Question

I use a sinlgeton in my application for managing data that is available to the whole application, which accessed via:

static MMProductManager *sharedInstance = nil;
+(MMProductManager*)SharedInstance {
    dispatch_once( &resultsToken, ^(void) {
        if ( ! sharedInstance ) {
            sharedInstance = [[MMProductManager alloc] init];
        }
    });
    return sharedInstance;
}

Everything is working as expected.

In Objective C, there does not seem to be a way to hide any object’s init method, and in my case having more than instance of MMProductManager would lead to data being duplicated (in the best case scenario).

What I would like to do is guard against instantiating more than one instance. Other languages seem to have this feature; i.e. marking certain methods/classes as private. I am thinking of implementing something along like:

-(id)init {
    // guard against instantiating a more than one instance
    if ( sharedInstance )
        return sharedInstance;

    if ( (self = [super init]) ) {
        self->_resultsQueue = dispatch_queue_create( kMMResultQLAbel, NULL );
        self->_initialized  = FALSE;

        [[NSNotificationCenter defaultCenter] addObserver:self
                                                 selector:@selector(handleNotification:)
                                                     name:UIApplicationDidReceiveMemoryWarningNotification
                                                   object:0];

        [self initialize];
    }

    return self;
}

Does this approach seem reasonable?

What would happen in the case of someone allocating this class, then calling the init described above? Would it be reasonable to override +(id)alloc? If so How would I go about doing that?

I know the convention of exposing a SharedInstance method is an implicit message to other developers to go through this method, but I would like a bit more control if possible.

Answer 1

You don’t want to override - init (if not for some other reason) – - init is not the method that creates the instance. You want to override + alloc for this:

@implementation SingletonClass

+ (id)alloc
{
    static id instance = nil;
    if (instance == nil) {
        instance = [super alloc];
    }
    return instance;
}

@end

This way you can actually prevent (almost) completely creating multiple instances of SingletonClass.

(Unless somebody falls back to calling

id trickyDifferentInstance = class_createInstance(objc_getClass("SingletonClass"), 0));

but that’s very unlikely.)