One-to-one communication initiated by the owner of the delegate.
We use delegation to define a set of APIs for one-to-one communication between two instances. In this case, we can conform to a protocol that will notify an object that the application finished launching.
public protocol UIApplicationDelegate : NSObjectProtocol {
optional func applicationDidFinishLaunching(_ application: UIApplication)
}Like delegates, they are a one-to-one communication where the owner of the property initiates the communication. Closures differ in that they can capture state, which makes implementation more convenient for the client.
Closures offer additional flexibility in the number of objects involved in communication. While each closure will be a one-to-one relationship, each can have a different client. Closures also provide access to state, which can be more convenient to clients. In this case, we can define a closure that can be set, which will be called when the application finished launching.
var applicationDidFinishLaunching: ((UIApplication) -> Void)?One-to-many communication where subscribers subscribe to a publisher. It is a one-way relationship.
Notifications are used to broadcast to any subscribers that are interested in events that a publisher advertises. In this case, a notification is defined that broadcasts that the application finished launching.
public class let didFinishLaunchingNotification: NSNotification.Name