NSArray @property backed by a NSMutableArray

Saw a bunch of posts today on Stack Overflow asking how to implement this:

NSArray @property backed by a NSMutableArray

Hiding privately mutable properties behind immutable interfaces in Objective-C (my answer)

Protect from adding object to NSMutableArray in public interface

Having readonly nsarray property and nsmutable array not readonly with the same name and _ in xcode 4.2 vs 4.5

How to expose an NSMutableArray as an NSArray as a return type from a method

The main reason is to expose an array as immutable in the public interface, to convey that it shouldn’t be modified externally, but can be mutated internally. This is a typical thing you would want to do in a Model class, e.g. having an insertObject method and then notifying controllers via a delegate and notifications. One of the stumbling blocks I noticed is people were trying to implement is solely with their knowledge of properties, where you can redefine a readonly property as readwrite internally using a category, unfortunately you cannot change its type. With some understanding of ObjC before automatic synthesis the solution is to manually implement the ivar that backs the property, like this:

.h file:

@interface Model : NSObject

@property (nonatomic, strong) NSArray *objects;

-(void)insertObject:(id)object

// todo: delegate
@end

.m file:

@implementation Model{
    NSMutableArray* _objects;
}

-(void)insertObject:(id)object{
    if(!_objects){
        _objects = [[NSMutableArray alloc] init];
    }
    [_objects addObject:object];
}

@end

Just one of the many reasons developers have such a difficulty extracting their model from the view controller in Apple’s default project templates.