.searchForFaces(on: image)
    }
}

import UIKit

@available(iOS 11.0, *)
@objc public protocol SmartImageViewDelegate: class {

    /// Asks the delegate for a view object to display in every face found.
    @objc optional func smartImageView(_ smartImageView: SmartImageView, viewForFaceIn frame: CGRect) -> UIView

    /// Sent after SmartImageView finishes searching for faces.
    @objc optional func smartImageView(_ smartImageView: SmartImageView, numberOfFacesFound: Int)

    /// Sent if an error was found at any time during the task.
    @objc optional func smartImageView(_ smartImageView: SmartImageView, encountered error: Error)

}

extension ViewController: SmartImageViewDelegate {

    func smartImageView(_ smartImageView: SmartImageView, numberOfFacesFound: Int) {
        print("Found: \(numberOfFacesFound) face(s)")
    }

    func smartImageView(_ smartImageView: SmartImageView, viewForFaceIn frame: CGRect) -> UIView {
        let customView = UIView(frame: frame)
        customView.backgroundColor = .yellow
        customView.alpha = 0.5
        return customView
    }

    func smartImageView(_ smartImageView: SmartImageView, encountered error: Error) {
        print(error.localizedDescription)
    }

}