The document discusses techniques for better object initialization and nullability checking in Objective-C, as related to creating a class for brewing beer components. It introduces the NS_DESIGNATED_INITIALIZER, NS_UNAVAILABLE, and nullability annotations like nonnull. It shows how to formalize initialization, prevent invalid initialization, and catch null values through compile-time checking. Audited regions with NS_ASSUME_NONNULL_BEGIN/END are presented as a cleaner way to specify nullability expectations for a class.

1. Better Brewing with…
Sam Meadley
@sammeadley

2. Overview
• Macros
• NS_DESIGNATED_INITIALIZER
• NS_UNAVAILABLE
• NS_ASSUME_NONNULL_BEGIN/END
• Behaviour changes between Xcode 6 vs Xcode 7

3. The real overview
• We’re going to start a brewery. Right here, tonight.

4. Any brewers in the
house?

5. Brewing beer; a primer
• 4 fundamental ingredients (experimentation encouraged)
• Water (let’s make this easier)
• Hops
• Malt
• Yeast

6. Let’s start a brewery
@interface CBCBrewComponents : NSObject
@property (copy, nonatomic) NSString *identifier;
@property (copy, nonatomic) NSString *hopVariety;
@property (copy, nonatomic) NSString *maltVariety;
@property (copy, nonatomic) NSString *yeastStrain;
- (instancetype)initWithIdentifier:(NSString *)identifier
hopVariety:(NSString *)hopVariety
maltVariety:(NSString *)maltVariety
yeastStrain:(NSString *)yeastStrain NS_DESIGNATED_INITIALIZER;
@end

7. NS_DESIGNATED_INITIALIZER
• Formalises object initialisation
- (instancetype)initWithIdentifier:(NSString *)identifier
hopVariety:(NSString *)hopVariety
maltVariety:(NSString *)maltVariety
yeastStrain:(NSString *)yeastStrain
{
self = [super init];
if (self) {
_identifier = [identifier copy];
_hopVariety = [hopVariety copy];
_maltVariety = [maltVariety copy];
_yeastStrain = [yeastStrain copy];
}
return self;
}

8. NS_DESIGNATED_INITIALIZER
• Formalises object initialisation
- (instancetype)initWithIdentifier:(NSString *)identifier
hopVariety:(NSString *)hopVariety
maltVariety:(NSString *)maltVariety
yeastStrain:(NSString *)yeastStrain
{
self = [super init];
if (self) {
_identifier = [identifier copy];
_hopVariety = [hopVariety copy];
_maltVariety = [maltVariety copy];
_yeastStrain = [yeastStrain copy];
}
return self;
}
- (instancetype)initWithHopVariety:(NSString *)hopVariety
maltVariety:(NSString *)maltVariety
yeastStrain:(NSString *)yeastStrain
{
NSUUID *UUID = [NSUUID UUID];
return [self initWithIdentifier:UUID.UUIDString
hopVariety:hopVariety
maltVariety:maltVariety
yeastStrain:yeastStrain];
}

9. NS_DESIGNATED_INITIALIZER
• Formalises object initialisation
• Generates warnings if convenience initialisers do
not call the designated initialiser
• Communicates intent to other developers
• Even more warnings if you don’t override the
designated initialiser inherited from superclass
(more on that later)

10. Well that was easy…
• Let’s down tools, it’s brew time.
@interface CBCBrewDay : NSObject
- (instancetype)initWithDate:(NSDate *)date NS_DESIGNATED_INITIALIZER;
- (CBCBrew *)brewFromComponents:(CBCBrewComponents *)brewComponents;
@end

11. Let’s start with a Pale Ale
CBCBrewDay *brewDay = [[CBCBrewDay alloc] initWithDate:[NSDate date]];
CBCBrewComponents *brewComponents = [[CBCBrewComponents alloc] init];
brewComponents.identifier = @"Pale Ale";
brewComponents.hopVariety = @"Fuggles";
brewComponents.maltVariety = @"Maris Otter";
brewComponents.yeastStrain = @“WLP002";
CBCBrew *brew = [brewDay brewFromComponents:brewComponents];
Nailed it… Cheers! 🍻

12. Second time around…
• First batch won’t make it past the brewery gates,
better brew a second batch.
CBCBrewDay *brewDay = [[CBCBrewDay alloc] initWithDate:[NSDate date]];
CBCBrewComponents *brewComponents = [[CBCBrewComponents alloc] init];
CBCBrew *brew = [brewDay brewFromComponents:brewComponents];
Erm, something is wrong here

13. Second time around…
CBCBrewDay *brewDay = [[CBCBrewDay alloc] initWithDate:[NSDate date]];
CBCBrewComponents *brewComponents = [[CBCBrewComponents alloc] init];
brewComponents.identifier = @"Pale Ale";
brewComponents.hopVariety = @"Fuggles";
brewComponents.maltVariety = @"Maris Otter";
brewComponents.yeastStrain = @“WLP002";
CBCBrew *brew = [brewDay brewFromComponents:brewComponents];
Hell no, H20

14. What went wrong?
CBCBrewDay *brewDay = [[CBCBrewDay alloc] initWithDate:[NSDate date]];
CBCBrewComponents *brewComponents = [[CBCBrewComponents alloc] init];
brewComponents.identifier = @"Pale Ale";
brewComponents.hopVariety = @"Fuggles";
brewComponents.maltVariety = @"Maris Otter";
brewComponents.yeastStrain = @“WLP002";
CBCBrew *brew = [brewDay brewFromComponents:brewComponents];
• No error checking code in the brewFromComponents:
method to ensure that all the required components are set
• Callers can initialise CBCBrewComponents using the
default init inherited from NSObject

15. Better Brewing with
NS_UNAVAILABLE;
@interface CBCBrewComponents : NSObject
@property (copy, nonatomic) NSString *identifier;
@property (copy, nonatomic) NSString *hopVariety;
@property (copy, nonatomic) NSString *maltVariety;
@property (copy, nonatomic) NSString *yeastStrain;
- (instancetype)initWithIdentifier:(NSString *)identifier
hopVariety:(NSString *)hopVariety
maltVariety:(NSString *)maltVariety
yeastStrain:(NSString *)yeastStrain NS_DESIGNATED_INITIALIZER;
- (instancetype)init NS_UNAVAILABLE;
@end

16. Better Brewing with
NS_UNAVAILABLE;
CBCBrewDay *brewDay = [[CBCBrewDay alloc] initWithDate:[NSDate date]];
CBCBrewComponents *brewComponents = [[CBCBrewComponents alloc] initWithIdentifier:@"Pale Ale"
hopVariety:@"Fuggles"
maltVariety:@"Maris Otter"
yeastStrain:@"WLP002"];
CBCBrew *brew = [brewDay brewFromComponents:brewComponents];
• Communicate minimum valid object
• Compile time checking
• Earlier resolution, more beer for everyone

17. The dining philosophers
forgetful brewer problem
@interface CBCBrewComponents : NSObject
- (instancetype)initWithIdentifier:(NSString *)identifier
hopVariety:(NSString *)hopVariety
maltVariety:(NSString *)maltVariety
yeastStrain:(NSString *)yeastStrain NS_DESIGNATED_INITIALIZER;
- (instancetype)init NS_UNAVAILABLE;
@end
CBCBrewDay *brewDay = [[CBCBrewDay alloc] initWithDate:[NSDate date]];
CBCBrewComponents *brewComponents = [[CBCBrewComponents alloc] initWithIdentifier:@"Pale Ale"
hopVariety:nil
maltVariety:@"Maris Otter"
yeastStrain:@"WLP002"];
CBCBrew *brew = [brewDay brewFromComponents:brewComponents];
😭

18. Better Brewing with
nonnull
• Nullability type specifiers introduced in Xcode 6.3
• Properties
• Parameters
• Method return values
• and more…
• nonnull and nullable
• NS_ASSUME_NONNULL_BEGIN/NS_ASSUME_NONNULL_END

19. Better Brewing with
nonnull
@interface CBCBrewComponents : NSObject
@property (copy, nonatomic, nonnull) NSString *identifier;
@property (copy, nonatomic, nonnull) NSString *hopVariety;
@property (copy, nonatomic, nonnull) NSString *maltVariety;
@property (copy, nonatomic, nonnull) NSString *yeastStrain;
- (instancetype)initWithIdentifier:(nonnull NSString *)identifier
hopVariety:(nonnull NSString *)hopVariety
maltVariety:(nonnull NSString *)maltVariety
yeastStrain:(nonnull NSString *)yeastStrain NS_DESIGNATED_INITIALIZER;
- (instancetype)init NS_UNAVAILABLE;
@end

20. Better Brewing with
nonnull
@interface CBCBrewComponents : NSObject
@property (copy, nonatomic, nonnull) NSString *identifier;
@property (copy, nonatomic, nonnull) NSString *hopVariety;
@property (copy, nonatomic, nonnull) NSString *maltVariety;
@property (copy, nonatomic, nonnull) NSString *yeastStrain;
- (instancetype)initWithIdentifier:(nonnull NSString *)identifier
hopVariety:(nonnull NSString *)hopVariety
maltVariety:(nonnull NSString *)maltVariety
yeastStrain:(nonnull NSString *)yeastStrain NS_DESIGNATED_INITIALIZER;
- (instancetype)init NS_UNAVAILABLE;
@end

21. Better Brewing with
nonnull
• Nullability type specifiers should not be applied partially.
• Along with properties and parameters, they can also
applied to method return types.
• Xcode helps with this.

22. Forgetful Brewer revisited
@interface CBCBrewComponents : NSObject
@property (copy, nonatomic, nonnull) NSString *identifier;
@property (copy, nonatomic, nonnull) NSString *hopVariety;
@property (copy, nonatomic, nonnull) NSString *maltVariety;
@property (copy, nonatomic, nonnull) NSString *yeastStrain;
- (nonnull instancetype)initWithIdentifier:(nonnull NSString *)identifier
hopVariety:(nonnull NSString *)hopVariety
maltVariety:(nonnull NSString *)maltVariety
yeastStrain:(nonnull NSString *)yeastStrain NS_DESIGNATED_INITIALIZER;
- (nonnull instancetype)init NS_UNAVAILABLE;
@end
Whaddayasay Xcode?

23. Audited Regions
• That’s a whole lot of nonnull
• Audited Regions can help
• NS_ASSUME_NONNULL_BEGIN
• NS_ASSUME_NONNULL_END

24. Audited Regions
@interface CBCBrewComponents : NSObject
@property (copy, nonatomic, nonnull) NSString *identifier;
@property (copy, nonatomic, nonnull) NSString *hopVariety;
@property (copy, nonatomic, nonnull) NSString *maltVariety;
@property (copy, nonatomic, nonnull) NSString *yeastStrain;
- (nonnull instancetype)initWithIdentifier:(nonnull NSString *)identifier
hopVariety:(nonnull NSString *)hopVariety
maltVariety:(nonnull NSString *)maltVariety
yeastStrain:(nonnull NSString *)yeastStrain NS_DESIGNATED_INITIALIZER;
- (nonnull instancetype)init NS_UNAVAILABLE;
@end
Is equivalent to;
NS_ASSUME_NONNULL_BEGIN
@interface CBCBrewComponents : NSObject
@property (copy, nonatomic) NSString *identifier;
@property (copy, nonatomic) NSString *hopVariety;
@property (copy, nonatomic) NSString *maltVariety;
@property (copy, nonatomic) NSString *yeastStrain;
- (instancetype)initWithIdentifier:(NSString *)identifier
hopVariety:(NSString *)hopVariety
maltVariety:(NSString *)maltVariety
yeastStrain:(NSString *)yeastStrain NS_DESIGNATED_INITIALIZER;
- (instancetype)init NS_UNAVAILABLE;
@end
NS_ASSUME_NONNULL_END

25. Audited Regions
• You can mix and match nullability type specifier
within an audited region.
NS_ASSUME_NONNULL_BEGIN
@interface CBCBrewComponents : NSObject
@property (copy, nonatomic) NSString *identifier;
@property (copy, nonatomic, nullable) NSString *hopVariety;
@property (copy, nonatomic) NSString *maltVariety;
@property (copy, nonatomic) NSString *yeastStrain;
- (instancetype)initWithIdentifier:(NSString *)identifier
hopVariety:(NSString *)hopVariety
maltVariety:(NSString *)maltVariety
yeastStrain:(NSString *)yeastStrain NS_DESIGNATED_INITIALIZER;
- (instancetype)init NS_UNAVAILABLE;
@end
NS_ASSUME_NONNULL_END

26. Audited Regions
• Even with all this help. Be sure to still implement
error checking. The compiler doesn’t catch it all.
• Be sure to check your inputs are valid.
CBCBrewDay *brewDay = [[CBCBrewDay alloc] initWithDate:[NSDate date]];
NSString *hopVariety = nil;
CBCBrewComponents *brewComponents = [[CBCBrewComponents alloc] initWithIdentifier:@"Pale Ale"
hopVariety:hopVariety
maltVariety:@"Maris Otter"
yeastStrain:@"WLP002"];
CBCBrew *brew = [brewDay brewFromComponents:brewComponents];

27. Better Brewing with
nonnull
• Communicate correct usage of your class
• More awesome compile-time help
• Less bugs! (you’d hope, right?)

28. Changes in Xcode 7
(beta)

29. Adopting
NS_DESIGNATED_INITIALIZER
• Xcode 6.x
• add macro to designated initialiser
• Xcode 7 beta
• add macro to designated initialiser
• explicitly override superclass designated
initialiser(s)
• … even if marked unavailable

30. Adopting
NS_DESIGNATED_INITIALIZER
• Override inherited initialiser with some sensible
defaults (not like these…)
- (instancetype)init
{
return [self initWithIdentifier:[NSString string]
hopVariety:[NSString string]
maltVariety:[NSString string]
yeastStrain:[NSString string]];
}

31. Adopting
NS_DESIGNATED_INITIALIZER
• Even if superclass initialiser is marked
NS_UNAVAILABLE, Xcode still issues warning
• https://openradar.appspot.com/21302875
• Call doesNotRecognizeSelector and return nil
- (instancetype)init
{
[self doesNotRecognizeSelector:_cmd];
return nil;
}

32. Summary
• Use NS_DESIGNATED_INITIALIZER to formalise
object initialisation
• Use NS_UNAVAILABLE to hide inherited members
not relevant to your subclass
• Guard against nil inputs and communicate intent
with NS_ASSUME_NONNULL_BEGIN/END

33. Summary
• https://openradar.appspot.com/21302875
• https://developer.apple.com/library/mac/
documentation/General/Conceptual/
CocoaEncyclopedia/Initialization/Initialization.html
• craftbeercraftcode.com
• https://github.com/sammeadley/brewery

34. Cheers! 🍻