Introduction to Objective-C Runtime and its benefits

1. Objective C Runtime
Khoa Pham
2359Media

2. Today
● self = [super init]
● objc_msgSend
● ObjC Runtime
● How to use the Runtime API
● Use cases

3. self = [super init]
ETPAnimal *cat = [ETPAnimal cat];
NSInteger recordCount = [ETPCoreDataManager
recordCount];

4. self = [super init]
@interface ETPCat : ETPAnimal
@end
ETPCat *cat = [[ETPCat alloc] init]

5. self = [super init]
- (id)init
{
self = [super init];
if (self) {
// ETPCat does it own initialization here
}
return self;
}

6. self = [super init]
[super init] calls the superclass implementation of init with
the (hidden) self argument.
It can do one of these things
+ set some properties on self, and return self
+ return a different object (factory, singleton)
+ return nil

7. self = [super init]
ETPCat *cat = [ETPCat alloc] // 0x1111111a
[cat init] // 0x1111111b
[cat meomeo] // 0x1111111a

8. self = [super init]
Demo

9. objc_msgSend
ETPCat *cat = [[ETPCat alloc] init]
[cat setName:@”meo”]
objc_msgSend(cat, @selector(setName:), @”meo”)

10. objc_msgSend
Demo

11. objc_msgSend

12. @selector
SEL selector1 = @selector(initWithName:)
SEL selector2 = @selector(initWithFriends1Name::)
typedef struct objc_selector *SEL
Read more at Objective C Runtime Reference -> Data
Structure -> Class definition Data structure -> SEL

13. @selector
Demo

14. Objective C Runtime
The Objective-C Runtime is a Runtime Library, it's a library
written mainly in C & Assembler that adds the Object
Oriented capabilities to C to create Objective-C.

15. Objective C Runtime
Source code http://www.opensource.apple.
com/source/objc4/objc4-532/runtime/objc-class.mm
There are two versions of the Objective-C runtime—
“modern” and “legacy”. The modern version was introduced
with Objective-C 2.0 and includes a number of new
features.

16. Objective C Runtime
Dynamic feature
Object oriented capability

17. Objective C Runtime
Features
● Class elements (categories, methods, variables,
property, …)
● Object
● Messaging
● Object introspection

18. Objective C Runtime
@interface ETPAnimal : NSObject
@end
typedef struct objc_class *Class;

19. Objective C Runtime (old)
struct objc_class {
Class isa;
Class super_class OBJC2_UNAVAILABLE;
const char *name OBJC2_UNAVAILABLE;
long version OBJC2_UNAVAILABLE;
long info OBJC2_UNAVAILABLE;
long instance_size OBJC2_UNAVAILABLE;
struct objc_ivar_list *ivars OBJC2_UNAVAILABLE;
struct objc_method_list **methodLists OBJC2_UNAVAILABLE;
struct objc_cache *cache OBJC2_UNAVAILABLE;
struct objc_protocol_list *protocols OBJC2_UNAVAILABLE;
}

20. Objective C Runtime
typedef struct class_ro_t
{
const char * name;
const ivar_list_t * ivars;
} class_ro_t
typedef struct class_rw_t
{
const class_ro_t *ro;
method_list_t **methods;
struct class_t *firstSubclass;
struct class_t *nextSiblingClass;
} class_rw_t;

21. Objective C Runtime
ETPAnimal *animal = [[ETPAnimal alloc] init]
struct objc_object
{
Class isa;
// variables
};

22. Objective C Runtime
id someAnimal = [[ETPAnimal alloc] init]
typedef struct objc_object
{
Class isa;
} *id;

23. Objective C Runtime
Class is also an object, its isa pointer points to its meta
class
The metaclass is the description of the class object

24. Objective C Runtime

26. Objective C Runtime
Demo

27. Objective C Runtime
● Dynamic typing
● Dynamic binding
● Dynamic method resolution
● Introspection

28. Objective C Runtime
Dynamic typing
Dynamic typing enables the runtime to determine the type
of an object at runtime
id cat = [[ETPCat alloc] init]
- (void)acceptAnything:(id)anything;

29. Objective C Runtime
Dynamic binding
Dynamic binding is the process of mapping a message to a
method at runtime, rather than at compile time

30. Objective C Runtime
Dynamic method resolution
Provide the implementation of a method dynamically.
@dynamic

31. Objective C Runtime
Introspection
isKindOfClass
respondsToSelector
conformsToProtocol

32. How to use the Runtime API
Objective-C programs interact with the runtime system to
implement the dynamic features of the language.
● Objective-C source code
● Foundation Framework NSObject methods
● Runtime library API

33. Use cases
Method swizzle (IIViewDeckController)
JSON Model (Torin ‘s BaseModel)
Message forwarding
Meta programming

34. Use cases
Method swizzle

35. Use cases
Method swizzle (IIViewDeckController)
SEL presentVC = @selector(presentViewController:animated:completion:);
SEL vdcPresentVC = @selector(vdc_presentViewController:animated:completion:);
method_exchangeImplementations(class_getInstanceMethod(self, presentVC),
class_getInstanceMethod(self, vdcPresentVC));

36. Use cases
Method swizzle (IIViewDeckController)
- (void)vdc_presentViewController:(UIViewController *)viewControllerToPresent animated:(BOOL)
animated completion:(void (^)(void))completion {
UIViewController* controller = self.viewDeckController ?: self;
[controller vdc_presentViewController:viewControllerToPresent animated:animated completion:
completion]; // when we get here, the vdc_ method is actually the old, real method
}

37. Use cases
JSON Model (Torin ‘s BaseModel)
updateWithDictionary
class_copyIvarList
ivar_getName

38. Use cases
JSON Model (Torin ‘s BaseModel)
@interface ETPItem : BaseModel
@property (nonatomic, copy) NSString * ID;
@property (nonatomic, copy) NSString *name;
@end
ETPItem *item = [[ETPItem alloc] init];
[item updateWithDictionary:@{@”ID”: @”1”, @”name”: @”item1”}];

39. Message forwarding

40. Use cases
Meta programming
● Dynamic method naming
● Validation
● Template
● Mocking

41. Reference
1. http://cocoasamurai.blogspot.com/2010/01/understanding-objective-c-runtime.html
2. http://www.slideshare.net/mudphone/what-makes-objective-c-dynamic
3. http://www.cocoawithlove.com/2009/04/what-does-it-mean-when-you-assign-super.html
4. http://nshipster.com/method-swizzling/
5. http://stackoverflow.com/questions/415452/object-orientation-in-c
6. http://stackoverflow.com/questions/2766233/what-is-the-c-runtime-library
7. http://gcc.gnu.org/onlinedocs/gcc/Modern-GNU-Objective-C-runtime-API.html
8. http://www.opensource.apple.com/source/objc4/objc4-532/runtime/objc-class.mm
9. http://www.friday.com/bbum/2009/12/18/objc_msgsend-part-1-the-road-map/
10. https://www.mikeash.com/pyblog/friday-qa-2010-01-29-method-replacement-for-fun-and-profit.html
11. Pro Objective C, chapter 7, 8, 9
12. Effective Objective C, chapter 2
13. http://wiki.gnustep.org/index.php/ObjC2_FAQ

42. Thank you
Q&A