didSet

didSet is a property observer in Swift that is called immediately after a value has been set to a property. It is part of a pair of property observers, the other being willSet. didSet is invoked every time the property's value is changed, even if the new value is the same as the old value.

When didSet is called, it provides access to the old value of the property. This can be useful for various tasks such as updating UI elements, performing calculations based on the previous state, or triggering other actions. The old value is available as a constant named oldValue within the didSet closure.

To use didSet, you declare it within the property's definition, similar to how you would declare a computed property. For example, you can observe changes to a variable named myProperty by writing:

var myProperty: Int = 0 {

didSet {

print("myProperty changed from \(oldValue) to \(myProperty)")

}

}

In this example, whenever myProperty is assigned a new value, the closure associated with didSet will execute, printing the old and new values to the console.

It is important to note that didSet is not called when a property is initialized. It is only triggered by subsequent assignments. Furthermore, if a property observer calls itself, an infinite loop can occur. Swift prevents this by ensuring that property observers are not called during initialization. When a property's value is set within a property observer, it does not trigger another call to that same observer.