Which properties can you add `willSet` / `didSet` observers to?

Answer: You can add observers to stored properties you define, and to inherited stored or computed properties by overriding them in a subclass.

Property kind	Can add observers?
Stored var you define	✅ Yes
Inherited stored property (via override)	✅ Yes
Inherited computed property (via override)	✅ Yes
Computed property you define	❌ No — put the logic in its set
let constant	❌ No — it never changes

Code Example:

class Vehicle {
    var currentSpeed = 0.0
    var description: String { "traveling at \(currentSpeed) mph" }
}

class Car: Vehicle {
    var gear = 1

    // Observing an inherited stored property
    override var currentSpeed: Double {
        didSet {
            gear = Int(currentSpeed / 10) + 1
        }
    }
}

let car = Car()
car.currentSpeed = 35
print(car.gear) // 4

Key Points:

• An overriding property observer cannot also provide its own setter — choose one or the other.
• You cannot add observers to an inherited read-only computed property, since it is never set.
• The superclass's own observers still run; the subclass's observers run around them.