Private properties for iPhone Objective-C

Keeping members private is extremely important in Object Oriented design as it allows us to adhere to the principles of encapsulation and information hiding, and makes it easier to keep our objects thread safe. We often want to use @properties in Objective-C to take care of retain counts in setters for us. However, unless you are careful these properties will always be public. This 'kitchen sink' approach results in all of your internal object state being exposed on your public API. This is dangerous because:

• Developers can modify the internal state of your objects when they shouldn't
  
• Developers are 'tempted' to access things that they shouldn't as they can see 'apparently useful' things on the interface.
  
• Developers are confused when they read the interface - they need to somehow work out which public properties they can use and which they can't.

In short: NEVER expose class internals publicly - it is very bad OO practice.

Private setters are discussed in Scott Stevenson's Objective-C tutorial, but the post does not explain how to make both the setter and getter private - furthermore I was tripped up by a coupled of points that are not illustrated in Scott's examples. So how do we create completely private properties in Objective-C? The steps are as follows:

1. Make the member variable @private. This is not essential to create the property but if we're making the property private then it'd be wise to make the member variable accessible only from within our class.
   
2. DO NOT declare the property in the main @interface in the header (.h) file
   
3. Create an @interface for an extension (unnamed category) in the .m file ABOVE the @implementation
   
4. Declare your private @property in the extension interface
   
5. @synthesize the private property in your @implementation as normal.

See code below for an example:

ClassWithPrivateProperty.h

    @interface ClassWithPrivateProperty : NSObject {

    @private

        NSString* member;

    }

    - (void) trySettingPrivateProperty;

    @end

ClassWithPrivateProperty.m

    #import "ClassWithPrivateProperty.h"

 

    @interface ClassWithPrivateProperty ()

    @property (nonatomic,retain) NSString* member; 

    @end

 

    @implementation ClassWithPrivateProperty

    @synthesize member;

    - (void) trySettingPrivateProperty {

        self.member = @"A Value";

        NSLog(@"myClass.member = %@", self.member);

    }

    @end