Objective C, why declare method in the interface

You have to declare your methods in the interface (header) to make it public (accessible by/from other classes). If you're using a method internally in your class only, you don't have to declare it in the interface.

Objective-C is a highly dynamic language (and runtime) that defers the decision-making to as late as possible.

The code runs fine because at runtime the object does actually respond to the selector you've specified.

By declaring methods, however, you can help the Objective-C compiler to provide you with help in checking that you won't encounter any unexpected situations at runtime.

For example, take the following class:

@interface MDFoo : NSObject
- (void)blah;
@end


@implementation MDFoo

- (void)blah {
    NSLog(@"[%@ %@]", NSStringFromClass([self class]), NSStringFromSelector(_cmd));
}

- (void)boo {
    NSLog(@"[%@ %@]", NSStringFromClass([self class]), NSStringFromSelector(_cmd));
}

@end

Let's say that in another controller class, you create an instance of MDFoo and want to send it some messages.

MDFoo *foo = [[MDFoo alloc] init];
[foo blah];
[foo boo]; // compiler warning
[foo yell]; // compiler warning

When you compile the program, the compiler should give you a warning that both of the last two methods can't be found and that an instance of MDFoo may not respond to those selectors.

If you then run the program, the following is what's logged:

foo[10540:303] [MDFoo blah]
foo[10540:303] [MDFoo boo]
foo[10540:303] -[MDFoo yell]: unrecognized selector sent to instance 0x1001082f0
foo[10540:303] *** Terminating app due to uncaught exception
            'NSInvalidArgumentException', reason: '-[MDFoo yell]:
                 unrecognized selector sent to instance 0x1001082f0'

So at runtime the warning about boo turned out to be nothing, but the warning about yell was correct and caused the application to terminate.

From a design standpoint, it's also how you let other objects know what messages can be called. (See Objective-C Programming Language: Defining a Class - The Role of the Interface).