iOS: The singleton pattern

person.h

```
#import <Foundation/Foundation.h>

@interface Person : NSObject 

+ (id)sharedInstance;

@end
```

person.m

```
+ (id)sharedInstance {
    static Person *theSharedInstance = nil;
    static dispatch_once_t onceToken;
    dispatch_once(&onceToken, ^{
        theSharedInstance = [[self alloc] init];
    });
    return theSharedInstance;
}
```
