A quick deep dive into method swizzling, one of Objective-C's most powerful tricks.

1. Black magic and swizzling in Objective-C

2. Me
iOS developer since 2008—first app, NetSketch
was on the App Store the day it went live.
Published more than 20 apps, which have been
featured on the App Store, demoed on stage at
SXSW, covered by Macworld.
Currently growing Kodiak, a platform that makes it
easy for you to build great educational apps.

3. 5 minute agenda
Today, we’ll learn about Objective-C method
dispatching (and it’s beautiful simplicity.)
We’ll talk about Method Swizzling, a technique for
doing very bad things.
We’ll see an example of some really cool stuff you
can do with it.

4. Obj-C is a thin layer
Objective-C is a thin wrapper on top of C, and the
objc.h headers declare C functions that implement
method invocation, properties and more.
Objective-C calls can be translated into their C
counterparts:
[target selector];
objc_msgSend(target, @selector(selector));

5. Methods in tables
Each class in Objective-C has
a lookup table associating
method names with selectors
(which point to functions.)
Objective-C categories are
simple: they allow you to add
methods to the lookup table.
You can add methods to
classes that already exist!

6. Method Swizzling
Categories let you add things to the method
lookup table, which is cool and somewhat unique
to Objective-C.
But what if I want to replace a method?
You can’t. Thanks for listening.

7. But I can write C!
@implementation NSString (Category)
- (NSString)betterDescription {
return @"Haha I win.";
}
@end
origMethod = class_getInstanceMethod([NSString class], @selector(description:));
altMethod = class_getInstanceMethod([NSString class], @selector(betterDescription:));
IMP temp = origMethod->method_imp;
origMethod->method_imp = altMethod->method_imp;
altMethod->method_imp = temp;
// What do you think this will do?
NSLog([@”Hello World” description]);

8. But... why?
Reroute calls to methodA: to methodB: instead.
Generally, you make methodB: call through to
methodA:, so the default behavior still exists.
Useful for logging, overriding behaviors, stubbing
methods for unit testing.
Use with extreme care. You will break things badly.
HTTP://DARKDUST.NET/WRITINGS/OBJECTIVE-C/METHOD-SWIZZLING

9. One use case: Spark
The Spark Inspector has been a side project of
mine for about a year, and is finally going through
App Store review.
Swizzles dozens of methods on core classes like
UIView to provide an awesome debug view.
Demo time! https://sparkinspector.com/

10. Secret Sauce
So how did that work? We added a framework to
our app and ran a line of code. What did that code
do?
The Spark Inspector swizzles setFrame:,
setNeedsDisplay: (and lots more), and sends
information about changes to a Mac app.
The mac app uses this information to rebuild the
view hierarchy.

11. Takeaways
Objective-C is a beautifully simple language, and
you can do some really powerful things with it.
In general, if you find yourself wanting to swizzle
something, you’re doing it wrong. But there are
great reasons to break the rules sometimes.
I’d be happy to send out free copies of the Spark
Inspector app once it’s live. Follow it on Twitter
@sparkinspector, and me @bengotow

12. http://cocoadev.com/wiki/MethodSwizzling