Question

I am actually a newby at xcode. I can make out a few things be myself but have questions about what some things do and why they are put there. I have tried to read many e-books, watched tutorials, but they never go into the basics, alway just say “Add this, Click here etc”

Could someone give me some answers to a few questions please.
Ok, I know an ios app is mostly made out of Views, views are controlled by controllers. Each controller has a header (.h) file and a module?class? file (.m). The .h file contains the declarations of variables and functions used in the .m file.
The whole app is controlled by a master “controller” called the “delegate”.
Definitions in .h file may be for example an action IBAction or IBLabel or something.

What raises questions for me is for example these lines:

@class FlipsideViewController;

@protocol FlipsideViewControllerDelegate
- (void)flipsideViewControllerDidFinish:(FlipsideViewController *)controller;
@end

@interface FlipsideViewController : UIViewController

@property (nonatomic, assign) id <FlipsideViewControllerDelegate> delegate;

- (IBAction)done:(id)sender;

and why are sometimes in another view controller the delegate class loaded

@class MainViewController;

what does the following do, meaning what is the @interface declaration?

@interface flipAppDelegate : NSObject <UIApplicationDelegate>

what is

nonatomic, retain

sorry for asking really stupid questions, but every tutorial just skips these things.
I can follow a youtube video or a manual, but it doesn’t teach me a lot…

Answer 1

Let me try to answer your questions, one at a time.

what is the @interface declaration?

The interface declares a class. By declaring a class, I mean it specifies the instance variables and private/public methods that it contains. Again, the header file only contains the declaration of the methods, and the implementation/body of the methods lies in the module class. So, here-

@interface FlipsideViewController : UIViewController

The class FlipsideViewController derives from/subclasses/extends UIViewController. i.e Is a type of UIViewController but adds its own features.

Similarly

@interface flipAppDelegate : NSObject <UIApplicationDelegate>

Subclasses NSObject and implements the UIApplicationDelegate protocol. A protocol is essentially a set of methods that a class promises to implement (although there can be optional methods).

why are sometimes in another view controller the delegate class loaded

The delegate pattern allows a class to delegate its work to another class that implements the delegate protocol. So, here, FlipsideViewController keeps an instance of the delegate object, so that its flipsideViewControllerDidFinish: can be called.

what is nonatomic, retain

It means that when you set a value to your instance variable, the value’s refcount will be incremented and set to your variable. Also, it will not happen as an atomic operation. You need atomic only in a multi-threaded environment.

@synthesize is simply a shortcut to generate getters and setters for your variables.