The document discusses the Objective-C runtime and provides an overview of its main components and functions. It describes how the runtime works with classes, instances, methods, and message passing. It also covers topics like method resolution, forwarding, and method swizzling. Code examples are provided to illustrate concepts like forwarding messages and swizzling methods. URLs are included for additional Objective-C runtime references and documentation.

1. The Objective-c Runtime
Павел Альбицкий
p.albitsky@gmail.com

2. Open source
libobjc.dylib
http://www.opensource.apple.com/source/objc
4/objc4-680/

3. Functions
class_getName
class_getSuperclass
class_setSuperclass
class_isMetaClass
class_getInstanceSize
class_getInstanceVariable
class_getClassVariable
class_addIvar
class_copyIvarList
class_getIvarLayout
class_setIvarLayout
class_getWeakIvarLayout
class_setWeakIvarLayout
class_getProperty
class_copyPropertyList
class_addMethod
class_getInstanceMethod
class_getClassMethod
class_copyMethodList
class_replaceMethod
class_getMethodImplementation
class_getMethodImplementation_stret
class_respondsToSelector
class_addProtocol
class_addProperty
class_replaceProperty
class_conformsToProtocol
class_copyProtocolList
class_getVersion
class_setVersion
objc_getFutureClass
objc_setFutureClass
Working with Classes

4. Adding Classes
objc_allocateClassPair
objc_disposeClassPair
objc_registerClassPair
objc_duplicateClass
Instantiating Classes
class_createInstance
objc_constructInstance
objc_destructInstance
Working with Instances
object_copy
object_dispose
object_setInstanceVariable
object_getInstanceVariable
object_getIndexedIvars
object_getIvar
object_setIvar
object_getClassName
object_getClass
object_setClass
Obtaining Class Definitions
objc_getClassList
objc_copyClassList
objc_lookUpClass
objc_getClass
objc_getRequiredClass
objc_getMetaClass
Working with Instance Variables
ivar_getName
ivar_getTypeEncoding
ivar_getOffset
Associative References
objc_setAssociatedObject
objc_getAssociatedObject
objc_removeAssociatedObjects

5. NSObject.mm
+ (BOOL)isKindOfClass:(Class)cls {
for (Class tcls = object_getClass((id)self); tcls; tcls = tcls->superclass) {
if (tcls == cls) return YES;
}
return NO;
}
- (BOOL)respondsToSelector:(SEL)sel {
if (!sel) return NO;
return class_respondsToSelector_inst([self class], sel, self);
}
- (BOOL)conformsToProtocol:(Protocol *)protocol {
if (!protocol) return NO;
for (Class tcls = [self class]; tcls; tcls = tcls->superclass) {
if (class_conformsToProtocol(tcls, protocol)) return YES;
}
return NO;
}
- (id)performSelector:(SEL)sel {
if (!sel) [self doesNotRecognizeSelector:sel];
return ((id(*)(id, SEL))objc_msgSend)(self, sel);
}

6. NSObject
@interface NSObject <NSObject> {
Class isa;
}
typedef struct objc_class *Class;
struct objc_class {
Class isa;
#if !__OBJC2__
Class super_class
const char *name
long version
long info
long instance_size
struct objc_ivar_list *ivars
struct objc_method_list **methodLists
struct objc_cache *cache
struct objc_protocol_list *protocols
#endif
};

7. NSObject
typedef struct objc_object *id;
typedef struct objc_selector *SEL;
typedef struct objc_method *Method;
typedef struct objc_ivar *Ivar;
typedef struct objc_category *Category;
typedef struct objc_property *objc_property_t;
typedef struct objc_object Protocol;

8. Class hierarchy
@interface BaseExample : NSObject {
NSString *_ivarBase;
}
@property(nonatomic, strong) NSString *baseProperty;
- (void)test;
+ (NSString *)baseClassMethod;
@end
#import "BaseExample.h"
@interface Example : BaseExample {
NSString *_ivarExample;
}
@property(nonatomic, strong) NSString *exampleProperty;
- (void)test;
- (NSString *)execWithParam:(NSString *)param;
+ (NSString *)exampleClassMethod;
@end

9. Instance of Example Class Example Metaclass Example
isa isa
Class BaseExample
isa
Class NSObject
isa
Metaclass
BaseExample
Metaclass NSObject
super_class
super_class
super_class
super_class
Class hierarchy

10. struct objc_method {
SEL method_name; // signature of method
char *method_types; // types of the parameters to the method
IMP method_imp; // memory address of the start of code block
};
typedef struct objc_method *Method;
A selector is the name used to select a method to execute for an object, or the
unique identifier that replaces the name when the source code is compiled.
Selector – C-string;
IMP - typedef id (*IMP)(id self,SEL _cmd,...);
Method

11. NSString *result = [example execWithParam:@"param"];
id objc_msgSend(id self, SEL op, ...)
result = objc_msgSend(example, @selector(execWithParam:), @"param");
Tail-call: typedef id (*IMP)(id self,SEL _cmd,...);
void objc_msgSend(void /* id self, SEL op, ... */ )
NSString *result = ((NSString* (*)(id, SEL, NSString *))objc_msgSend)(example,
@selector(execWithParam:), @"param");
Sending Messages

12. id objc_msgSend(id self, SEL op, ...)
Sends a message with a simple return value to an instance of a class.
double objc_msgSend_fpret(id self, SEL op, ...)
Sends a message with a floating-point return value to an instance of a class.
void objc_msgSend_stret(void * stretAddr, id theReceiver, SEL theSelector, ...)
Sends a message with a data-structure return value to an instance of a class.
id objc_msgSendSuper(struct objc_super *super, SEL op, ...)
Sends a message with a simple return value to the superclass of an instance of a class.
void objc_msgSendSuper_stret(struct objc_super *super, SEL op, ...) Sends a message with a
data-structure return value to the superclass of an instance of a class.
objc_msgSend

13. Message forwarding
resolveInstanceMethod
forwardingTargetForSelector
forwardInvocation
Message handled
Message
not handled
Return NO
Return YES
Return nil
Return
replacement
receiver

14. resolveInstanceMethod
1. Class cache -> class dispatch table -> all super classes
2. resolveInstanceMethod for @dynamic properties
id dynamicGetter(id self, SEL _cmd);
void dynamicSetter(id self, SEL _cmd, id value);
+ (BOOL)isDynamicProperty:(NSString *)selectorString;
+ (BOOL)resolveInstanceMethod:(SEL)sel {
NSString *selectorString = NSStringFromSelector(sel);
if ([self isDynamicProperty:selectorString]) {
if ([selectorString hasPrefix:@"set"]) {
class_addMethod(self, sel, (IMP)dynamicSetter, "v@:@");
} else {
class_addMethod(sel, sel, (IMP)dynamicGetter, "@@:");
}
return YES;
}
return [super resolveInstanceMethod:sel];
}
// "v@:@", "@@:" - Objective-C type encodings

15. forwardingTargetForSelector
3. The Runtime then calls forwardingTargetForSelector
Returns the object to which unrecognized messages should first be directed.
- (id)forwardingTargetForSelector:(SEL)aSelector {
if (aSelector == @selector(magicMethod:)) {
return magicObject;
} else {
return [super forwardingTargetForSelector:aSelector];
}
}

16. forwardInvocation
@implementation InvClass
- (NSString *)invWithFirst:(NSString *)first second:(NSNumber *)second
third:(NSArray *)third {
return [NSString stringWithFormat:@"first: %@, second: %@, third: %@", first, second,
third];
}
@end
// perform unrecognized selector
Example *example = [Example new];
SEL selector = @selector(invWithFirst:);
NSString *result = [example performSelector:selector
withObject:@"first"];

17. forwardInvocation
+ (BOOL)resolveInstanceMethod:(SEL)sel {
if (sel == @selector(invWithFirst:)) {
return NO;
}
return YES;
}
- (id)forwardingTargetForSelector:(SEL)aSelector {
if (aSelector == @selector(invWithFirst:)) {
return nil;
}
return [super forwardingTargetForSelector:aSelector];
}
- (NSMethodSignature *)methodSignatureForSelector:(SEL)aSelector {
NSMethodSignature* signature = [super methodSignatureForSelector:aSelector];
if (!signature) {
SEL customSelector = @selector(invWithFirst:second:third:);
signature = [_invObject methodSignatureForSelector:customSelector];
}
return signature;
}

18. forwardInvocation
- (void)forwardInvocation:(NSInvocation *)anInvocation {
SEL selector = anInvocation.selector;
if ([_invObject respondsToSelector:selector]) {
[anInvocation setTarget:_invObject];
SEL customSelector = @selector(invWithFirst:second:third:);
[anInvocation setSelector:customSelector];
NSString *first = @"first";
NSNumber *second = @2;
NSArray *third = @[@”3", @”33”, @”333"];
[anInvocation setArgument:&first atIndex:2];
[anInvocation setArgument:&second atIndex:3];
[anInvocation setArgument:&third atIndex:4];
[anInvocation invoke];
} else {
[self doesNotRecognizeSelector:selector];
}
}

19. Method Swizzling

20. #import <objc/runtime.h>
@import Crashlytics;
@implementation UIViewController (Tracking)
+ (void)load {
static dispatch_once_t onceToken;
dispatch_once(&onceToken, ^{
Class class = [self class];
SEL originalSelector = @selector(viewWillAppear:);
SEL swizzledSelector = @selector(swizzled_viewWillAppear:);
Method originalMethod = class_getInstanceMethod(class, originalSelector);
Method swizzledMethod = class_getInstanceMethod(class, swizzledSelector);
BOOL didAddMethod =
class_addMethod(class,
originalSelector,
method_getImplementation(swizzledMethod),
method_getTypeEncoding(swizzledMethod));
if (didAddMethod) {
class_replaceMethod(class,
swizzledSelector,
method_getImplementation(originalMethod),
method_getTypeEncoding(originalMethod));
} else {
method_exchangeImplementations(originalMethod, swizzledMethod);
}
});
}
- (void)swizzled_viewWillAppear:(BOOL)animated {
CLS_LOG(@"CLSLOG viewWillAppear: %@", self);
[self swizzled_viewWillAppear:animated];
}
@end

21. URLs
• Mike Ash
• Objective-C Runtime Reference
• Objective-C Runtime Programming Guide
• opensource.apple.com

22. Спасибо за внимание!
Павел Альбицкий
p.albitsky@gmail.com