If you have done any development with UIKit you have used a life cycle function. These UIKit life cycle functions would be viewDidLoad, viewWillAppear, viewWillDisappear, and viewDidDisappear. In SwiftUI we lose these life cycle functions because there are no more ViewControllers in SwiftUI, only views. SwiftUI replaces these functions with two methods called onAppear( ) and onDisappear( ).
onAppear( ): is a function that runs every time a view appears on screen. The onAppear( ) function is similar to the viewWillAppear function used in UIKit. A good example of how we can use the onAppear( ) function is for any network calls we would need for that view.
onDisappear( ): is a function that runs when a view disappears from our apps screen. This function is similar to the viewWillDisappear function in UIKit. A example for when to use the onDisappear( ) function is for clearing any data before dismissing that view.
Thanks for reading and I hope this helps in your learning of SwiftUI!
👨🏻💻 Happy Programming 👨🏻💻