Don’t you just hate when a small change in the innards of your view controller forces you to change its header file just to conform to a delegate protocol? For example, adding emailing functionality requires you to implement the MFMailComposeViewControllerDelegate protocol and @import <MessageUI/MessageUI.h>. Talk about breaking encapsulation…

Thankfully, you can do that in your .m implementation file instead. (Even though Apple sample code doesn’t.) All you need to do is use the empty category:

// MyViewController.m
#import "MyViewController.h"
#import 

@interface MyViewController ()
     // privately conform to protocol
@property (nonatomic, strong) UIView *somePrivateSubview;
@end

@implementation MyViewController
// synthesize and methods implementations
@end

The empty category MyViewController () allows you to define private ivars, properties, methods, and even protocols – all in the privacy of your .m file, transparent to your clients.