Question

I’ve found this snippet of code on the net. It sets up an NSMutableArray in a way I’ve not seen before ( I’m an Obj-C newb). Can someone explain what it’s doing and why you would do it this way? Particularly the @syncronized, static and little plus sign on the method signature.

add the following to the .h file:
+(NSMutableArray *)allMySprites;

add the following to he .m file after implementation:

static NSMutableArray * allMySprites = nil;

+(NSMutableArray *)allMySprites {
    @synchronized(allMySprites) {
        if (allMySprites == nil)
            allMySprites = [[NSMutableArray alloc] init];
        return allMySprites;
    }
 return nil;

}

Answer 1

Adding to the other responses … the posted code is wrong. It should be more like this:

@implementation SpriteManager
+ (NSMutableArray*) allMySprites {
    @synchronized(self) {
        if (allMySprites == nil) {
            allMySprites = [[NSMutableArray alloc] init];
        }
    }
    return allMySprites;
}
@end

It makes no sense to @synchronize on nil. Using self in a class method refers to the class and not the instance. Also the ‘return nil’ in the original code is pointless.

A better approach where the @synchronized can be completely avoided is to use a class initializer method:

@implementation SomeClass
+ (void) initialize
{
    allMySprites = [[NSMutableArray alloc] init];
}
@end

The initialize methods are guaranteed to be called before the class is used.