Categories allow you to add methods to existing classes. The category allows new methods to be added to an existing class.
One of favorite illustrations of Objective-c categories in action is NSString. NSString is defined in the Foundation framework, which has no notion of views or windows. However, if you use an NSString in a Cocoa application you'll notice it responds to messages like
@implementation NSString (removeNumbers)
- (NSString *)removeNumbersFromString:(NSString *)string
{
NSString *trimmedString = nil;
trimmedString = [string stringByTrimmingCharactersInSet:numbersSet];
return trimmedString;
}
@interface MyClass : NSObject
{
}
@interface MyClass : NSObject
{
}
-(NSString *)removeNumbersinString:(NSString *) string;
-(NSString *)removeNumbersinString:(NSString *)string
{
NSString *stringWithNums =string;
stringWithNums = [stringWithNums removeNumbersFromString:stringWithNums];
return stringWithNums;
}
int main(int argc, const char * argv[])
{
MyClass *myClassObject=[[MyClass alloc]init];
}
return 0;
}
int main(int argc, const char * argv[])
{
MyClass *myClassObject=[[MyClass alloc]init];
}
return 0;
}
- Categories let you add application or domain specific methods to existing classes. It can be quite powerful and convenient.
- The clue is in the restriction that categories only add methods, you can't add variables to a class using categories. If the class needs more properties, then it has to be subclassed.(edit: you can use associative storage, I believe).
- Categories are a nice way to add functionality while at the same time conforming to an object oriented principle to prefer composition over inheritance.
One of favorite illustrations of Objective-c categories in action is NSString. NSString is defined in the Foundation framework, which has no notion of views or windows. However, if you use an NSString in a Cocoa application you'll notice it responds to messages like
– drawInRect:withAttributes:.Steps to create Categories
Step 1: Create the Category
Now that your project is set up, let's create a category that adds additional functionality to the
NSStringclass. Click File > New > File and choose a Cocoa Touch Objective-C category from the window. Click "Next." Name your category "RemoveNums" and select NSString from the "Category on" drop down menu (you may need to type this in manually). Click "Next" followed by "Create."
Declare Category:
Step 2:Declare the Category Method:
Back in your Xcode project, click "NSString+RemoveNums.h" to view the new category's header file. Add the following code to the interface to declare the method.
@interface NSString (removeNumbers)
{
}
- (NSString *)removeNumbersFromString:(NSString *)string;
#import
{
@end
Implement the Category Method:
#import "NSString+removeNumbers.h"
{
/*--
- Implemetation logic for new category method
* This category method creates a NSCharacterSet of 0-9
* It trims any occurences of number characters from the input string
* It returns the new string value to the receiver
--*/
NSCharacterSet *numbersSet = [NSCharacterSet characterSetWithCharactersInString:@"0123456789"];
}
@end
First we define an
NSCharacterSetof the numbers zero through nine which we'll use as a reference to compare against the original input string. In this case, the original string "Tekglintsoftsol iPhone Training 12345" will have the numbers "12345" removed from the string because the category method uses the NSString method stringByTrimmingCharactersInSet:.
my NSString+removeNumbers.h category .h and .m is over now i have created one class i.e
MyClass
After declaring the class i have imported Category into Myclass. the declaration is look like this
#import "NSString+removeNumbers.h"
#import
{
@end
After importing the Category i have created method declaration in Myclass interface .h
#import "NSString+removeNumbers.h"
#import
{
@end
MyClass Implementation .m
#import "MyClass.h"
@implementation MyClass
{
//Create the string
NSLog(@"stringWithNums --> %@",stringWithNums);
//Run string through category method
//Output trimmed string to the console
NSLog(@"trimmed stringWithNums --> %@",stringWithNums);
}
@end
The above removeNumbersFromString is Category method whenever we use this method the original string "Tekglintsoftsol iPhone Training 12345" will have the numbers "12345" removed from the string because the category method uses the
NSString method stringByTrimmingCharactersInSet:.
After MyClass implementation is over now i am importing MyClass into main method and creating the MyClass object my code is shown below.
#import
#import "MyClass.h"
{
@autoreleasepool {
return 0;
}
Step 3:Test the Category
#import
#import "MyClass.h"
{
@autoreleasepool {
[myClassObject removeNumbersinString:@"Tekglint iPhone Training 12345"];
return 0;
}
The local variable
stringWithNums contains a combination of letters and numbers. The next line takes the string variable and runs it through the category method removeNumbersFromString. Finally, NSLog outputs the returned value of the newly trimmed string without any numbers.Step 4:Run The Application
Click Product > Run, or click the "Run" arrow in the top left corner, to test the code. Notice the console shows the original input string, "Tekglint iPhone Training 12345" as well as the string after it has been altered by the category method and the numbers have been removed.
2013-07-22 12:37:33.118 RemoveNumbers[1062:303] stringWithNums --> Tekglint iPhone
Training 12345
2013-07-22 12:37:33.128 RemoveNumbers[1062:303] trimmed stringWithNums --> Tekglint iPhone Training
Step 5: Conclusion
Subclassing is one way to add functionality to an object, but avoiding unnecessary subclassing by using a category will help reduce the amount of code and keep your projects more organized. There are a number of scenarios where using a category is beneficial.
