TweetFollow Us on Twitter

Jun 02 Cocoa

Volume Number: 18 (2002)
Issue Number: 06
Column Tag: Cocoa Development

by Dan Wood, Alameda CA

The Beauty of Categories

Use This Objective-C Feature to Make Your Cocoa Code Cleaner

Ask any experienced Cocoa programmer what they like the most about Objective-C and the answer will invariably be “categories.” Categories is one of the features of Objective-C, not found in Java or C++, that raises the body temperature of developers if you suggest they use another language.

What is It, and Why Use It?

A category is an extension of an existing class. But unlike inheritance, in which you create a new class that descends from another class, a category is like a remora, attaching itself to the belly of a shark and getting a free ride. By creating a category, you add new methods to an existing class, without needing to create a new one.

Writing in a language without categories, the programmer is often faced with the need to perform minor operations, acting upon an object for which source code is unavailable. These routines might end up as methods in the application class that needs to perform those operations, although that doesn’t promote reuseability, since the operations are tied in with the enclosing class. A better approach, one more commonly used, is to collect these operations into a utility class.

On an open-source web application framework that I worked on, called Janx (available at www.bearriver.com) there is a string utilities class, for example. This class has operations to parse strings representing dollars and cents, encode a string for HTML display, generate a hexadecimal representation, build an MD5 digest from a string, and so forth. Each of these methods takes a string to operate upon as one of its parameters.

This “utility class” approach isn’t particularly elegant either. Dissimilar operations tend to be grouped together into the same class. Each method must be passed in the object to operated upon as a parameter, which means that the functions that you write look and operate differently from methods that are part of the class, even if they perform similar operations.

Another approach to extending functionality is to create a subclass of an existing framework object, and add your new functionality into the subclass. For instance, you might subclass an existing “image” class to add operations. The problem is that you must now be sure to work only with instances of your new class; any objects that aren’t must be converted.

If you are programming in a language such as C++ or Java without categories, though, you just deal with these limitations; they may not seem like limitations at all.

When you write an application in Cocoa using Objective C, you have the ability to put such functions directly into an existing class by creating a category on that class. No, you don’t recompile the class with new methods in the file; in fact you usually don’t have the source code to the class you are adding to.

Utilities vs. Categories

Let’s take a look at how this might be done by implementing a utility function to strip quote marks off of a string. (We’ll implement them both in Objective-C just to keep the playing field level.) We implement it as a method in a string utility class in listing 1 and 2; we implement it as a category on NSString in listing 3 and 4.

Listing 1: StringUtilities.h

#import 
@interface StringUtilities
+ (NSString *) stripQuotes:(NSString *)inString;
@end

Listing 2: StringUtilities.m

#import "StringUtilities.h"
@implementation StringUtilities

+ (NSString *) stripQuotes:(NSString *)inString
{
   NSString *result = inString;      // Return inString if no stripping needed
   int len = [inString length];
   if (len >= 2
      && '"' == [inString characterAtIndex:0]
      && '"' == [inString characterAtIndex:len-1])
   {
      // Get the substring that doesn’t include first and last character
      result =
         [inString substringWithRange:NSMakeRange(1,len-2)];
   }
   return result;
}

Listing 3: NSString+misc.h

#import 
@interface NSString ( misc )
- (NSString *) stripQuotes;
@end

Listing 4: NSString+misc.m

#import "NSString+misc.h"
@implementation NSString ( misc )

- (NSString *) stripQuotes
{
   NSString *result = self;      // Return self if no stripping needed
   int len = [self length];
   if (len >= 2
      && '"' == [self characterAtIndex:0]
      && '"' == [self characterAtIndex:len-1])
   {
      // Get the substring that doesn’t include first and last character
      result = [self substringWithRange:NSMakeRange(1,len-2)];
   }
   return result;
}

The implementations of these category looks much like the utility class; the main difference is that the string to operate upon is not passed in as a parameter; it is accessed with the self keyword. Things start to look different when you compare code that uses the category instead of a utility class. Here are snippets that use each approach.

Snippet using a utility class

   NSString *stripped =
      [StringUtilities stripQuotes:theValue];
   [lineDict setObject:stripped
      forKey:[theKey uppercaseString]];

Snippet using a category

   NSString *stripped =
      [theValue stripQuotes];
   [lineDict setObject:stripped
      forKey:[theKey uppercaseString]];

The code using the category is quite a bit cleaner because we don’t have to be conscious of a separate utility class; it is just another operation on the string, just like the built-in uppercaseString method on the last line.

Writing Categories

A category must have an @interface and @implementation section, just as a class. After the name of the class being added to is an arbitrary name which describes what the category is for, in parentheses. The example above uses "misc" as its name.

Normally, a category on a class gets its own “.h” and “.m” file; a convention is to name the file based on the class name concatenated with “+” to the category name. For example, the file NSImage+bitmap.m would be expected to hold @implementation NSImage ( bitmap ). This is not strictly neccesary; however; you could make a quick category interface and implementation right in your class file that makes use of the category; this would only be practical if it was not needed outside of the associated class.

Methods are declared and implemented just as they would be for any standard Objective-C’s methods. Keep in mind, however that self is the class that you are implementing; feel free to send messages to self to operate on that object.

The one big limitation on categories is that you can only add functionality; you cannot add new data members to the class. There are no curly braces in the @interface section of a category. If you feel the need to add data members, you may want to consider subclassing instead.

Using Categories

The best thing about categories is that you can add whatever features to Cocoa you’d like to that you feel are “missing.” Frustrated that NSImage lacks the +[NSImage imageFromData:] method? Add it in yourself! You can write generic categories and use them on all your projects, and make use of them as if you were using functionality of the classes provided by Apple. Or, you can create categories on an object as needed, whenever it seems more intuitive to extend the functionality of a Cocoa class rather than write a function to act upon that object.

You can even use categories on your own code, to help factor your application’s classes into smaller, more manageable chunks. For instance, you might create separate categories to partition your document controller into preferences management, window management, and general functionality. Doing so makes your files smaller and makes your project more navigable. Cocoa itself makes heavy use of categories in this manner; it allows classes to be created in one library (such as Foundation Kit) and then extended in another (such as Application Kit).

One of the best places to use a category is to split up your class’s private methods from its public ones, to overcome a limitation in Objective-C. Unlike C++ and Java, there’s no way to specify the access of a method using keywords. So the solution is to create a new @interface for your category at the top of your class’s “.m” file, holding the methods you do not want to be exposed in the “.h” file. This category would have a name such as “private” to indicate its purpose. Below that, the @implementation section of your class can then hold the implementation of both the public methods (declared in the “.h” file) and the private methods (declared in your private category). Other classes will not be able to see your private methods.

Usually, you will find yourself adding categories to classes in the Foundation Kit, because this kit tends to hold containers and utilities. You can even add categories to NSObject so that any object can respond to your new functionality. When there is a technique that requires bridging into Carbon or Core Foundation to accomplish your task, you could wrap it into a category on a related class (or even find one online that somebody else has already written) , so that if such functionality were to make its way into a future version of Cocoa, your code wouldn’t have to change much.

Examples

Where you make use of categories is limited only by your imagination. It is useful to look at other people’s source code just to get a sense of what kinds of categories are possible. Many source code packages are available for downloading at softrak.stepwise.com.

Here are a few examples that I have used in my own code. To make use of these, you would need to create @interface and @implementation sections following the guidelines above.

Category for NSImage

A method to set an image size to be the size of its associated NSBitmapImageRepresentation so that the image displays at full size of 72 DPI. It finds the first bitmap it can, and sets the size of the bitmap and of the image to the pixel width and height.

- (NSImage *) normalizeSize
{
   NSBitmapImageRep   *theBitmap = nil;
   NSArray               *reps = [self representations];
   NSSize                  newSize;
   int                     i;
   
   for (i = 0 ; i < [reps count] ; i++ )
   {
      NSImageRep *theRep = [reps objectAtIndex:i]; 
      if ([theRep isKindOfClass:[NSBitmapImageRep class]])
      {
         theBitmap = (NSBitmapImageRep *)theRep;
         break;
      }
   }
   if (nil != theBitmap)      // Found a bitmap to resize
   {
      newSize.width = [theBitmap pixelsWide];
      newSize.height = [theBitmap pixelsHigh];
      [theBitmap setSize:newSize];      // resize bitmap
      [self setSize:newSize];            // resize image
   }
   return self;
}

Category for NSBundle, NSDictionary, NSString, etc.

A comparison method (passing in another object of the same) so that you can sort an array of those objects by some property, using -[NSMutableArray sortUsingSelector:]. For example, you could sort an array of dictionaries by the value of their “name” key by passing in the selector for the following method.

- (NSComparisonResult) compareSymbolName:
      (NSDictionary *) inDict
{
   NSString *myName = [self objectForKey:@"name"];
   NSString *otherName = [inDict objectForKey:@"name"];
   return [myName caseInsensitiveCompare:otherName];
}

Category for NSString

A method to return an attributed string as a blue underlined hyperlink, so that text fields can respond to link clicks as in a web browser. Text in an NSTextView with these attributes will send the message of textView: clickedOnLink: atIndex: to the view’s delegate.

- (NSAttributedString *)hyperlink
{
   NSDictionary *attributes=
      [NSDictionary dictionaryWithObjectsAndKeys:
         [NSNumber numberWithInt:NSSingleUnderlineStyle],
            NSUnderlineStyleAttributeName,
         self, NSLinkAttributeName,            // link to the string itself
         [NSFont systemFontOfSize:[NSFont smallSystemFontSize]],
            NSFontAttributeName,
         [NSColor blueColor], NSForegroundColorAttributeName,
         nil];
   NSAttributedString *result= 
      [[[NSAttributedString alloc]
         initWithString:self
         attributes:attributes] autorelease];
   return result;
}

Category for NSWorkspace

A method to return the path of the current user’s temporary directory. This makes use of the Carbon FindFolder() API, and then converts the C string into an NSString.

- (NSString *) temporaryDirectory
{
   char         s[1024];
   FSSpec      spec;
   FSRef      ref;
   short      vRefNum;
   long         dirID;
   
   if ( FindFolder(
      kOnAppropriateDisk, kChewableItemsFolderType, true,
         &vRefNum, &dirID ) == noErr )
   {
      FSMakeFSSpec( vRefNum, dirID, "", &spec );
      if ( FSpMakeFSRef(&spec, &ref) == noErr )
      {
         FSRefMakePath(&ref, s, sizeof(s));
         return [NSString stringWithCString:s];
      }
   }
   return nil;
}

Category for NSSet, NSArray, etc.

A method to build a string listing the strings in a collection, separated by commas. It enumerates through all objects in the structure, adding each string and then adding a comma. It then removes the extra comma (and space) at the end, after the list is traversed.

- (NSString *) show
{
   NSString               *result = @"";      // empty string if none in collection
   NSMutableString      *buffer = [NSMutableString string];
   NSEnumerator         *theEnum = [self objectEnumerator];
   NSString               *theIdentifier;

   while (nil != (theIdentifier = [theEnum nextObject]) )
   {
      [buffer appendString:theIdentifier];
      [buffer appendString:@", "];
   }
   // Delete final comma+space from the string
   if (![buffer isEqualToString:@""])
   {
      [buffer deleteCharactersInRange:NSMakeRange(
         [buffer length]-2, 2)];
      result = [NSString stringWithString:buffer];
   }
   return result;
}

Conclusion

Hopefully you have been convinced that categories are a useful construct for programming in Cocoa. If you’re not using Objective-C, you can certainly function without them. But if you are, then categories are a great way to make your code more readable, more reuseable, more maintainable, and simpler.


Dan Wood wrote Watson for Mac OS X, a Cocoa application that connects to a variety of Web services. You can reach him at dwood@karelia.com.

 
AAPL
$113.11
Apple Inc.
+1.33
MSFT
$47.89
Microsoft Corpora
+0.23
GOOG
$520.04
Google Inc.
+3.69

MacTech Search:
Community Search:

Software Updates via MacUpdate

NeoOffice 2014.6 - Mac-tailored, OpenOff...
NeoOffice is a complete office suite for OS X. With NeoOffice, users can view, edit, and save OpenOffice documents, PDF files, and most Microsoft Word, Excel, and PowerPoint documents. NeoOffice 3.x... Read more
LibreOffice 4.3.5.2 - Free Open Source o...
LibreOffice is an office suite (word processor, spreadsheet, presentations, drawing tool) compatible with other major office suites. The Document Foundation is coordinating development and... Read more
CleanApp 5.0.0 Beta 5 - Application dein...
CleanApp is an application deinstaller and archiver.... Your hard drive gets fuller day by day, but do you know why? CleanApp 5 provides you with insights how to reclaim disk space. There are... Read more
Monolingual 1.6.2 - Remove unwanted OS X...
Monolingual is a program for removing unnecesary language resources from OS X, in order to reclaim several hundred megabytes of disk space. It requires a 64-bit capable Intel-based Mac and at least... Read more
NetShade 6.1 - Browse privately using an...
NetShade is an Internet security tool that conceals your IP address on the web. NetShade routes your Web connection through either a public anonymous proxy server, or one of NetShade's own dedicated... Read more
calibre 2.13 - Complete e-library manage...
Calibre is a complete e-book library manager. Organize your collection, convert your books to multiple formats, and sync with all of your devices. Let Calibre be your multi-tasking digital librarian... Read more
Mellel 3.3.7 - Powerful word processor w...
Mellel is the leading word processor for OS X and has been widely considered the industry standard since its inception. Mellel focuses on writers and scholars for technical writing and multilingual... Read more
ScreenFlow 5.0.1 - Create screen recordi...
Save 10% with the exclusive MacUpdate coupon code: AFMacUpdate10 Buy now! ScreenFlow is powerful, easy-to-use screencasting software for the Mac. With ScreenFlow you can record the contents of your... Read more
Simon 4.0 - Monitor changes and crashes...
Simon monitors websites and alerts you of crashes and changes. Select pages to monitor, choose your alert options, and customize your settings. Simon does the rest. Keep a watchful eye on your... Read more
BBEdit 11.0.2 - Powerful text and HTML e...
BBEdit is the leading professional HTML and text editor for the Mac. Specifically crafted in response to the needs of Web authors and software developers, this award-winning product provides a... Read more

Latest Forum Discussions

See All

Crossbow Warrior – The Legend of William...
Crossbow Warrior – The Legend of William Tell Review By Lee Hamlet on December 22nd, 2014 Our Rating: :: MISSES THE MARKUniversal App - Designed for iPhone and iPad Crossbow Warrior details the entertaining adventures of legendary... | Read more »
Workflow: Powerful Automation Made Simpl...
Workflow: Powerful Automation Made Simple Review By Campbell Bird on December 22nd, 2014 Our Rating: :: GO WITH THE FLOWUniversal App - Designed for iPhone and iPad This powerful app lets users accomplish multiple tasks at touch of... | Read more »
My Little Monster Review
My Little Monster Review By Jordan Minor on December 22nd, 2014 Our Rating: :: IT'S ALIVE!Universal App - Designed for iPhone and iPad What’s it worth to make your own monster?   | Read more »
Galaxy Trucker Pocket (Games)
Galaxy Trucker Pocket 1.0.8 Device: iOS iPhone Category: Games Price: $2.99, Version: 1.0.8 (iTunes) Description: Galaxy Truckers Wanted!================================================================= (5/5) "Galaxy Trucker isn’t... | Read more »
Make your own Tribez Figures (and More)...
Make your own Tribez Figures (and More) with Toyze Posted by Jessica Fisher on December 19th, 2014 [ permalink ] Universal App - Designed for iPhone and iPad | Read more »
So Many Holiday iOS Sales Oh My Goodness...
The holiday season is in full-swing, which means a whole lot of iOS apps and games are going on sale. A bunch already have, in fact. Naturally this means we’re putting together a hand-picked list of the best discounts and sales we can find in order... | Read more »
It’s Bird vs. Bird in the New PvP Mode f...
It’s Bird vs. Bird in the New PvP Mode for Angry Birds Epic Posted by Jessica Fisher on December 19th, 2014 [ permalink ] Universal App - Designed for iPhone and iPad | Read more »
Telltale Games and Mojang Announce Minec...
Telltale Games and Mojang Announce Minecraft: Story Mode – A Telltale Games Series Posted by Jessica Fisher on December 19th, 2014 [ permalink ] | Read more »
WarChest and Splash Damage Annouce Their...
WarChest and Splash Damage Annouce Their New Game: Tempo Posted by Jessica Fisher on December 19th, 2014 [ permalink ] WarChest Ltd and Splash Damage Ltd are teaming up again to work | Read more »
BulkyPix Celebrates its 6th Anniversary...
BulkyPix Celebrates its 6th Anniversary with a Bunch of Free Games Posted by Jessica Fisher on December 19th, 2014 [ permalink ] BulkyPix has | Read more »

Price Scanner via MacPrices.net

Apple resellers offer free overnight shipping
The Apple Store is now offering free next-day shipping on all in stock items if ordered before 12/23/14 at 10:00am PT. Local store pickup is also available within an hour of ordering for any in stock... Read more
Holiday sale continues: 15-inch Retina MacBoo...
 B&H Photo has the new 2014 15″ Retina MacBook Pros on sale for up to $300 off MSRP for a limited time. Shipping is free, and B&H charges NY sales tax only: - 15″ 2.2GHz Retina MacBook Pro: $... Read more
Holiday sale: 13-inch 128GB MacBook Air for $...
 Best Buy has the 2014 13-inch 1.4GHz 128GB MacBook Air on sale for $849.99, or $150 off MSRP, on their online store. Choose free home shipping or free local store pickup (if available). Price valid... Read more
13-inch 2.6GHz Retina MacBook Pro on sale for...
Best Buy has lowered their price on the 2014 13″ 2.6GHz/128GB Retina MacBook Pro to $1149.99 on their online store for a limited time. That’s $150 off MSRP and the lowest price available for this... Read more
Kodak Returns to CES With New Consumer Produ...
Former photography colossus Kodak is returning to CES for the first time in three years where the Kodak booth (#21818 South Hall 1) will showcase a wide range of innovative, imaging-related products... Read more
Invaluable Launches New Eponymously -Named A...
Invaluable, the world’s largest online live auction marketplace, hhas announced the official launch of the Invaluable app for iPad, now available for download in the iTunes App Store. Invaluable... Read more
IDC Reveals Worldwide Mobile Enterprise Appli...
International Data Corporation (IDC) last week hosted the IDC FutureScape: Worldwide Mobile Enterprise Applications and Solutions 2015 Predictions Web conference. The session provided organizations... Read more
Hello Vino Wine App Launches “Safe Ride Home”...
Hello Vino has announced addition of a new “Get a Safe Ride Home” feature in its Food & Drink app with a direct connection to Uber, the technology platform that connects users with rides. The... Read more
DEVON-technologies Releases DEVONthink To Go...
Coeur d’Alene, Idaho based DEVON-technologies, LLC has updated DEVONthink To Go, its mobile companion to DEVONthink, to version 1.5. The update includes an iOS 8 extension, compatibility with the... Read more
The Apple Store offering free next-day shippi...
The Apple Store is now offering free next-day shipping on all in stock items if ordered before 12/23/14 at 10:00am PT. Local store pickup is also available within an hour of ordering for any in stock... Read more

Jobs Board

*Apple* Store Leader Program (US) - Apple, I...
…Summary Learn and grow as you explore the art of leadership at the Apple Store. You'll master our retail business inside and out through training, hands-on experience, Read more
Project Manager, *Apple* Financial Services...
**Job Summary** Apple Financial Services (AFS) offers consumers, businesses and educational institutions ways to finance Apple purchases. We work with national and Read more
*Apple* Retail - Multiple Positions (US) - A...
Sales Specialist - Retail Customer Service and Sales Transform Apple Store visitors into loyal Apple customers. When customers enter the store, you're also the Read more
*Apple* Retail - Multiple Positions (US) - A...
Sales Specialist - Retail Customer Service and Sales Transform Apple Store visitors into loyal Apple customers. When customers enter the store, you're also the Read more
*Apple* Retail - Multiple Positions (US) - A...
Job Description: Sales Specialist - Retail Customer Service and Sales Transform Apple Store visitors into loyal Apple customers. When customers enter the store, Read more
All contents are Copyright 1984-2011 by Xplain Corporation. All rights reserved. Theme designed by Icreon.