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.

 

Community Search:
MacTech Search:

Software Updates via MacUpdate

Corel Painter 20.1.0.285 - Digital art s...
Corel Painter lets you advance your digital art style with painted textures, subtle glazing brushwork, interactive gradients, and realistic Natural-Media. Easily transition from traditional to... Read more
iTubeDownloader 6.5.19 - Easily download...
iTubeDownloader is a powerful-yet-simple YouTube downloader for the masses. Because it contains a proprietary browser, you can browse YouTube like you normally would. When you see something you want... Read more
OmniFocus 3.8 - GTD task manager with iO...
OmniFocus is an organizer app. It uses projects to organize tasks naturally, and then add tags to organize across projects. Easily enter tasks when you’re on the go, and process them when you have... Read more
Hazel 4.4.5 - Create rules for organizin...
Hazel is your personal housekeeper, organizing and cleaning folders based on rules you define. Hazel can also manage your trash and uninstall your applications. Organize your files using a familiar... Read more
Macs Fan Control 1.5.7 - Monitor and con...
Macs Fan Control allows you to monitor and control almost any aspect of your computer's fans, with support for controlling fan speed, temperature sensors pane, menu-bar icon, and autostart with... Read more
Acorn 6.6 - Bitmap image editor.
Acorn is a new image editor built with one goal in mind - simplicity. Fast, easy, and fluid, Acorn provides the options you'll need without any overhead. Acorn feels right, and won't drain your bank... Read more
OnyX 3.8.2 - Maintenance and optimizatio...
OnyX is a multifunction utility that you can use to verify the startup disk and the structure of its system files, to run miscellaneous maintenance and cleaning tasks, to configure parameters in the... Read more
macOS Catalina 10.15.5 - An Operating Sy...
macOS Catalina gives you more of everything you love about Mac. Experience three all-new media apps: Apple Music, Apple TV, and Apple Podcasts. Locate a missing Mac with the new Find My app. And now... Read more
macOS High Sierra Security Updates 10.13...
macOS High Sierra introduces new core technologies that improve the most important functions of your Mac. From rearchitecting how it stores your data to improving the efficiency of video streaming to... Read more
Numi 3.28 - Menu-bar calculator supports...
Numi is a calculator that magically combines calculations with text, and allows you to freely share your computations. Numi combines text editor and calculator Support plain English. For example, '5... Read more

Latest Forum Discussions

See All

Steam Link Spotlight - Signs of the Sojo...
Steam Link Spotlight is a feature where we look at PC games that play exceptionally well using the Steam Link app. Our last entry was XCOM: Chimera Squad. Read about how it plays using Steam Link's new mouse and keyboard support over here. | Read more »
Steampunk Tower 2, DreamGate's sequ...
Steampunk Tower 2 is a DreamGate's follow up to their previous tower defence game. It's available now for both iOS and Android as a free-to-play title and will see players defending their lone base by kitting it out with a variety of turrets. [... | Read more »
Clash Royale: The Road to Legendary Aren...
Supercell recently celebrated its 10th anniversary and their best title, Clash Royale, is as good as it's ever been. Even for lapsed players, returning to the game is as easy as can be. If you want to join us in picking the game back up, we've put... | Read more »
Pokemon Go Fest 2020 will be a virtual e...
Niantic has announced that Pokemon Go Fest will still take place this year although understandably it won't be a physical event. Instead, it will become a virtual celebration and is set to be held on 25th and 26th July. [Read more] | Read more »
Marvel Future Fight's major May upd...
Marvel Future Fight's latest update has now landed, and it sounds like a big one. The focus this time around is on Marvel's Guardians of the Galaxy, and it introduces all-new characters, quests, and uniforms for players to collect. [Read more] | Read more »
SINoALICE, Yoko Taro and Pokelabo's...
Yoko Taro and developer Pokelabo's SINoALICE has now opened for pre-registration over on the App Store. It's already amassed 1.5 million Android pre-registrations, and it's currently slated to launch on July 1st. [Read more] | Read more »
Masketeers: Idle Has Fallen's lates...
Masketeers: Idle Has Fallen is the latest endeavour from Appxplore, the folks behind Crab War, Thor: War of Tapnarok and Light A Way. It's an idle RPG that's currently available for Android in Early Access and will head to iOS at a later date. [... | Read more »
Evil Hunter Tycoon celebrates 2 million...
Evil Hunter Tycoon has proved to be quite the hit since launching back in March, with its most recent milestone being 2 million downloads. To celebrate the achievement, developer Super Planet has released a new updated called Darkness' Front Yard... | Read more »
Peak's Edge is an intriguing roguel...
Peak's Edge is an upcoming roguelike puzzle game from developer Kenny Sun that's heading for both iOS and Android on June 4th as a free-to-play title. It will see players rolling a pyramid shape through a variety of different levels. [Read more] | Read more »
Clash Royale: The Road to Legendary Aren...
Supercell recently celebrated its 10th anniversary and their best title, Clash Royale, is as good as it's ever been. Even for lapsed players, returning to the game is as easy as can be. If you want to join us in picking the game back up, we've put... | Read more »

Price Scanner via MacPrices.net

New 16″ MacBook Pros now on sale for up to $2...
Apple reseller DataVision is now offering new 16″ Apple MacBook Pros for up to $255 off MSRP, each including free shipping. Prices start at $2194. DataVision charges sales tax for NY, NJ, PA, and CA... Read more
Apple now offering Certified Refurbished iPho...
Apple is now offering Certified Refurbished iPhone Xr models in the refurbished section of their online store starting at $499. Each iPhone comes with Apple’s standard one-year warranty, ships free,... Read more
Sale! Get a 10.2″ 32GB WiFi iPad for only $27...
Walmart has new 10.2″ 32GB WiFi iPads on sale for $50 off Apple’s MSRP, only $279. These are the same iPads sold by Apple in their retail and online stores. Be sure to select Walmart as the seller... Read more
Apple resellers offer new 2020 Mac minis for...
Apple resellers are offering new 2020 Mac minis for up to $50 off Apple’s MSRP with prices available starting at $759. Shipping is free: (1) B&H Photo: – 2020 4-Core Mac mini: $759 $40 off MSRP... Read more
Sprint is offering the Apple iPhone 11 free t...
Did you miss out on Sprint’s recent free iPhone SE promotion? No worries. Sprint has the 64GB iPhone 11 available for $0 per month for new lines when you trade-in a qualifying phone in any condition... Read more
Apple has clearance 2019 13″ 1.4GHz MacBook P...
Apple has Certified Refurbished 2019 13″ 1.4GHz 4-Core Touch Bar MacBook Pros available today starting at $979 and up to $440 off original MSRP. Apple’s one-year warranty is included, shipping is... Read more
Apple restocks 2019 MacBook Airs starting at...
Apple has clearance, Certified Refurbished, 2019 13″ MacBook Airs available again starting at $779. Each MacBook features a new outer case, comes with a standard Apple one-year warranty, and is... Read more
Apple restocks clearance Mac minis for only $...
Apple has restocked Certified Refurbished 2018 4-Core Mac minis for only $599. Each mini comes with a new outer case plus a standard Apple one-year warranty. Shipping is free: – 3.6GHz Quad-Core... Read more
Apple’s new 2020 13″ MacBook Airs on sale for...
B&H Photo has Apple’s new 2020 13″ 4-Core and 6-Core MacBook Airs on sale today for $50-$100 off Apple’s MSRP, starting at $949. Expedited shipping is free to many addresses in the US. The... Read more
B&H continues to offer clearance 2019 13″...
B&H Photo has clearance 2019 13″ 4-Core MacBook Pros available for up to $300 off Apple’s original MSRP, with prices starting at $1149. Expedited shipping is free to many addresses in the US. B... Read more

Jobs Board

Security Officer - *Apple* Store - NANA (Un...
**Security Officer \- Apple Store** **Description** About NMS Built on a culture of safety and integrity, NMSdelivers award\-winning, integrated support services to Read more
Transition Into Practice Program (TIP) - Sept...
…Academy-Transition into Practice (TIP) Residency program at St Mary Medical Center in Apple Valley, CA. **We are seekingRegistered Nurses who are:** + New graduate Read more
Essbase Developer - *Apple* - Theorem, LLC...
Job Summary Apple is seeking an experienced, detail-minded Essbase developer to join our worldwide business development and strategy team. If you are someone who Read more
Senior Software Engineer @ *Apple* - Theore...
Job Summary Apple is looking for a seasoned senior software engineer to join our worldwide business development and strategy team. This is an opportunity to lead a Read more
Cub Foods - *Apple* Valley - Now Hiring Par...
Cub Foods - Apple Valley - Now Hiring Part Time! United States of America, Minnesota, Apple Valley Retail Operations Post Date May 18, 2020 Requisition # 119230 Read more
All contents are Copyright 1984-2011 by Xplain Corporation. All rights reserved. Theme designed by Icreon.