Property Observers

struct ScoreTracker {
    var score: Int = 0 {
        didSet {
            print("Score changed from \(oldValue) to \(score)")
        }
    }
}
var tracker = ScoreTracker()
tracker.score = 10
tracker.score = 25

struct Thermostat {
    var temperature: Int = 70 {
        willSet {
            print("About to change from \(temperature) to \(newValue)")
        }
    }
}
var thermostat = Thermostat()
thermostat.temperature = 75

1. Trying to add willSet/didSet to a computed property; observers only work on stored properties (or overridden inherited properties).
2. Forgetting didSet receives the OLD value as oldValue, while willSet receives the upcoming NEW value as newValue.
3. Setting the property again inside its own didSet without a guard condition, risking infinite recursion in more complex cases.

• willSet runs just before a stored property's value changes; didSet runs just after.
• Inside willSet, the implicit parameter newValue holds the value about to be set.
• Inside didSet, the implicit parameter oldValue holds the previous value.
• Property observers only apply to stored properties, not computed ones.