Main point of Application Delegate class

Question

What's the main point behind putting variables and method signatures inside ApplicationDelegate.h in Objective-C ? By doing this, all those methods and variables are seen by another view controller classes? Is that the point?

And also: is there only one application delegate class inside each project?

Williham Totland · Answer 1 · 2010-04-14T14:08:41.950

3

The Application Delegate (and yes, there is only one per application) is the backbone of the Controller layer in Cocoa and Cocoa Touch applications. Its main function is to customize the behaviour of NS/UIApplication in certain circumstances (such as application termination, application opening, etc.). It is also commonly used to assign delegates to other responders (such as table views, outline views, etc.).

In small applications, it will usually act as both delegate and dataSource to most interface elements, but as Felixyz points out, this can deteriorate quickly if the application grows.

edited Apr 14 '10 at 14:08

answered Apr 14 '10 at 13:45

Williham Totland

28,471
6
52
68

@ahmet732: what kind of comment is that? – Felixyz Apr 14 '10 at 13:58
1

@Felixyz he missed a '?', it's easier to understand when you imagine there are six. – Tim Snowhite Apr 14 '10 at 14:08
sorry for that :) ı wanted to ask "what about identifying some variables and methods inside app delegate?" Felixyz very thanks for your answer .. It was very undestandable.. – user310000 Apr 14 '10 at 18:07
Thanks for adding a definition of what an appDelegate's function is. – jroyce Nov 14 '15 at 22:14

Felixyz · Accepted Answer · 2010-06-06T04:03:17.300

"By doing this, all those methods and variables are seen by another view controller classes? Is that the point?"

Of course, any class that imports the app controller's header can access public properties and methods (and ivars), but that goes for any class, not just the app delegate.

It's pretty common to put a few important objects and methods that are needed all over the app in the app delegate. In that way, yes that's often the reason why you see variables and methods defined on the app delegate class.

This makes sense sometimes, but it can very quickly deteriorate into very bad design. You should make sure not to use the app delegate as some basket where you can just throw all shared state and functionality in the app. I'm working with some code where someone did this right now, and it's extremely hard to change and refactor functionality.

This is just a version of the well-know problems of using global state for everything. In short, you should analyze functionality and divide it into separate classes, or groups of classes. Try to keep your app delegate as slim as possible!

EDIT: ...and read Matt Gallagher's post on this issue.

Main point of Application Delegate class

2 Answers2

Linked