import UIKit

class PropertyObserver {
    var totalScore: Int = 0 {
        willSet {
            print("willSet: \(totalScore)")
        }

        didSet {
            print("didSet: \(totalScore)")
        }

    }

}

let propertyObserver = PropertyObserver()
propertyObserver.totalScore = 100

ผลลัพท์
//willSet: 0
//didSet: 100