Relation between Singleton class & AppDelegate class in iOS Objective-C

Question

I have a variable declared in AppDelegate class(.h file) whose value gets changed from multiple ViewController classes.Also,single application-wide instance for my AppDelegate class is shared throughout my application as follows :

AppDelegate *AppD = (AppDelegate *)[[UIApplication sharedApplication] delegate];

As I could access this variable declared in AppDelegate from any ViewController class, is AppDelegate class as an example of Singleton class in this scenario?

Can anyone help to site out the usage of singleton class with real-life example ?

Answer 1

AppDelegate is however a singleton class but you show only use it for declaring things that applies globally in your application.
For ex: If you want to change the color of navigation bar in your entire application you can use app delegate and set the color of navigation bar. Also app delegate is an object that handles different state transition in your app.
So if you want to create a variable that can be changed from multiple View controllers you should create a singleton class and declare that variable in that class.

Answer 2

The app delegate is not supposed to be a repository for all kinds of global variables. The app delegate is supposed to be used for things that affect the whole of the application, like launch / app termination, entering the background and returning from the background, that kind of thing.

If there is state that is shared by multiple view controllers, that should exist only once, but doesn't affect the application as a whole, then you could consider creating a singleton for that state. Then again, global state that is just an artefact of how you write your code should be avoided.

Answer 3

AppDelegate can be used just like singleton, but I don't recommend it. It's like you can put all your classes declarations and definitions in a class.h and a class.m file. Simply import the class.h file can invoke all classes. But it will be very inconvenient to read, understand and manage.

AppDelegate is mainly used for all kinds of app itself event, through UIApplicationDelegate method. Do not recommend deal with too much logic about global data here. Such as classes named XXManager, XXService, PublicData, is proposed to manage all kinds of singleton data.