The document discusses various techniques for working with objects and classes in Objective-C at runtime, including swizzling methods, introspecting properties, dynamically creating classes, and building a basic implementation of KVO. Code examples and demos are provided for each technique. Potential risks and limitations are also outlined, such as the dangers of automatic parsing with property introspection and the esoteric nature of dynamic class creation.

1. Objec&ve(Superman

2. $ whoami
iOS$Champion$@$wondermall.com

3. Looks%familiar?
// NSMutableDictionary *counters, id key
if (!counters[key]) {
counters[key] = @1;
} else {
NSUInteger i = [counters[key] integerValue];
counters[key] = @(i + 1);
}
NSInteger count = [counters[key] integerValue];

4. We#can#do#be*er!
// NSCountedSet *counters, id key
[counters addObject:key];
NSUInteger count = [counters countForObject:key];

5. When%the%scheiße%hits%the%
fan
• How%o&en%did%you%use%
NSCountedSet?
• Know%what%is%there%at%your%disposal.
• !%hits%the%fan%=%"

6. Swizzling

7. The$way$to$swizzle
As#done#by#those#fine#gentleman:
• Mike&Ash:&"Method&Replacement&for&
Fun&and&Profit"1
• Jonathan&"Wolf"&Rentzsch:&JRSwizzle2
2
"h$ps://github.com/rentzsch/jrswizzle
1
"h$ps://www.mikeash.com/pyblog/friday9qa920109019299method9
replacement9for9fun9and9profit.html

8. The$way$to$swizzle
@implementation UIViewController (EveryDayImSwizzling)
+ (void)load {
[UIViewController jr_swizzleMethod:@selector(viewDidLoad)
withMethod:@selector(xyz_viewDidLoad)
error:nil];
}
- (void)xyz_viewDidLoad {
NSLog(@"I was here");
[self xyz_viewDidLoad];
}
- (void)viewDidLoad {
NSLog(@"!");
}
@end

9. The$right$way$to$swizzle3
• Each&method&has&implicit&id&&&_cmd
• Read&about&other&people&suffering4
4
"h$p://petersteinberger.com/blog/2014/a9story9about9swizzling9the9
right9way9and9touch9forwarding
3
"h$p://blog.newrelic.com/2014/04/16/right:way:to:swizzle/

10. Demo%!

11. Swizzling()ps
• Prefix:(xyz_
• Calling(original(method:([self
xyz_swizzleMe]
• If(swizzling(ini9alisers,(hint(ARC(
seman9cs(by(adding(
__attribute__((objc_method_fa
mily(init)))5
• The(right(way(to(swizzle:(func9ons(or(
blocks,(remember(_cmd.
5
"h$p://clang.llvm.org/docs/A$ributeReference.html#objc>method>family

12. Proper&es(inspec&on6
6
"Ivars,"too."Remember"ivars?

13. Proper&es(introspec&on:(
cool(stuff.
• List&all&the&proper.es:&name&/&type.
• Type,&even&if&nil&or&incorrect&type.
• Accessors&(readonly,&copy,&
getter=isHidden…).
• Applica.ons:&automa.c&parsing,&
valida.on,&IOC7
&etc.
7
"h$ps://github.com/tomersh/AppleGuice

14. Proper&es(introspec&on:(
Mantle8
• Reduces(amount(of(boilerplate
• Serialize(to(JSON,(CoreData,(XML(!
• Deserialize(dates,(enums,(URLs(etc.
• ValidaBon.
• NSCopying(and(NSCoding.
8
"h$ps://github.com/Mantle/Mantle

15. Demo%!

16. Proper&es(introspec&on(
&ps
• Does&not&include&proper0es&from&
superclasses,&while5loop&that&class&
chain!
• Automa0c&parsing&is&dangerous&(just&
like&self5driving&cars).
• Assigning&through&KVC,&always&check&
for&readonly.
• Did&you&call&me&lazy?

17. Dynamic(class(crea-on

18. Dynamic(class(crea-on
• Just&like&a&regular&subclassing.
• Leave&no&trace:&class&does&not&exist&in&
assembly.
• Dynamically&assemble&class&on&the&fly.
• Simulate&mul>ple&inheritance.

19. Demo%!

20. Dynamic(class(crea-on:(
-ps
• UIKit'&'Founda/on'use'it'twice.'Twice!
• Seems'like'a'very'esoteric'exercise.
• You'want'to'know'it's'there'when'you'
need'it.
• When'do'you'need'it?

21. Let's&build&KVO

22. Let's&build&KVO
• We$want$to$know$when$any$given$
property$is$set
• What$a$coincidence,$we$just$learned$
everything$we$need!
• Create$a$custom$subclass.
• Swizzle$se@er$for$specified$property.
• Set$our$subclass$as$observed$object$
class.

23. Demo%!

24. Let's&build&KVO
• Curiosity
• KVO-for-ge1er
• KVO-for-collec4ons
• Inject-side-effects-into-se1ers-and-
ge1ers-of-all-subclasses-of-a-certain-
class,-e.g.-createdAt,-updatedAt,-
lastAccessedAt

25. Conclusion
• <objc/runtime.h>"!="App"Store.
• I"say,"test"it,"Mantle"is"a"produc7on"
grade"code.
• Swizzling"to"patch"UIKit"gotchas!9
• Objec7veDC"run7me"is"its"own"
kryptonite.
• "Wax"on,"wax"off""–"it"comes"together,"
when"you"need"it.
9
"h$p://petersteinberger.com

26. One$more$thing...

27. Unicode(2015:(Diversity

28. S"ll$no$black$cat$emoji$!

29. Thank&you
• This&talk&and&demos&github.com/zats/Presenta9ons
• Objec9ve=C&run9me&opensource.apple.com
• twiAer.com/zats
• github.com/zats
• 9nder.com/zats