Notice can be understood as the change property before or after.

willAndDidSet.swift

struct Product {

    let cost: Int
    var profit = 0
    var count = 0

    init(cost: Int, price: Int) {
        self.cost = cost
        self.price = price
    }

    var price: Int {
        willSet {
            count += 1
            profit = newValue - cost
        } didSet {
            print("\(oldValue) => \(price)")
        }
    }
}

var 🍊 = Product(cost: 100, price: 200)
🍊.price = 400
🍊.price = 300
🍊.price = 500

// 3 times change price. Profit is 400.
print("\(🍊.count) times change price. Profit is \(🍊.profit).")