What are AppDelegates in Objective-C?

Question

I'm working through an iPhone tutorial (link text and it has me put in some code (a few times throughout the various tutorials) but it doesn't explain it at all. In this code:

todoAppDelegate *appDelegate = (todoAppDelegate *)[[UIApplication sharedApplication] delegate];

What exactly is an appDelegate? What does the "delegate" at the end of the instantiation mean? Actually, what does the whole thing mean? (UIIapplication sharedApplication)?

I am a .Net programmer if that helps someone explain it better. I hate learning through tutorials because I always need to know what EVERYTHING does and no one explains everything.

Answer 1

Let's back up a little bit.

The square brackets ([ ]) are Objective-C's method calling syntax. So if Cocoa had a C# syntax, the equivalent syntax would be:

TodoAppDelegate appDelegate = UIApplication.sharedApplication.delegate;

In C#, you would use a static class for a class that only has a single instance. In Cocoa, the Singleton pattern is used to accomplish this. A class method (in this case, sharedApplication) is used to retrieve the single instance of that class.

Delegates in Cocoa are not like the delegate keyword in C#, so don't be confused by that. In C#, you use the delegate keyword to reference a method. The delegate pattern in Cocoa is provided as an alternative to subclassing.

Many objects allow you to specify another object as a delegate. Delegates implement methods that those objects will call to notify them of certain events. In this case, UIApplication is the class that represents the current running application (similar to System.Windows.Forms.Application, for example). It sends messages to its delegate when things that affect the application happen (e.g. when the application launches, quits, gains or loses focus, and so on.)

Another Objective-C concept is the protocol. It is similar in principle to a .NET interface, except that methods can be marked as @optional, meaning they classes are not required to implement the methods marked that way. Delegates in the iPhone SDK are simply objects that conform to a specific protocol. In the case of UIApplication, the protocol delegates must conform to is UIApplicationDelegate.

Because it's not required to implement every method, this gives the delegate flexibility to decide which methods are worth implementing. If you wanted to, for example, perform some actions when the application finishes launching, you can implement a class that conforms to the UIApplicationDelegate protocol, set it as the UIApplication instance's delegate, and then implement applicationDidFinishLaunching:.

UIApplication will determine if its delegate implements this method when the application finishes launching and, if it does, call that method. This gives you a chance to respond to this event without having to subclass UIApplication.

In iPhone applications, developers also frequently use the app delegate as a kind of top-level object. Since you don't usually subclass UIApplication, most developers keep their global application data in the app delegate.

Answer 2

A delegate is just an object that implements certain methods (basically callbacks). The NSApplication docs explain what its delegate is supposed to do and what messages it needs to respond to to.

And this isn't instantiation. The snippet you posted above doesn't create anything. It accesses whatever object is set as the application's delegate. [UIApplication sharedApplication] gets the object representing the application, and sending delegate to the application gets its delegate (if any).