Reader Submissions - New Year's 2013

In celebration of the forthcoming year++, I thought it’d be fun to compile a list of some of your favorite tips and tricks of the trade–to give y’all a chance to show off some of your NSHipster cred.

Thanks to Cédric Luthi, Jason Kozemczak, Jeff Kelley, Joel Parsons, Maximilian Tagher, Rob Mayoff, Vadim Shpakovski, & @alextud for answering the call with excellent submissions.

Associated Objects in Categories

This first tip is so nice it was mentioned twice, both by Jason Kozemczak & Jeff Kelley.

Categories are a well-known feature of Objective-C, allowing new methods to be added to existing classes. Much less well known is that with some objc runtime hacking, you can add new properties as well. Observe!

NSObject+IndieBandName.h

@interface NSObject (IndieBandName)
@property (nonatomic, strong) NSString *indieBandName;
@end

NSObject+IndieBandName.m

#import "NSObject+Extension.h"
#import <objc/runtime.h>

static const void *IndieBandNameKey = &IndieBandNameKey;

@implementation NSObject (IndieBandName)
@dynamic indieBandName;

- (NSString *)indieBandName {
    return objc_getAssociatedObject(self, IndieBandNameKey);
}

- (void)setIndieBandName:(NSString *)indieBandName {
    objc_setAssociatedObject(self, IndieBandNameKey, indieBandName, OBJC_ASSOCIATION_RETAIN_NONATOMIC);
}

@end

This way, all of your objects can store and retrieve the name of their band, which–by the way–is performing this Wednesday night, and you should totally come.

While this is a cool trick and all, it should only be used as a method of last resort. Before you go this route, ask yourself if a particular property can’t either be derived from existing values, or should be managed by another class.

A good example of an associated object is how AFNetworking adds a property for an image request operation in its UIImageView category.

LLDB View Hierarchy Dump

Rob Mayoff responded with an obscure and powerful incantation to make debugging views a delight. Create .lldbinit in your home directory, if it doesn’t already exist, and add the following:

~/.lldbinit

command regex rd 's/^[[:space:]]*$/po [[[UIApplication sharedApplication] keyWindow] recursiveDescription]/' 's/^(.+)$/po [%1 recursiveDescription]/'

Now you can get a recursive hierarchy of any view in your iOS application with the LLDB debugger. You can try this for yourself by setting a breakpoint in a view controller, and type rd self.view. You may be surprised by what’s under the hood with some of the built-in UI controls!

LLDB Print Contents of a CGPathRef

While we’re on the subject of LLDB, Rob Mayoff sent in a useful incantation for printing out the contents of a CGPathRef from the debugger:

p (void)CGPathPrint(pathRef, 0)

If you’re doing any kind of complex Core Graphics drawing, be sure to keep this one handy.

Use +initialize, Not +load

Vadim Shpakovski wrote in with some advice about class loading and initialization. There are two magical class methods in Objective-C: +load and +initialize, which are automatically called by virtue of the class being used. The difference between the two methods, however, has significant performance implications for your application.

Mike Ash has a great explanation of this:

+load is invoked as the class is actually loaded, if it implements the method. This happens very early on. If you implement +load in an application or in a framework that an application links to, +load will run before main(). If you implement +load in a loadable bundle, then it runs during the bundle loading process.

The +initialize method is invoked in a more sane environment and is usually a better place to put code than +load. +initialize is interesting because it’s invoked lazily and may not be invoked at all. When a class first loads, +initialize is not called. When a message is sent to a class, the runtime first checks to see if +initialize has been called yet. If not, it calls it before proceeding with the message send.

tl;dr: Implement +initialize, not +load, if you need this automatic behavior.

Xcode Snippets

Maximilian Tagher gave a shout-out to the benefits of Xcode Snippets.

Great developers take pride in knowing their tools, and being able to use them to maximum effect. For better or for worse, this means knowing Xcode like the back of our hand. Verbose as Objective-C is, “do more by typing less” rings especially true as a productivity mantra, and Xcode Snippets are one of the best ways to do this.

If you’re looking for a place to start, try downloading and forking these Xcode Snippets.

Macro for Measuring Execution Time

Here’s a helpful macro for easily measuring the elapsed time for executing a particular block of code, sent in from @alextud:

NS_INLINE void MVComputeTimeWithNameAndBlock(const char *caller, void (^block)()) {
    CFTimeInterval startTimeInterval = CACurrentMediaTime();
    block();
    CFTimeInterval nowTimeInterval = CACurrentMediaTime();
    NSLog(@"%s - Time Running is: %f", caller, nowTimeInterval - startTimeInterval);
}

#define MVComputeTime(...) MVComputeTimeWithNameAndBlock(__PRETTY_FUNCTION__, (__VA_ARGS__))

Block Enumeration Methods

Joel Parsons submitted a great tip about using -enumerateObjectsWithOptions:usingBlock: in NSArray and other collection classes. By passing the NSEnumerationConcurrent option, you can get significant performance benefits over NSFastEnumeration’s for...in-style enumeration by executing the block concurrently.

However, be warned! Not all enumerations lend themselves to concurrent execution, so don’t go around replacing all of your for...in blocks with NSEnumerationConcurrent willy-nilly, unless random crashing is something you like in an app.

Reverse-Engineered Implementation of NSString Equality Methods

Displaying his characteristic brilliance and familiarity of Cocoa internals Cédric Luthi submitted a reverse-engineered implementation of the NString equality methods. Fascinating!

Animate NSLayoutConstraint.constant

This one goes out to all you fans of Cocoa Auto Layout, from Vadim Shpakovski:

viewConstraint.constant = <#Constant Value From#>;
[view layoutIfNeeded];

viewConstraint.constant = <#Constant Value To#>;
[view setNeedsUpdateConstraints];

[UIView animateWithDuration:ConstantAnimationDuration animations:^{
     [view layoutIfNeeded];
}];

Attentive readers may have already noted this, but the code above would make an excellent Xcode Snippet, by the way.

Printing NSCache Usage

Finishing up this batch of tips and tricks is Cédric Luthi again, this time unearthing the private method cache_print as a way to get some visibility into NSCache:

extern void cache_print(void *cache);

- (void) printCache:(NSCache *)cache {
    cache_print(*((void **)(__bridge void *)cache + 3));
}

This code sample has only been tested on iOS, and should only be used for debugging (i.e. take this out before submitting to Apple!).

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Thanks again to everyone for their submissions this time around. We’ll definitely be doing this again, so feel free to send your favorite piece of Objective-C trivia, framework arcana, hidden Xcode feature, or anything else you think is cool to @NSHipster!

And thank you, dear reader, for your support of NSHipster over these last wonderful months. We have a lot of insanely great things planned for NSHipster in 2013, and we look forward to being able to share it all with all of you.