Initialize properties in Singletons in objective-c - iphone

Question

So I'm kind of unsure about something. What I want is one class to know about the data through all my different view controllers. This one class should have an array of my objects so that if I have a detailViewController, I would just be looking at one instance in my array that the DataManager would hold. I thought that this DataManager would be a singleton. I followed Apple's documentation on creating a singleton, but now I'm a bit confused on the properties.
static DataManager *sharedDmgr = nil;
+ (id)sharedInstance {
if (sharedDmgr == nil) {
sharedDmgr = [[super allocWithZone:NULL] init];
}
return sharedDmgr;
}
If I want an NSMutableArray property, what is the proper way to initialize it? Do I do something like
+ (id)sharedInstance {
if (sharedDmgr == nil) {
sharedDmgr = [[super allocWithZone:NULL] init];
[self sharedInit];
}
return sharedDmgr;
}
- (void)sharedInit {
// initialize all my properties for the singleton here?
}
Thanks!

Answer 1

Let's say a DataManager object has a NSMutableArray attribute named "array", then your objective is to be able to do [DataManager sharedDataManager].array in all your code.
The way to do it would be to:
Declare the NSMutableArray *array as an attribute on the DataManager.h #interface block.
Declare a #property for that attribute.
On the - [DataManager init] method initialize the mutable array. Something like self.array = [NSMutableArray array];
Then your sharedInstance method would be:
static DataManager *sharedDmgr = nil;
+ (id)sharedInstance {
if (sharedDmgr == nil) {
sharedDmgr = [[DataManager alloc] init];
}
return sharedDmgr;
}
All done. Let me know if you need some example code.

Answer 2

You can use the same lazy initialization pattern for your property getters. So for a mutable array...
#interface DataManager ()
#property (strong, nonatomic) NSMutableArray *array;
#end
#implementation DataManager
#synthesize array=_array;
// shared instance method like #fbernardo's suggestion
- (NSMutableArray *)array {
if (!_array) {
_array = [[NSMutableArray alloc] init];
}
return _array;
}