error with Xcode 6 'Duplicate interface definition for class 'ViewController'

Question

I'm programming an app following a basic tutorial from 2012 using Xcode 6. The tutorial was made using Xcode 4.3 and I'm sure I have followed it exactly as I have double checked by watching through the problem areas. I'm quite new to this type of programming as I usually deal with game development and robots but have done a little before.

error:

Duplicate interface definition for class 'ViewController'

This is the code:

 #import "ViewController.h"

 @interface ViewController // First error here.

 @end

 @implementation ViewController

 - (void)viewDidLoad
{
    [super viewDidLoad];
    // Do any additional setup after loading the view, typically from a nib.
}

   -(void) presentMessage:(NSString *)message {
       UIAlertView *alert = [[UIAlertView alloc] initWithTitle:@"My Title"         message: message delegate: nil cancelButtonTitle:@"Ok" otherButtonTitles: nil ];

[alert show];
[alert release]; // second error.
}

-(void) scheduleLocalNotificationWithDate:(NSDate *)fireDate {
UILocalNotification *notification = [[UILocalNotification alloc] init];

...    

[notification release]; // third error
}

-(IBAction) buttonTapped:(id)sender {

    dateFormatter...

[dateFormatter release]; // fourth error
 }
   @end

Sorry about the weird formatting but I couldn't get this to format into code.

Thank you in advance

Answer 1

You need to add () to your @interface ViewController line.

@interface ViewController()

This is called a private category in iOS, and it's used to define private methods and properties in your implementation file.

In your .h file, you'll find the interface declared as @interface ViewController, and that's why the compiler thinks you're declaring it twice. Using a private category (@interface ViewController()) tells the compiler that you're actually extending the functionality of your already defined interface (called ViewController), adding private methods and properties.