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Best way to define private methods for a class in Objective-C

Asked 2008-10-05T20:29:53.803
363

I just started programming Objective-C and, having a background in Java, wonder how people writing Objective-C programs deal with private methods.

I understand there may be several conventions and habits and think about this question as an aggregator of the best techniques people use dealing with private methods in Objective-C.

Please include an argument for your approach when posting it. Why is it good? Which drawbacks does it have (that you know of) and how you deal with them?

As for my findings so far.

It is possible to use categories [e.g. MyClass (Private)] defined in MyClass.m file to group private methods.

This approach has 2 issues:

  1. Xcode (and compiler?) does not check if you define all methods in private category in corresponding @implementation block
  2. You have to put @interface declaring your private category in the begin of MyClass.m file, otherwise Xcode complains with a message like "self may not respond to message "privateFoo".

The first issue can be worked around with empty category [e.g. MyClass ()].
The second one bothers me a lot. I'd like to see private methods implemented (and defined) near the end of the file; I do not know if that's possible.

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2 Answers

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every objects in Objective C conform to NSObject protocol, which holds onto the performSelector: method. I was also previously looking for a way to create some "helper or private" methods that I did not need exposed on a public level. If you want to create a private method with no overhead and not having to define it in your header file then give this a shot...

define the your method with a similar signature as the code below...

-(void)myHelperMethod: (id) sender{
     // code here...
}

then when you need to reference the method simply call it as a selector...

[self performSelector:@selector(myHelperMethod:)];

this line of code will invoke the method you created and not have an annoying warning about not having it defined in the header file.

answered 2010-11-19T16:21:21.230
2

If you wanted to avoid the @interface block at the top you could always put the private declarations in another file MyClassPrivate.h not ideal but its not cluttering up the implementation.

MyClass.h

interface MyClass : NSObject {
 @private
  BOOL publicIvar_;
  BOOL privateIvar_;
}

@property (nonatomic, assign) BOOL publicIvar;
//any other public methods. etc
@end

MyClassPrivate.h

@interface MyClass ()

@property (nonatomic, assign) BOOL privateIvar;
//any other private methods etc.
@end

MyClass.m

#import "MyClass.h"
#import "MyClassPrivate.h"
@implementation MyClass

@synthesize privateIvar = privateIvar_;
@synthesize publicIvar = publicIvar_;

@end
answered 2010-08-03T23:48:02.477

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