Categories in Objective-C aren't working

I'm developing an iOS application that needs to deploy to iOS 3.1.3. I need to extend some of the functionality of the NSData class and am using the following code inside NSData+Base64 (truncated to show the interesting part):

NSData+Base64.h

[...]

@interface NSData (Base64)

+ (NSData *)dataFromBase64String:(NSString *)aString;
- (NSString *)base64EncodedString;

@end

NSData+Base64.m

@implementation NSData (Base64)

[...]

//
// base64EncodedString
//
// Creates an NSString object that contains the base 64 encoding of the
// receiver's data. Lines are broken at 64 characters long.
//
// returns an autoreleased NSString being the base 64 representation of the
//  receiver.
//
- (NSString *)base64EncodedString
{
    size_t outputLength;
    char *outputBuffer =
        NewBase64Encode([self bytes], [self length], true, &outputLength);
    
    NSString *result =
        [[[NSString alloc]
            initWithBytes:outputBuffer
            length:outputLength
            encoding:NSASCIIStringEncoding]
        autorelease];
    free(outputBuffer);
    return result;
}

@end

However, when I try to message this selector:

NSData *HMAC = [[NSData alloc] initWithBytes:cHMAC length:sizeof(cHMAC)];
NSString *hash = [HMAC base64EncodedString];

I get the following error:

 -[NSConcreteData base64EncodedString]: unrecognized selector sent to instance 0x6146e70
2010-11-09 13:44:41.443 SpringboardApplication[21318:40b] *** Terminating app due to uncaught exception 'NSInvalidArgumentException', reason: '-[NSConcreteData base64EncodedString]: unrecognized selector sent to instance 0x6146e70'

I read a lot about iOS 3.1.x having problems with categories. I tried adding the flags -all_load and -ObjC (both separately and together) to no avail. I would really appreciate some direction of how to get开发者_C百科 this selector to work.

Thanks!

It really seems like your category isn't being compiled or linked into the same target that you're using it from. You should make sure that NSData+Base64.m is marked to be compiled by the same target that it's being used from by getting info on the two files and comparing the targets they're assigned to.

A test you can perform is to add a line with an #error error message to NSData+Base64.m, which will cause the build to fail when it gets to that file. Like this:

#error We're now compiling NSData+Base64.m

Then look and see which target fails to compile.

I had the same issue with ARC project which was linking with non-ARC module having category extension.

Fixed the issue by adding "Other Linker Flags: -all_load" in parent ARC project.

Have you #imported the header file for your category? I know it sounds simple, but I forget nearly every time.

There is a great post on The Carbon Emitter about about handling categories in iOS. It details an easy way to handle importing categories to your project.

Make a file containing all of your category imports, in this example it is Extensions.h:

#import "NSDate+Formatting.h"
#import "UIFonts+MyFonts.h"
#import "UIViewController+Tourbot.h"

Add import your file in AppName-Prefix.pch:

#import <Availability.h>

#ifndef __IPHONE_3_0
#warning "This project uses features only available in iPhone SDK 3.0 and later."
#endif

#ifdef __OBJC__
#import <UIKit/UIKit.h>
#import <Foundation/Foundation.h>
#import <QuartzCore/QuartzCore.h>
#import <CoreText/CoreText.h>
#import "Extensions.h" // Add import here
#endif

In My case when I got this error I simply added the .m file in the Compiled resources, and it get worked. This can be achieved by selecting target project->Build Phases->Compile Sources. Then you click on the + button from its bottom left. In this case you may add 'NSData+Base64.m' file to the compile sources. Then you clean your project and run. I guess this may help.