Why can I refer to UIKit classes after removing the import of UIKit.h?

Question

I've been watching a video that stated that UIAlertView works only if UIKit.h has been imported. However, if I comment out the import statement in the header file:

//#import <UIKit/UIKit.h>
@interface ViewController : UIViewController
@end

the alert still works when I add it to the implementation file:

- (void)viewDidLoad
{
    [super viewDidLoad];
    UIAlertView *alert = [[UIAlertView alloc] initWithTitle:@"Title" message:@"Message" delegate:self cancelButtonTitle:@"OK" otherButtonTitles:nil, nil];
[alert show];
}

Please explain why this works? What is the true role of UIKit?

Answer 1

It's because it's probably already declared in your Prefix.pch file, which looks something like this by default:

#ifdef __OBJC__
    #import <Foundation/Foundation.h>
    #import <UIKit/UIKit.h>
#endif

Answer 2

UIKit.h is just the file where the classes etc. are defined.

Without it, the compiler wouldn't know what UIAlertView is.

However, in your case it probably works anyway, as the #import <UIKit/UIKit.h> is usually included in your project's prefix file, which is included by default in your sources.