24 May 2021 . 2min read       @gurjitpt

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 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

We want to print a statement before and after the value of the score property is changed in this situation. We use the willSet observer to print a new score value with the default name newValue and the current score.

Similarly, the didSet observer prints the previous score and the current score with the default name oldValue. Finally, we got the following result in the console, and we noticed that willSet and didSet observers are calling every time the score value changes.

New score : 20, current score: 0
Previous score: 0, current score: 20
New score : 40, current score: 20
Previous score: 20, current score: 40

You can set your own names in the willSet and didSet observers if you don't want to use the default names for oldValue and newValue to make your code cleaner and more readable.

var score: Int {
  willSet(newScore) {
    print("New score : \(newScore)")
  }
  didSet(previousScore) {
    print("Previous score: \(previousScore)")
  }
}

Conclusion

Property observers with willSet and didSet, offer a powerful way to monitor changes in the value of a property without having to write any additional code or abstraction.

Don’t hesitate to contact me if you have any questions or queries. Follow me on twitter @gurjitpt for any updates.

Thanks!

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