Objective-C - readonly NSMutableArray and NSCoding?

Question

I have an NSMutableArray, that should be only modified by the owner object (MyObject). so the first attemp was to make it a readonly property. The problem is that this class implements NSCoding, and NSCoding requires the archived objects to be a readwrite property.

Then I thought of having a private property (declared in m file), and have a public method that return my private array. But then this would be a reference to the array, and other classes would be able to modify it.

My Methods should not return a copy of this array either, because I want the other classes to be able to modify each item= in this array, but not the array itself.

QUESTION: How can I have a public property, that is readonly, and at the same time be able to archive and unarchive it?

Other classes would still be able to modify the mutable array even if it's read only property. The only thing that is read only is the pointer itself. The objects inside the array can be modified freely from the outside. — David Rönnqvist, Jul 11 '12 at 18:14
I want to be able to modify the items in the array, what I'm trying to avoid is other classes adding and removing objects from this array — aryaxt, Jul 11 '12 at 18:14
If you expose the array as mutable (even when being read only) other classes _will be_ able to add or remove items from the array — David Rönnqvist, Jul 11 '12 at 18:21
Well, you're right, readonly refers to the property and not the object itself — aryaxt, Jul 11 '12 at 18:26

Andrew Madsen · Accepted Answer · 2013-07-10T13:45:12.313

There are a few ways to do this, but one reasonable way is to maintain a private, mutable array, then provide a readonly accessor for the public. Then you just have the public accessor return an immutable copy of the internal array. That would look something like this:

In the .h file:

@interface MyClass : NSObject

@property (readonly) NSArray *publicArray;

@end

In the .m file:

@interface MyClass ()

@property NSMutableArray *privateArray;

@end

@implementation MyClass

+ (NSSet *)keyPathsForValuesAffectingValueForKey:(NSString *)key
{
    NSSet *keyPaths = [super keyPathsForValuesAffectingValueForKey:key];

    if ([key isEqualToString:@"publicArray"]) {
        keyPaths = [keyPaths setByAddingObject:@"privateArray"];
    }

    return keyPaths;
}


@synthesize privateArray = _privateArray;

- (NSArray *)publicArray { return [self.privateArray copy]; }

@end

You could do without the copy if you feel comfortable counting on the compiler to warn about code that attempts to call mutation methods on the result of -publicArray rather than runtime exceptions being thrown. Another caveat is that without a copy, any changes to the private array will be 'seen' even in a previously obtained reference to the supposedly immutable array.

Objective-C - readonly NSMutableArray and NSCoding?

1 Answers1