Question

I’ve been trudging through some code for two days trying to figure out why I couldn’t fetch a global NSMutableArray variable I declared in the .h and implemented in .m and set in a the viewDidLoad function.

It finally dawned on me: there’s no such thing as a global variable in Objective-C, at least not in the PHP sense I’ve come to know. I didn’t ever really read the XCode error warnings, but there it was, even if not quite plain English: “Instance variable ‘blah’ accessed in class method.”

My question: What am I supposed to do now? I’ve got two View Controllers that need to access a central NSMutableDictionary I generate from a JSON file via URL. It’s basically an extended menu for all my Table View drill downs, and I’d like to have couple other “global” (non-static) variables.

Do I have to grab the JSON each time I want to generate this NSMutableDictionary or is there some way to set it once and access it from various classes via #import? Do I have to write data to a file, or is there another way people usually do this?

Answer 1

If you have two view controllers that access the shared NSMutableDictionary, can you pass a pointer to the common dictionary into their respective init messages?

So in your AppDelegate:

- (BOOL)application:(UIApplication *)application didFinishLaunchingWithOptions:(NSDictionary *)launchOptions 
{
  // the app delegate doesn't keep a reference this, it just passes it to the 
  // view controllers who retain the data (and it goes away when both have been released)
  NSMutableDictionary * commonData = [[NSMutableDictionary new] autorelease];

  // some method to parse the JSON and build the dictionary
  [self populateDataFromJSON:commonData];

   // each view controller retains the pointer to NSMutableDictionary (and releases it on dealloc)
   self.m_viewControllerOne = [[[UIViewControllerOne alloc] initWithData:commonData] autorelease];
   self.m_viewControllerTwo = [[[UIViewControllerTwo alloc] initWithData:commonData] autorelease];
}

And in your respective UIViewControllerOne and UIViewControllerTwo implementations

- (id)initWithData:(NSMutableDictionary*)data
{
    // call the base class ini
    if (!(self=[super init]))
        return nil;

    // set your retained property
    self.sharedData = data;
}

// don't forget to release the property
- (void)dealloc {
    [sharedData release];
    [super dealloc];
}