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Use NSMutableArray as a queue

Objective-C defines a small but powerful set of extensions to the ANSI C programming language that enables sophisticated object-oriented programming. Objective-C is the native language for Cocoa programming�it�s the language that the frameworks are written in, and the language that most applications are written in. You can also use some other languages�such as Python and Ruby�to develop programs using the Cocoa frameworks. It�s useful, though, to have at least a basic understanding of Objective-C because Apple�s documentation and code samples are typically written in terms of this language.

Sometimes you might need to use a queue on your iPhone or iPad app. There are many ways to accomplish this task and I would like to discuss one of the ways we are using here at Walden Systems, because we believe this is the most straightforward method to create a queue.

Strategy to using a NSMutableArray as a queue is fairly simple: First, we need use categories on NSmutableArray by creating a new .h and .m file. Second, we need to declare 2 methods, one for adding to the top and one for deleting from the bottom of the queue. Third, we need to implement the method for add and remove. Finally, we can use the new methods we created for NSMutableArray.

Here are actual code samples:

The first thing you need to do is create the .h and .m file. To do this in Xcode, do the following:

   1.    Right-Click on your project and select New File.
   2.    Select Cocoa Touch class and click on Next.
   3.    Give it a class name and select NSObject as the 
         subclass and click on Next.

Then you need to make changes to the .h file by deleting everything on the .h file and replacing it with the following to declare the methods :

1 #import 
2 
3 @interface NSMutableArray ( Queue )
4    - ( id ) deleteFromBottom ;
5    - ( void ) addToTop : ( id ) p_obj ;
6 
7 @end

Then you will have implement the new methods by opening up the .m file and making the following changes :

 1 #import "YOUR_CLASS_FILE.h"
 2 
 3 @implementation NSMutableArray ( Queue )
 4 
 5    - ( id ) deleteFromBottom
 6    {
 7       @synchronized ( self )
 8       {
 9          if ( [ self count ] == 0 )
10          {
11             return nil ;
12          }
13 
14          id queueObject = [ self objectAtIndex : 0 ] ;
15 
16          [ self removeObjectAtIndex : 0 ] ;
17 
18          return queueObject ;
19       }
20    }
21    - ( void ) addToTop : ( id ) obj
22    {
23       @synchronized ( self )
24       {
25          [ self addObject : obj ] ;
26       }
27    }
28    
29 @end
30 
31 



32 
33 So, this is how the final code will look like:
34 

35     Queue.h :
36 

37 
38 #import 
39 
40 @interface NSMutableArray ( Queue )
41    - ( id ) deleteFromBottom ;
42    - ( void ) addToTop : ( id ) p_obj ;
43 
44 @end

Queue.m :

 1 #import "YOUR_CLASS_FILE.h"
 2 
 3 @implementation NSMutableArray ( Queue )
 4 
 5    - ( id ) deleteFromBottom
 6    {
 7       @synchronized ( self )
 8       {
 9          if ( [ self count ] == 0 )
10          {
11             return nil ;
12          }
13 
14          id queueObject = [ self objectAtIndex : 0 ] ;
15 
16          [ self removeObjectAtIndex : 0 ] ;
17 
18          return queueObject ;
19       }
20    }
21    - ( void ) addToTop : ( id ) obj
22    {
23       @synchronized ( self )
24       {
25          [ self addObject : obj ] ;
26       }
27    }
28    
29 @end
30 
31 



32 
33 To use the category,  import the .h file in your class by adding the following line :
34     import "Queue.h"
35 


36 Now when you want to use a NSMutableArray as a queue,  you can simply do the following to declare the queue:
37 

38 
39 NSMutableArray *     queueArray ;
40 
41 queueArray - [ [ NSMutableArray alloc ] init ] ;

To add the the queue, do the following :

    [ queueArray addToTop: @"SOME_VALUE" ] ;

To delete from the queue, do the following :

1  NSString *    someValue ;
2 
3 someValue = [ queue deleteFromBottom ] ;