Presentation given to the MN iOS Developer Meetup in July 2012. Related Code Can Be Found Here: https://gist.github.com/3150536

1. the objective-c runtime
ryan johnson
ryan@mds.tc @_ryanjohnson_

2. enough about me,
lets talk about you.

3. what is objective-c?

4. what is objective-c?
objc = C + smalltalk

5. what is objective-c?
objc = C + smalltalk
objc = C + messaging + OOP

6. what is objective-c?
objc = C + smalltalk
objc = C + messaging + OOP
objective c is dynamic, and that’s as awesome as it sounds.

7. so, what’s the runtime?

8. so, what’s the runtime?
C and assembly library which adds oop

9. so, what’s the runtime?
C and assembly library which adds oop
allows objects to be aware of and manipulate their own state

10. so, what’s the runtime?
C and assembly library which adds oop
allows objects to be aware of and manipulate their own state
two runtimes: modern (we all use this) and legacy (32-bit)

11. definitions
• selector
@selector - name of the method
SEL - compiled selector, unique identifier that replaces the name
• invocation (IMP) - what actually performs the magic

12. before using anything
you learn here

13. before using anything
you learn here
ask yourself
is this the easiest, most elegant way to achieve my goals?
it’s probably not.
you don’t get a gold star for over engineering, and
no one is impressed by needlessly complicated code.

14. better ways to accomplish things

15. better ways to accomplish things
• categories

16. better ways to accomplish things
• categories
• subclasses

17. better ways to accomplish things
• categories
• subclasses
• protocols

18. better ways to accomplish things
• categories
• subclasses
• protocols
that’s great, but I was promised a talk about the runtime you jerk.

19. highway to the danger zone

20. highway to the danger zone
using the runtime functions isn’t going to get you rejected

21. highway to the danger zone
using the runtime functions isn’t going to get you rejected
but.....

22. highway to the danger zone
using the runtime functions isn’t going to get you rejected
but.....
using them the wrong way or to do prohibited things will

23. highway to the danger zone
using the runtime functions isn’t going to get you rejected
but.....
using them the wrong way or to do prohibited things will
you may be smart, but your idea to call a private method with
objc_msgSend isn’t

24. messaging
when you write:
[foo bar:variable];
the compiler turns it into:
objc_msg(foo, @selector(bar:),variable);

25. what objc_msgSend does
• finds the implementation, calling the procedure and returning the
value.
• also, passes hidden arguments:
self - the receiver of the message
_cmd - the @selector for the method

26. _
abusing objc_msgSend

27. _
abusing objc_msgSend
• calling private apis - useful for debugging

28. _
abusing objc_msgSend
• calling private apis - useful for debugging
• avoiding annoying ARC performSelector: warnings.

29. _
abusing objc_msgSend
• calling private apis - useful for debugging
• avoiding annoying ARC performSelector: warnings.
• looking bad ass

30. method swizziling
• in short, changing what the selector maps to
• useful when a single method needs replacement, and doesn’t warrant
subclassing

31. how and when to swizzle

32. how and when to swizzle
• when
customizing UIKit, but might get you rejected :(
mock objects for unit tests - (OCMock)

33. how and when to swizzle
• when
customizing UIKit, but might get you rejected :(
mock objects for unit tests - (OCMock)
• other options:
categories allow you to replace methods completely but can’t call
original implementation and not guaranteed your code will be called.

34. how and when to swizzle
• when
customizing UIKit, but might get you rejected :(
mock objects for unit tests - (OCMock)
• other options:
categories allow you to replace methods completely but can’t call
original implementation and not guaranteed your code will be called.
• dangers

35. associated objects

36. associated objects
great way to add instance variables to a class you don’t own or is
difficult to subclass

37. associated objects
great way to add instance variables to a class you don’t own or is
difficult to subclass
and, it’s arc friendly.

38. associated objects
great way to add instance variables to a class you don’t own or is
difficult to subclass
and, it’s arc friendly.

39. using associated objects

40. using associated objects
• setting
objc_setAssociatedObject(object, key, value, policy);
id object <- the target for the association
void * key <- the key(name) of the association - e.g. “type”
id value <- the object which will be associated with the target
objc_AssociationPolicy policy <- the memory management policy to use
options: OBJC_ASSOCIATION_ASSIGN, OBJC_ASSOCIATION_RETAIN_NONATOMIC,
OBJC_ASSOCIATION_COPY_NONATOMIC, OBJC_ASSOCIATION_RETAIN,
OBJC_ASSOCIATION_COPY

41. using associated objects
• setting
objc_setAssociatedObject(object, key, value, policy);
id object <- the target for the association
void * key <- the key(name) of the association - e.g. “type”
id value <- the object which will be associated with the target
objc_AssociationPolicy policy <- the memory management policy to use
options: OBJC_ASSOCIATION_ASSIGN, OBJC_ASSOCIATION_RETAIN_NONATOMIC,
OBJC_ASSOCIATION_COPY_NONATOMIC, OBJC_ASSOCIATION_RETAIN,
OBJC_ASSOCIATION_COPY
• getting
objc_getAssociatedObject(object, key);
id object <- source object for the association
void * key <- the key(name) of the association

42. learning is fun.
Slightly beyond our scope but check these out on your own:
• NSProxy - can create ‘placeholder’ objects
• NSInvocation - objectifies messages, used a lot by NSTimer

43. resources
• Objective-C Runtime Programing Guide
https://developer.apple.com/library/mac/#documentation/Cocoa/Conceptual/ObjCRuntimeGuide/
Introduction/Introduction.html#//apple_ref/doc/uid/TP40008048-CH1-SW1
• Mike Ash’s Blog
http://www.mikeash.com
• Code Samurai
http://cocoasamurai.blogspot.com/2010/01/understanding-objective-c-runtime.html