Objective-C Runtime overview

Objective C Runtime
Khoa Pham
2359Media

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

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

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

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

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

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

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

@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

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.

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.

Objective C Runtime
Dynamic feature
Object oriented capability

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

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

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;
}

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;

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

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

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

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

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;

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

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

Objective C Runtime
Introspection
isKindOfClass
respondsToSelector
conformsToProtocol

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

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

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

Use cases
- (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
}

Use cases
updateWithDictionary
class_copyIvarList
ivar_getName

Use cases
@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”}];

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

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

Objective-C Runtime overview

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