How to use Property observers in Swift

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It’s a common pattern or technique in various programming languages when we want to perform some action when a value is changed. Property observers are used in Swift when we want to perform an action when the value of a property changes. Even if the new value is similar to the old value, property observers will respond every time they are called.

Property observers can be defined in two ways:

willSet: It runs just before the property’s new value is about to be set.

didSet: It runs immediately after the property’s new value has been set.

In our code, we can use both willSet and didSet observers or only one of them. willSet is rarely used in our code, but didSet is very common in the Swift programming language.

To demonstrate how property observers function, we implement a struct Player with two stored properties in this example.

struct Player {
  var level: Int
  var score: Int {
    willSet {
      print("New score : \(newValue), current score: \(score)")
    }
    didSet {
      print("Previous score: \(oldValue), current score: \(score)")
    }
  }
}
var player = Player(level: 1, score: 0)
player.score = 20
player.score = 40