What are lazy properties and property observers?

A stored property can run code when it is set or read through observers and modifiers.

• willSet and didSet observe changes to a stored property. willSet receives the new value, didSet the old, and neither fires during initialisation.
• lazy defers creation until first access. It is useful for expensive setup that depends on other properties, and the property must be var.

class ViewModel {
    lazy var formatter = DateFormatter()

    var score = 0 {
        didSet {
            print("changed from \(oldValue) to \(score)")
        }
    }
}

Computed properties with get and set do not store a value and are recalculated on each access; a get-only computed property is read-only. Observers are not called when setting from within the initialiser or from a designated initialiser delegation.

Lazy properties are not thread-safe by themselves. Use observers for lightweight side effects, not heavy work.